69 Character Tables

Called with a group G, CharacterTable calls the attribute OrdinaryCharacterTable (see OrdinaryCharacterTable). Called with first argument a group G or an ordinary character table ordtbl, and second argument a prime p, CharacterTable calls the operation BrauerTable (see BrauerTable). Called with a string name and perhaps optional parameters param, CharacterTable delegates to CharacterTableFromLibrary, which tries to access the GAP Character Table Library (see the manual of this library for an overview of admissible strings name).

Probably the most interesting information about the character table is its list of irreducibles, which can be accessed as the value of the attribute Irr (see Irr). If the argument of CharacterTable is a string name then the irreducibles are just read from the library file, therefore the returned table stores them already. However, if CharacterTable is called with a group G or with an ordinary character table ordtbl, the irreducible characters are not computed by CharacterTable. They are only computed when the Irr value is accessed for the first time, for example when Display is called for the table (see Printing Character Tables). This means for example that CharacterTable returns its result very quickly, and the first call of Display for this table may take some time because the irreducible characters must be computed at that time before they can be displayed together with other information stored on the character table. The value of the filter HasIrr indicates whether the irreducible characters have been computed already.

The reason why CharacterTable does not compute the irreducible characters is that there are situations where one only needs the ``table head'', that is, the information about class lengths, power maps etc., but not the irreducibles. For example, if one wants to inspect permutation characters of a group then all one has to do is to induce the trivial characters of subgroups one is interested in; for that, only class lengths and the class fusion are needed. Or if one wants to compute the Molien series (see MolienSeries) for a given complex matrix group, the irreducible characters of this group are in general of no interest.

For details about different algorithms to compute the irreducible characters, see Computing the Irreducible Characters of a Group.

If the group G is given as an argument, CharacterTable accesses the conjugacy classes of G and therefore causes that these classes are computed if they were not yet stored (see The Interface between Character Tables and Groups).

Called with an ordinary character table ordtbl or a group G, BrauerTable returns its p-modular character table if GAP can compute this table, and fail otherwise. The p-modular table can be computed for p-solvable groups (using the Fong-Swan Theorem) and in the case that ordtbl is a table from the GAP character table library for which also the p-modular table is contained in the table library.

The default method for a group and a prime delegates to BrauerTable for the ordinary character table of this group. The default method for ordtbl uses the attribute ComputedBrauerTables for storing the computed Brauer table at position p, and calls the operation BrauerTableOp for computing values that are not yet known.

So if one wants to install a new method for computing Brauer tables then it is sufficient to install it for BrauerTableOp.

The \mod operator for a character table and a prime (see Operators for Character Tables) delegates to BrauerTable.

gap> g:= SymmetricGroup( 4 );
Sym( [ 1 .. 4 ] )
gap> tbl:= CharacterTable( g );;  HasIrr( tbl );
false
gap> tblmod2:= CharacterTable( tbl, 2 );
BrauerTable( Sym( [ 1 .. 4 ] ), 2 )
gap> tblmod2 = CharacterTable( tbl, 2 );
true
gap> tblmod2 = BrauerTable( tbl, 2 );
true
gap> tblmod2 = BrauerTable( g, 2 );
true
gap> CharacterTable( "A5" );
CharacterTable( "A5" )
gap> CharacterTable( "Symmetric", 4 );
CharacterTable( "Sym(4)" )
gap> ComputedBrauerTables( tbl );
[ , BrauerTable( Sym( [ 1 .. 4 ] ), 2 ) ]

SupportedCharacterTableInfo is a list that contains at position 3i-2 an attribute getter function, at position 3i-1 the name of this attribute, and at position 3i a list containing one or two of the strings "class", "character", depending on whether the attribute value relies on the ordering of classes or characters. This allows one to set exactly the components with these names in the record that is later converted to the new table, in order to use the values as attribute values. So the record components that shall not be regarded as attribute values can be ignored. Also other attributes of the old table are ignored.

SupportedCharacterTableInfo is used when (ordinary or Brauer) character table objects are created from records, using ConvertToCharacterTable (see ConvertToCharacterTable).

New attributes and properties can be notified to SupportedCharacterTableInfo by creating them with DeclareAttributeSuppCT and DeclarePropertySuppCT instead of DeclareAttribute and DeclareProperty.

Let record be a record. ConvertToCharacterTable converts record into a component object (see Component Objects in ``Programming in GAP'') representing a character table. The values of those components of record whose names occur in SupportedCharacterTableInfo (see SupportedCharacterTableInfo) correspond to attribute values of the returned character table. All other components of the record simply become components of the character table object.

If inconsistencies in record are detected, fail is returned. record must have the component UnderlyingCharacteristic bound (see UnderlyingCharacteristic), since this decides about whether the returned character table lies in IsOrdinaryTable or in IsBrauerTable (see IsOrdinaryTable, IsBrauerTable).

ConvertToCharacterTableNC does the same except that all checks of record are omitted.

An example of a conversion from a record to a character table object can be found in Section PrintCharacterTable.

69.4 Character Table Categories

Every ``character table like object'' in GAP lies in the category IsNearlyCharacterTable. There are four important subcategories, namely the ordinary tables in IsOrdinaryTable, the Brauer tables in IsBrauerTable, the union of these two in IsCharacterTable, and the incomplete ordinary tables in IsCharacterTableInProgress.

We want to distinguish ordinary and Brauer tables because a Brauer table may delegate tasks to the ordinary table of the same group, for example the computation of power maps. A Brauer table is constructed from an ordinary table and stores this table upon construction (see OrdinaryCharacterTable).

Furthermore, IsOrdinaryTable and IsBrauerTable denote character tables that provide enough information to compute all power maps and irreducible characters (and in the case of Brauer tables to get the ordinary table), for example because the underlying group (see UnderlyingGroup!for character tables) is known or because the table is a library table (see the manual of the GAP Character Table Library). We want to distinguish these tables from partially known ordinary tables that cannot be asked for all power maps or all irreducible characters.

The character table objects in IsCharacterTable are always immutable (see Mutability and Copyability). This means mainly that the ordering of conjugacy classes used for the various attributes of the character table cannot be changed; see Sorted Character Tables for how to compute a character table with a different ordering of classes.

The GAP objects in IsCharacterTableInProgress represent incomplete ordinary character tables. This means that not all irreducible characters, not all power maps are known, and perhaps even the number of classes and the centralizer orders are known. Such tables occur when the character table of a group G is constructed using character tables of related groups and information about G but for example without explicitly computing the conjugacy classes of G. An object in IsCharacterTableInProgress is first of all mutable, so nothing is stored automatically on such a table, since otherwise one has no control of side-effects when a hypothesis is changed. Operations for such tables may return more general values than for other tables, for example class functions may contain unknowns (see Chapter Unknowns) or lists of possible values in certain positions, the same may happen also for power maps and class fusions (see Parametrized Maps). @Incomplete tables in this sense are currently not supported and will be described in a chapter of their own when they become available.@ Note that the term ``incomplete table'' shall express that GAP cannot compute certain values such as irreducible characters or power maps. A table with access to its group is therefore always complete, also if its irreducible characters are not yet stored.

gap> g:= SymmetricGroup( 4 );;
gap> tbl:= CharacterTable( g );  modtbl:= tbl mod 2;
CharacterTable( Sym( [ 1 .. 4 ] ) )
BrauerTable( Sym( [ 1 .. 4 ] ), 2 )
gap> IsCharacterTable( tbl );  IsCharacterTable( modtbl );
true
true
gap> IsBrauerTable( modtbl );  IsBrauerTable( tbl );
true
false
gap> IsOrdinaryTable( tbl );  IsOrdinaryTable( modtbl );
true
false
gap> IsCharacterTable( g );  IsCharacterTable( Irr( g ) );
false
false

is the info class (see Info Functions) for computations with character tables.

Every character table like object lies in this family (see Families).

69.5 Conventions for Character Tables

The following few conventions should be noted.

• The class of the identity element is expected to be the first one; thus the degree of a character is the character value at position 1.
• The trivial character of a character table need not be the first in the list of irreducibles.
• Most functions that take a character table as an argument and work with characters expect these characters as an argument, too. For some functions, the list of irreducible characters serves as the default, i.e, the value of the attribute Irr (see Irr); in these cases, the Irr value is automatically computed if it was not yet known.
• For a stored class fusion, the image table is denoted by its Identifier value (see Identifier!for character tables); each library table has a unique identifier by which it can be accessed (see ctbllib:Access to Library Character Tables in the manual for the GAP Character Table Library), tables constructed from groups get an identifier that is unique in the current GAP session.

69.6 The Interface between Character Tables and Groups

For a character table with underlying group (see UnderlyingGroup!for character tables), the interface between table and group consists of three attribute values, namely the group, the conjugacy classes stored in the table (see ConjugacyClasses below) and the identification of the conjugacy classes of table and group (see IdentificationOfConjugacyClasses below).

Character tables constructed from groups know these values upon construction, and for character tables constructed without groups, these values are usually not known and cannot be computed from the table.

However, given a group G and a character table of a group isomorphic to G (for example a character table from the GAP table library), one can tell GAP to use the given table as the character table of G (see ConnectGroupAndCharacterTable).

Tasks may be delegated from a group to its character table or vice versa only if these three attribute values are stored in the character table.

For an ordinary character table ordtbl of a finite group, the group can be stored as value of UnderlyingGroup.

Brauer tables do not store the underlying group, they access it via the ordinary table (see OrdinaryCharacterTable).

For a character table tbl with known underlying group G, the ConjugacyClasses value of tbl is a list of conjugacy classes of G. All those lists stored in the table that are related to the ordering of conjugacy classes (such as sizes of centralizers and conjugacy classes, orders of representatives, power maps, and all class functions) refer to the ordering of this list.

This ordering need not coincide with the ordering of conjugacy classes as stored in the underlying group of the table (see Sorted Character Tables). One reason for this is that otherwise we would not be allowed to use a library table as the character table of a group for which the conjugacy classes are stored already. (Another, less important reason is that we can use the same group as underlying group of character tables that differ only w.r.t. the ordering of classes.)

The class of the identity element must be the first class (see Conventions for Character Tables).

If tbl was constructed from G then the conjugacy classes have been stored at the same time when G was stored. If G and tbl were connected later than in the construction of tbl, the recommended way to do this is via ConnectGroupAndCharacterTable (see ConnectGroupAndCharacterTable). So there is no method for ConjugacyClasses that computes the value for tbl if it is not yet stored.

Brauer tables do not store the (p-regular) conjugacy classes, they access them via the ordinary table (see OrdinaryCharacterTable) if necessary.

For an ordinary character table tbl with known underlying group G, IdentificationOfConjugacyClasses returns a list of positive integers that contains at position i the position of the i-th conjugacy class of tbl in the list ConjugacyClasses( G ).

gap> g:= SymmetricGroup( 4 );;
gap> repres:= [ (1,2), (1,2,3), (1,2,3,4), (1,2)(3,4), () ];;
gap> ccl:= List( repres, x -> ConjugacyClass( g, x ) );;
gap> SetConjugacyClasses( g, ccl );
gap> tbl:= CharacterTable( g );;   # the table stores already the values
gap> HasConjugacyClasses( tbl );  HasUnderlyingGroup( tbl );
true
true
gap> UnderlyingGroup( tbl ) = g;
true
gap> HasIdentificationOfConjugacyClasses( tbl );
true
gap> IdentificationOfConjugacyClasses( tbl );
[ 5, 1, 2, 3, 4 ]

Let G be a group and tbl a character table of (a group isomorphic to) G, such that G does not store its OrdinaryCharacterTable value and tbl does not store its UnderlyingGroup value. ConnectGroupAndCharacterTable calls CompatibleConjugacyClasses, trying to identify the classes of G with the columns of tbl.

If this identification is unique up to automorphisms of tbl (see AutomorphismsOfTable) then tbl is stored as CharacterTable value of G, in tbl the values of UnderlyingGroup, ConjugacyClasses, and IdentificationOfConjugacyClasses are set, and true is returned.

Otherwise, i.e., if GAP cannot identify the classes of G up to automorphisms of G, false is returned.

If a record arec is present as third argument, its meaning is the same as for CompatibleConjugacyClasses (see CompatibleConjugacyClasses).

If a list bijection is entered as third argument, it is used as value of IdentificationOfConjugacyClasses, relative to ConjugacyClasses( G ), without further checking, and true is returned.

In the first form, ccl must be a list of the conjugacy classes of the group G, and tbl the ordinary character table of G. Then CompatibleConjugacyClasses returns a list l of positive integers that describes an identification of the columns of tbl with the conjugacy classes ccl in the sense that l[i] is the position in ccl of the class corresponding to the i-th column of tbl, if this identification is unique up to automorphisms of tbl (see AutomorphismsOfTable); if GAP cannot identify the classes, fail is returned.

In the second form, tbl must be an ordinary character table, and CompatibleConjugacyClasses checks whether the columns of tbl can be identified with the conjugacy classes of a group isomorphic to that for which tbl is the character table; the return value is a list of all those sets of class positions for which the columns of tbl cannot be distinguished with the invariants used, up to automorphisms of tbl. So the identification is unique if and only if the returned list is empty.

The usual approach is that one first calls CompatibleConjugacyClasses in the second form for checking quickly whether the first form will be successful, and only if this is the case the more time consuming calculations with both group and character table are done.

The following invariants are used.

1. element orders (see OrdersClassRepresentatives),
2. class lengths (see SizesConjugacyClasses),
3. power maps (see PowerMap, ComputedPowerMaps),
4. symmetries of the table (see AutomorphismsOfTable).

If the optional argument arec is present then it must be a record whose components describe additional information for the class identification. The following components are supported.

natchar

if G is a permutation group or matrix group then the value of this component is regarded as the list of values of the natural character (see NaturalCharacter) of G, w.r.t. the ordering of classes in tbl,

bijection

a list describing a partial bijection; the i-th entry, if bound, is the position of the i-th conjugacy class of tbl in the list ccl.

gap> g:= AlternatingGroup( 5 );
Alt( [ 1 .. 5 ] )
gap> tbl:= CharacterTable( "A5" );
CharacterTable( "A5" )
gap> HasUnderlyingGroup( tbl );  HasOrdinaryCharacterTable( g );
false
false
gap> CompatibleConjugacyClasses( tbl );   # unique identification
[  ]
gap> ConnectGroupAndCharacterTable( g, tbl );
true
gap> HasConjugacyClasses( tbl );  HasUnderlyingGroup( tbl );
true
true
gap> IdentificationOfConjugacyClasses( tbl );
[ 1, 2, 3, 4, 5 ]
gap> # Here is an example where the identification is not unique.
gap> CompatibleConjugacyClasses( CharacterTable( "J2" ) );
[ [ 17, 18 ], [ 9, 10 ] ]

69.7 Operators for Character Tables

The following infix operators are defined for character tables.

tbl1 * tbl2

the direct product of two character tables (see CharacterTableDirectProduct),

tbl / list

the table of the factor group modulo the normal subgroup spanned by the classes in the list list (see CharacterTableFactorGroup),

tbl mod p

the p-modular Brauer character table corresponding to the ordinary character table tbl (see CharacterTable),

tbl.name

the position of the class with name name in tbl (see ClassNames).

69.8 Attributes and Properties of Character Tables

Several attributes for groups are valid also for character tables. These are on one hand those that have the same meaning for both group and character table, and whose values can be read off or computed, respectively, from the character table, such as Size, IsAbelian, or IsSolvable. On the other hand, there are attributes whose meaning for character tables is different from the meaning for groups, such as ConjugacyClasses.

In the first two forms, CharacterDegrees returns a collected list of the degrees of the absolutely irreducible characters of the group G; the optional second argument p must be either zero or a prime integer denoting the characteristic, the default value is zero. In the third form, tbl must be an (ordinary or Brauer) character table, and CharacterDegrees returns a collected list of the degrees of the absolutely irreducible characters of tbl.

(The default method for the call with only argument a group is to call the operation with second argument 0.)

For solvable groups, the default method is based on Con90b.

gap> CharacterDegrees( SymmetricGroup( 4 ) );
[ [ 1, 2 ], [ 2, 1 ], [ 3, 2 ] ]
gap> CharacterDegrees( SymmetricGroup( 4 ), 2 );
[ [ 1, 1 ], [ 2, 1 ] ]
gap> CharacterDegrees( CharacterTable( "A5" ) );
[ [ 1, 1 ], [ 3, 2 ], [ 4, 1 ], [ 5, 1 ] ]
gap> CharacterDegrees( CharacterTable( "A5" ) mod 2 );
[ [ 1, 1 ], [ 2, 2 ], [ 4, 1 ] ]

Called with a group G, Irr returns the irreducible characters of the ordinary character table of G. Called with a group G and a prime integer p, Irr returns the irreducible characters of the p-modular Brauer table of G. Called with an (ordinary or Brauer) character table tbl, Irr returns the list of all complex absolutely irreducible characters of tbl.

For a character table tbl with underlying group, Irr may delegate to the group. For a group G, Irr may delegate to its character table only if the irreducibles are already stored there.

(If G is p-solvable (see IsPSolvable) then the p-modular irreducible characters can be computed by the Fong-Swan Theorem; in all other cases, there may be no method.)

Note that the ordering of columns in the Irr matrix of the group G refers to the ordering of conjugacy classes in CharacterTable( G ), which may differ from the ordering of conjugacy classes in G (see The Interface between Character Tables and Groups). As an extreme example, for a character table obtained from sorting the classes of CharacterTable( G ), the ordering of columns in the Irr matrix respects the sorting of classes (see Sorted Character Tables), so the irreducibles of such a table will in general not coincide with the irreducibles stored as Irr( G ) although also the sorted table stores the group G.

In the following example we temporarily increase the line length limit from its default value 80 to 85 in order to get a nicer output format.

gap> Irr( SymmetricGroup( 4 ) );
[ Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 1, -1, 1, 1, -1 ] ), 
  Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 3, -1, -1, 0, 1 ] ), 
  Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 2, 0, 2, -1, 0 ] ), 
  Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 3, 1, -1, 0, -1 ] ), 
  Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1, 1 ] ) ]
gap> Irr( SymmetricGroup( 4 ), 2 );
[ Character( BrauerTable( Sym( [ 1 .. 4 ] ), 2 ), [ 1, 1 ] ), 
  Character( BrauerTable( Sym( [ 1 .. 4 ] ), 2 ), [ 2, -1 ] ) ]
gap> SizeScreen([ 85, ]);;
gap> Irr( CharacterTable( "A5" ) );
[ Character( CharacterTable( "A5" ), [ 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( "A5" ), [ 3, -1, 0, -E(5)-E(5)^4, -E(5)^2-E(5)^3 ] ), 
  Character( CharacterTable( "A5" ), [ 3, -1, 0, -E(5)^2-E(5)^3, -E(5)-E(5)^4 ] ), 
  Character( CharacterTable( "A5" ), [ 4, 0, 1, -1, -1 ] ), 
  Character( CharacterTable( "A5" ), [ 5, 1, -1, 0, 0 ] ) ]
gap> SizeScreen([ 80, ]);;
gap> Irr( CharacterTable( "A5" ) mod 2 );
[ Character( BrauerTable( "A5", 2 ), [ 1, 1, 1, 1 ] ), 
  Character( BrauerTable( "A5", 2 ), [ 2, -1, E(5)+E(5)^4, E(5)^2+E(5)^3 ] ), 
  Character( BrauerTable( "A5", 2 ), [ 2, -1, E(5)^2+E(5)^3, E(5)+E(5)^4 ] ), 
  Character( BrauerTable( "A5", 2 ), [ 4, 1, -1, -1 ] ) ]

LinearCharacters returns the linear (i.e., degree 1) characters in the Irr (see Irr) list of the group G or the character table tbl, respectively. In the second form, LinearCharacters returns the p-modular linear characters of the group G.

For a character table tbl with underlying group, LinearCharacters may delegate to the group. For a group G, LinearCharacters may delegate to its character table only if the irreducibles are already stored there.

The ordering of linear characters in tbl need not coincide with the ordering of linear characters in the irreducibles of tbl (see Irr).

gap> LinearCharacters( SymmetricGroup( 4 ) );
[ Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( Sym( [ 1 .. 4 ] ) ), [ 1, -1, 1, 1, -1 ] ) ]

OrdinaryCharacterTable returns the ordinary character table of the group G or the Brauer character table modtbl, respectively.

Since Brauer character tables are constructed from ordinary tables, the attribute value for modtbl is already stored (cf. Character Table Categories).

gap> OrdinaryCharacterTable( SymmetricGroup( 4 ) );
CharacterTable( Sym( [ 1 .. 4 ] ) )
gap> tbl:= CharacterTable( "A5" );;  modtbl:= tbl mod 2;
BrauerTable( "A5", 2 )
gap> OrdinaryCharacterTable( modtbl ) = tbl;
true

The following operations for groups are applicable to character tables and mean the same for a character table as for the group; see the chapter about groups for the definition.

AbelianInvariants

CommutatorLength

Exponent

IsAbelian

IsCyclic

IsFinite

IsMonomial

IsNilpotent

IsPerfect

IsSimple

IsSolvable

IsSupersolvable

NrConjugacyClasses

Size

gap> tables:= [ CharacterTable( CyclicGroup( 3 ) ),
>               CharacterTable( SymmetricGroup( 4 ) ),
>               CharacterTable( AlternatingGroup( 5 ) ) ];;
gap> List( tables, AbelianInvariants );
[ [ 3 ], [ 2 ], [  ] ]
gap> List( tables, CommutatorLength );
[ 1, 1, 1 ]
gap> List( tables, Exponent );
[ 3, 12, 30 ]
gap> List( tables, IsAbelian );
[ true, false, false ]
gap> List( tables, IsCyclic );
[ true, false, false ]
gap> List( tables, IsFinite );
[ true, true, true ]
gap> List( tables, IsMonomial );
[ true, true, false ]
gap> List( tables, IsNilpotent );
[ true, false, false ]
gap> List( tables, IsPerfect );
[ false, false, true ]
gap> List( tables, IsSimple );
[ true, false, true ]
gap> List( tables, IsSolvable );
[ true, true, false ]
gap> List( tables, IsSupersolvable );
[ true, false, false ]
gap> List( tables, NrConjugacyClasses );
[ 3, 5, 5 ]
gap> List( tables, Size );
[ 3, 24, 60 ]

The following three attributes for character tables would make sense also for groups but are in fact not used for groups. This is because the values depend on the ordering of conjugacy classes stored as value of ConjugacyClasses, and this value may differ for a group and its character table (see The Interface between Character Tables and Groups). Note that for character tables, the consistency of attribute values must be guaranteed, whereas for groups, there is no need to impose such a consistency rule.

is a list of orders of representatives of conjugacy classes of the character table tbl, in the same ordering as the conjugacy classes of tbl.

is a list that stores at position i the size of the centralizer of any element in the i-th conjugacy class of the character table tbl.

is a list that stores at position i the size of the i-th conjugacy class of the character table tbl.

gap> tbl:= CharacterTable( "A5" );;
gap> OrdersClassRepresentatives( tbl );
[ 1, 2, 3, 5, 5 ]
gap> SizesCentralizers( tbl );
[ 60, 4, 3, 5, 5 ]
gap> SizesConjugacyClasses( tbl );
[ 1, 15, 20, 12, 12 ]

The following attributes apply only to character tables, not to groups.

is the permutation group of all column permutations of the character table tbl that leave the set of irreducibles and each power map of tbl invariant (see also TableAutomorphisms).

gap> tbl:= CharacterTable( "Dihedral", 8 );;
gap> AutomorphismsOfTable( tbl );
Group([ (4,5) ])
gap> OrdersClassRepresentatives( tbl );
[ 1, 4, 2, 2, 2 ]
gap> SizesConjugacyClasses( tbl );
[ 1, 2, 1, 2, 2 ]

For an ordinary character table tbl, the result is 0, for a p-modular Brauer table tbl, it is p. The underlying characteristic of a class function psi is equal to that of its underlying character table.

The underlying characteristic must be stored when the table is constructed, there is no method to compute it.

We cannot use the attribute Characteristic (see Characteristic) to denote this, since of course each Brauer character is an element of characteristic zero in the sense of GAP (see Chapter Class Functions).

gap> tbl:= CharacterTable( "A5" );;
gap> UnderlyingCharacteristic( tbl );
0
gap> UnderlyingCharacteristic( tbl mod 17 );
17

ClassNames and CharacterNames return lists of strings, one for each conjugacy class or irreducible character, respectively, of the character table tbl. These names are used when tbl is displayed.

The default method for ClassNames computes class names consisting of the order of an element in the class and at least one distinguishing letter.

The default method for CharacterNames returns the list [ "X.1", "X.2", ... ], whose length is the number of irreducible characters of tbl.

The position of the class with name name in tbl can be accessed as tbl.name.

When ClassNames is called with two arguments, the second being the string "ATLAS", the class names returned obey the convention used in Chapter 7, Section 5 of the ATLAS of Finite Groups CCN85.

gap> tbl:= CharacterTable( "A5" );;
gap> ClassNames( tbl );
[ "1a", "2a", "3a", "5a", "5b" ]
gap> tbl.2a;
2

is a string that identifies the character table tbl in the current GAP session. It is used mainly for class fusions into tbl that are stored on other character tables. For character tables without group, the identifier is also used to print the table; this is the case for library tables, but also for tables that are constructed as direct products, factors etc. involving tables that may or may not store their groups.

The default method for ordinary tables constructs strings of the form "CTn", where n is a positive integer. LARGEST_IDENTIFIER_NUMBER is a list containing the largest integer n used in the current GAP session.

The default method for Brauer tables returns the concatenation of the identifier of the ordinary table, the string "mod", and the (string of the) underlying characteristic.

gap> Identifier( CharacterTable( "A5" ) );
"A5"
gap> tbl:= CharacterTable( Group( () ) );;
gap> Identifier( tbl );  Identifier( tbl mod 2 );
"CT8"
"CT8mod2"

is a mutable string with information about the character table tbl. There is no default method to create an info text.

This attribute is used mainly for library tables (see the manual of the GAP Character Table Library). Usual parts of the information are the origin of the table, tests it has passed (1.o.r. for the test of orthogonality, pow[p] for the construction of the p-th power map, DEC for the decomposition of ordinary into Brauer characters, TENS for the decomposition of tensor products of irreducibles), and choices made without loss of generality.

gap> Print( InfoText( CharacterTable( "A5" ) ), "\n" );
origin: ATLAS of finite groups, tests: 1.o.r., pow[2,3,5]

For a character table tbl, InverseClasses returns the list mapping each conjugacy class to its inverse class. This list can be regarded as (-1)-st power map of tbl (see PowerMap).

For a character table tbl, RealClasses returns the strictly sorted list of positions of classes in tbl that consist of real elements.

An element x is real iff it is conjugate to its inverse x^-1 = x^o(x)-1.

gap> InverseClasses( CharacterTable( "A5" ) );
[ 1, 2, 3, 4, 5 ]
gap> InverseClasses( CharacterTable( "Cyclic", 3 ) );
[ 1, 3, 2 ]
gap> RealClasses( CharacterTable( "A5" ) );
[ 1, 2, 3, 4, 5 ]
gap> RealClasses( CharacterTable( "Cyclic", 3 ) );
[ 1 ]

is the list of positions of those conjugacy classes of the character table tbl that are Galois conjugate to the cc-th class. That is, exactly the classes at positions given by the list returned by ClassOrbit contain generators of the cyclic group generated by an element in the cc-th class.

This information is computed from the power maps of tbl.

For a character table tbl, ClassRoots returns a list containing at position i the list of positions of the classes of all nontrivial p-th roots, where p runs over the prime divisors of Size( tbl ).

This information is computed from the power maps of tbl.

gap> ClassOrbit( CharacterTable( "A5" ), 4 );
[ 4, 5 ]
gap> ClassRoots( CharacterTable( "A5" ) );
[ [ 2, 3, 4, 5 ], [  ], [  ], [  ], [  ] ]
gap> ClassRoots( CharacterTable( "Cyclic", 6 ) );
[ [ 3, 4, 5 ], [  ], [ 2 ], [ 2, 6 ], [ 6 ], [  ] ]

The following attributes for a character table tbl correspond to attributes for the group G of tbl. But instead of a normal subgroup (or a list of normal subgroups) of G, they return a strictly sorted list of positive integers (or a list of such lists) which are the positions --relative to ConjugacyClasses( tbl )-- of those classes forming the normal subgroup in question.

correspond to NormalSubgroups and MaximalNormalSubgroups for the group of the ordinary character table ordtbl (see NormalSubgroups, MaximalNormalSubgroups).

The entries of the result lists are sorted according to increasing length. (So this total order respects the partial order of normal subgroups given by inclusion.)

corresponds to Agemo (see Agemo) for the group of the ordinary character table ordtbl.

corresponds to Centre (see Centre) for the group of the ordinary character table ordtbl.

Let tbl be the ordinary character table of the group G, say. Called with the only argument tbl, ClassPositionsOfDirectProductDecompositions returns the list of all those pairs [ l₁, l₂ ] where l₁ and l₂ are lists of class positions of normal subgroups N₁, N₂ of G such that G is their direct product and |N₁| £ |N₂| holds. Called with second argument a list nclasses of class positions of a normal subgroup N of G, ClassPositionsOfDirectProductDecompositions returns the list of pairs describing the decomposition of N as a direct product of two normal subgroups of G.

corresponds to DerivedSubgroup (see DerivedSubgroup) for the group of the ordinary character table ordtbl.

corresponds to ElementaryAbelianSeries (see ElementaryAbelianSeries) for the group of the ordinary character table ordtbl.

corresponds to FittingSubgroup (see FittingSubgroup) for the group of the ordinary character table ordtbl.

corresponds to LowerCentralSeries (see LowerCentralSeriesOfGroup) for the group of the ordinary character table ordtbl.

corresponds to UpperCentralSeries (see UpperCentralSeriesOfGroup) for the group of the ordinary character table ordtbl.

corresponds to SupersolvableResiduum (see SupersolvableResiduum) for the group of the ordinary character table ordtbl.

is the sorted list of the positions of all conjugacy classes of the ordinary character table ordtbl that form the normal closure (see NormalClosure) of the conjugacy classes at positions in the list classes.

gap> tbla5:= CharacterTable( "A5" );;
gap> tbls4:= CharacterTable( "Symmetric", 4 );;
gap> tbld8:= CharacterTable( "Dihedral", 8 );;
gap> ClassPositionsOfNormalSubgroups( tbls4 );
[ [ 1 ], [ 1, 3 ], [ 1, 3, 4 ], [ 1, 2, 3, 4, 5 ] ]
gap> ClassPositionsOfAgemo( tbls4, 2 );
[ 1, 3, 4 ]
gap> ClassPositionsOfCentre( tbld8 );
[ 1, 3 ]
gap> ClassPositionsOfDerivedSubgroup( tbld8 );
[ 1, 3 ]
gap> ClassPositionsOfElementaryAbelianSeries( tbls4 );
[ [ 1 .. 5 ], [ 1, 3, 4 ], [ 1, 3 ], [ 1 ] ]
gap> ClassPositionsOfElementaryAbelianSeries( tbla5 );
fail
gap> ClassPositionsOfFittingSubgroup( tbls4 );
[ 1, 3 ]
gap> ClassPositionsOfLowerCentralSeries( tbls4 );
[ [ 1 .. 5 ], [ 1, 3, 4 ] ]
gap> ClassPositionsOfLowerCentralSeries( tbld8 );
[ [ 1 .. 5 ], [ 1, 3 ], [ 1 ] ]
gap> ClassPositionsOfUpperCentralSeries( tbls4 );
[ [ 1 ] ]
gap> ClassPositionsOfUpperCentralSeries( tbld8 );
[ [ 1, 3 ], [ 1, 2, 3, 4, 5 ] ]
gap> ClassPositionsOfSupersolvableResiduum( tbls4 );
[ 1, 3 ]
gap> ClassPositionsOfNormalClosure( tbls4, [ 1, 4 ] );
[ 1, 3, 4 ]

69.9 Operations Concerning Blocks

For an ordinary character table ordtbl and a prime integer p, PrimeBlocks returns a record with the following components.

block

a list, the value j at position i means that the i-th irreducible character of ordtbl lies in the j-th p-block of ordtbl,

defect

a list containing at position i the defect of the i-th block,

height

a list containing at position i the height of the i-th irreducible character of ordtbl in its block,

relevant

a list of class positions such that only the restriction to these classes need be checked for deciding whether two characters lie in the same block,

exponents

a list containing at the positions in the component relevant an integer n such that the n-th power of a difference of characters is divisible by p if the two characters lie in the same block, and

centralcharacter

a list containing at position i a list whose values at the positions stored in the component relevant are the values of a central character in the i-th block.

The components relevant, exponents, and centralcharacters are used by SameBlock (see SameBlock).

If InfoCharacterTable has level at least 2, the defects of the blocks and the heights of the characters are printed.

The default method uses the attribute ComputedPrimeBlockss for storing the computed value at position p, and calls the operation PrimeBlocksOp for computing values that are not yet known.

Two ordinary irreducible characters c, y of a group G are said to lie in the same p-block if the images of their central characters w_c, w_y (see CentralCharacter) under the ring homomorphism *:R ® R / M are equal, where R denotes the ring of algebraic integers in the complex number field, and M is a maximal ideal in R with pR Í M. (The distribution to p-blocks is in fact independent of the choice of M, see Isa76.)

For |G| = p^a m where p does not divide m, the defect of a block is the integer d such that p^a-d is the largest power of p that divides the degrees of all characters in the block.

The height of a character c in the block is defined as the largest exponent h for which p^h divides c(1) / p^a-d.

gap> tbl:= CharacterTable( "L3(2)" );;
gap> pbl:= PrimeBlocks( tbl, 2 );
rec( block := [ 1, 1, 1, 1, 1, 2 ], defect := [ 3, 0 ], 
  height := [ 0, 0, 0, 1, 0, 0 ], relevant := [ 3, 5 ], 
  exponents := [ ,, 2,, 6 ], 
  centralcharacter := [ [ ,, 56,, 24 ], [ ,, -7,, 3 ] ] )

Let tbl be an ordinary character table, p a prime integer, omega1 and omega2 two central characters (or their values lists) of tbl. The remaining arguments relevant and exponents are lists as stored in the components relevantclasses and exponents of a record returned by PrimeBlocks (see PrimeBlocks).

SameBlock returns true if omega1 and omega2 are equal modulo any maximal ideal in the ring of complex algebraic integers containing the ideal spanned by p, and false otherwise.

gap> omega:= List( Irr( tbl ), CentralCharacter );;
gap> SameBlock( tbl, 2, omega[1], omega[2], pbl.relevant, pbl.exponents );
true
gap> SameBlock( tbl, 2, omega[1], omega[6], pbl.relevant, pbl.exponents );
false

For a Brauer character table modtbl, the value of BlocksInfo is a list of (mutable) records, the i-th entry containing information about the i-th block. Each record has the following components.

defect

the defect of the block,

ordchars

the list of positions of the ordinary characters that belong to the block, relative to Irr( OrdinaryCharacterTable( modtbl ) ),

modchars

the list of positions of the Brauer characters that belong to the block, relative to IBr( modtbl ).

basicset

a list of positions of ordinary characters in the block whose restriction to modtbl is maximally linearly independent, relative to Irr( OrdinaryCharacterTable( modtbl ) ),

decmat

the decomposition matrix of the block, it is stored automatically when DecompositionMatrix is called for the block (see DecompositionMatrix),

decinv

inverse of the decomposition matrix of the block, restricted to the ordinary characters described by basicset,

brauertree

a list that describes the Brauer tree of the block, in the case that the block is of defect 1.

gap> BlocksInfo( CharacterTable( "L3(2)" ) mod 2 );
[ rec( defect := 3, ordchars := [ 1, 2, 3, 4, 5 ], modchars := [ 1, 2, 3 ], 
      decinv := [ [ 1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ], 
      basicset := [ 1, 2, 3 ] ), 
  rec( defect := 0, ordchars := [ 6 ], modchars := [ 4 ], decinv := [ [ 1 ] ],
      basicset := [ 6 ] ) ]

Let modtbl be a Brauer character table.

In the first version DecompositionMatrix returns the decomposition matrix of modtbl, where the rows and columns are indexed by the irreducible characters of the ordinary character table of modtbl and the irreducible characters of modtbl, respectively,

In the second version DecompositionMatrix returns the decomposition matrix of the block of modtbl with number blocknr; the matrix is stored as value of the decmat component of the blocknr-th entry of the BlocksInfo list (see BlocksInfo) of modtbl.

An ordinary irreducible character is in block i if and only if all characters before the first character of the same block lie in i-1 different blocks. An irreducible Brauer character is in block i if it has nonzero scalar product with an ordinary irreducible character in block i.

DecompositionMatrix is based on the more general function Decomposition (see Decomposition).

gap> modtbl:= CharacterTable( "L3(2)" ) mod 2;
BrauerTable( "L3(2)", 2 )
gap> DecompositionMatrix( modtbl );
[ [ 1, 0, 0, 0 ], [ 0, 1, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 1, 1, 0 ], 
  [ 1, 1, 1, 0 ], [ 0, 0, 0, 1 ] ]
gap> DecompositionMatrix( modtbl, 1 );
[ [ 1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ], [ 0, 1, 1 ], [ 1, 1, 1 ] ]
gap> DecompositionMatrix( modtbl, 2 );
[ [ 1 ] ]

is a string that contains LaTeX code to print a decomposition matrix (see DecompositionMatrix) nicely.

The optional argument options, if present, must be a record with components phi, chi (strings used in each label for columns and rows), collabels, rowlabels (subscripts for the labels). The defaults for phi and chi are "\tt Y" and "\tt X", the defaults for collabels and rowlabels are the lists of positions of the Brauer characters and ordinary characters in the respective lists of irreducibles in the character tables.

The optional components nrows and ncols denote the maximal number of rows and columns per array; if they are present then each portion of nrows rows and ncols columns forms an array of its own which is enclosed in \[, \].

If the component decmat is bound in options then it must be the decomposition matrix in question, in this case the matrix is not computed from the information in modtbl.

For those character tables from the GAP table library that belong to the ATLAS of Finite Groups CCN85, AtlasLabelsOfIrreducibles constructs character labels that are compatible with those used in the ATLAS (see ctbllib:ATLAS Tables and  ctbllib:AtlasLabelsOfIrreducibles in the manual of the GAP Character Table Library).

gap> modtbl:= CharacterTable( "L3(2)" ) mod 2;;
gap> Print( LaTeXStringDecompositionMatrix( modtbl, 1 ) );
\[
\begin{array}{r|rrr} \hline
 & {\tt Y}_{1}
 & {\tt Y}_{2}
 & {\tt Y}_{3}
 \rule[-7pt]{0pt}{20pt} \\ \hline
{\tt X}_{1} & 1 & . & . \rule[0pt]{0pt}{13pt} \\
{\tt X}_{2} & . & 1 & . \\
{\tt X}_{3} & . & . & 1 \\
{\tt X}_{4} & . & 1 & 1 \\
{\tt X}_{5} & 1 & 1 & 1 \rule[-7pt]{0pt}{5pt} \\
\hline
\end{array}
\]
gap> options:= rec( phi:= "\\varphi", chi:= "\\chi" );;
gap> Print( LaTeXStringDecompositionMatrix( modtbl, 1, options ) );
\[
\begin{array}{r|rrr} \hline
 & \varphi_{1}
 & \varphi_{2}
 & \varphi_{3}
 \rule[-7pt]{0pt}{20pt} \\ \hline
\chi_{1} & 1 & . & . \rule[0pt]{0pt}{13pt} \\
\chi_{2} & . & 1 & . \\
\chi_{3} & . & . & 1 \\
\chi_{4} & . & 1 & 1 \\
\chi_{5} & 1 & 1 & 1 \rule[-7pt]{0pt}{5pt} \\
\hline
\end{array}
\]

69.10 Other Operations for Character Tables

In the following, we list operations for character tables that are not attributes.

For an ordinary character table tbl, IsInternallyConsistent checks the consistency of the following attribute values (if stored).

• Size, SizesCentralizers, and SizesConjugacyClasses.
• SizesCentralizers and OrdersClassRepresentatives.
• ComputedPowerMaps and OrdersClassRepresentatives.
• SizesCentralizers and Irr.
• Irr (first orthogonality relation).

For a Brauer table tbl, IsInternallyConsistent checks the consistency of the following attribute values (if stored).

• Size, SizesCentralizers, and SizesConjugacyClasses.
• SizesCentralizers and OrdersClassRepresentatives.
• ComputedPowerMaps and OrdersClassRepresentatives.
• Irr (closure under complex conjugation and Frobenius map).

If no inconsistency occurs, true is returned, otherwise each inconsistency is printed to the screen if the level of InfoWarning is at least 1 (see Info Functions), and false is returned at the end.

IsPSolvableCharacterTable for the ordinary character table tbl corresponds to IsPSolvable for the group of tbl (see IsPSolvable). p must be either a prime integer or 0.

The default method uses the attribute ComputedIsPSolvableCharacterTables for storing the computed value at position p, and calls the operation IsPSolvableCharacterTableOp for computing values that are not yet known.

gap> tbl:= CharacterTable( "Sz(8)" );;
gap> IsPSolvableCharacterTable( tbl, 2 );
false
gap> IsPSolvableCharacterTable( tbl, 3 );
true

For two ordinary character tables tbl and subtbl of a group G and its subgroup U, say, and a list fus of positive integers that describes the class fusion of U into G, IsClassFusionOfNormalSubgroup returns true if U is a normal subgroup of G, and false otherwise.

gap> tblc2:= CharacterTable( "Cyclic", 2 );;
gap> tbld8:= CharacterTable( "Dihedral", 8 );;
gap> fus:= PossibleClassFusions( tblc2, tbld8 );
[ [ 1, 3 ], [ 1, 4 ], [ 1, 5 ] ]
gap> List( fus, map -> IsClassFusionOfNormalSubgroup( tblc2, map, tbld8 ) );
[ true, false, false ]

If tbl is an ordinary character table then Indicator returns the list of n-th Frobenius-Schur indicators of the characters in the list characters; the default of characters is Irr( tbl ).

The n-th Frobenius-Schur indicator n_n(c) of an ordinary character c of the group G is given by n_n(c) = [ 1/(|G|)] å_{g Î G} c(gⁿ).

If tbl is a Brauer table in characteristic ¹ 2 and n = 2 then Indicator returns the second indicator.

The default method uses the attribute ComputedIndicators for storing the computed value at position n, and calls the operation IndicatorOp for computing values that are not yet known.

gap> tbl:= CharacterTable( "L3(2)" );;
gap> Indicator( tbl, 2 );
[ 1, 0, 0, 1, 1, 1 ]

returns the number and isomorphism type of polyhedral subgroups of the group with ordinary character table tbl which are generated by an element g of class c1 and an element h of class c2 with the property that the product gh lies in class c3.

According to p. 233 in NPP84, the number of polyhedral subgroups of isomorphism type V₄, D_2n, A₄, S₄, and A₅ can be derived from the class multiplication coefficient (see ClassMultiplicationCoefficient!for character tables) and the number of Galois conjugates of a class (see ClassOrbit).

The classes c1, c2 and c3 in the parameter list must be ordered according to the order of the elements in these classes.

gap> NrPolyhedralSubgroups( tbl, 2, 2, 4 );
rec( number := 21, type := "D8" )

returns the class multiplication coefficient of the classes i, j, and k of the group G with ordinary character table tbl.

The class multiplication coefficient c_i,j,k of the classes i, j, k equals the number of pairs (x,y) of elements x, y Î G such that x lies in class i, y lies in class j, and their product xy is a fixed element of class k.

In the center of the group algebra of G, these numbers are found as coefficients of the decomposition of the product of two class sums K_i and K_j into class sums,

Ki Kj = å k  cijk Kk.

Given the character table of a finite group G, whose classes are C₁, ..., C_r with representatives g_i Î C_i, the class multiplication coefficient c_ijk can be computed by the following formula.

cijk =  ||Ci||||Cj|| ||G|| å c Î Irr(G)  c(gi) c(gj) c(gk)   c(1) .

On the other hand the knowledge of the class multiplication coefficients admits the computation of the irreducible characters of G. (see IrrDixonSchneider).

returns the so-called class structure of the classes in the list classes, for the character table tbl of the group G. The length of classes must be at least 2.

Let C = (C₁, C₂, ..., C_n) denote the n-tuple of conjugacy classes of G that are indexed by classes. The class structure n(C) equals the number of n-tuples (g₁, g₂, ¼, g_n) of elements g_i Î C_i with g₁ g₂ ¼g_n = 1. Note the difference to the definition of the class multiplication coefficients in ClassMultiplicationCoefficient (see ClassMultiplicationCoefficient!for character tables).

n(C₁, C₂, ¼, C_n) is computed using the formula

n(C1, C2, ¼, Cn) =  ||C1||||C2||¼||Cn|| ||G|| å c Î Irr(G)   c(g1)c(g2)¼c(gn) c(1)n-2 ·

For an ordinary character table tbl and a class position i, MatClassMultCoeffsCharTable returns the matrix [ a_ijk ]_j,k of structure constants (see ClassMultiplicationCoefficient!for character tables).

gap> tbl:= CharacterTable( "L3(2)" );;
gap> ClassMultiplicationCoefficient( tbl, 2, 2, 4 );
4
gap> ClassStructureCharTable( tbl, [ 2, 2, 4 ] );
168
gap> ClassStructureCharTable( tbl, [ 2, 2, 2, 4 ] );
1848
gap> MatClassMultCoeffsCharTable( tbl, 2 );
[ [ 0, 1, 0, 0, 0, 0 ], [ 21, 4, 3, 4, 0, 0 ], [ 0, 8, 6, 8, 7, 7 ], 
  [ 0, 8, 6, 1, 7, 7 ], [ 0, 0, 3, 4, 0, 7 ], [ 0, 0, 3, 4, 7, 0 ] ]

69.11 Printing Character Tables

The default ViewObj (see ViewObj) method for ordinary character tables prints the string "CharacterTable", followed by the identifier (see Identifier!for character tables) or, if known, the group of the character table enclosed in brackets. ViewObj for Brauer tables does the same, except that the first string is replaced by "BrauerTable", and that the characteristic is also shown.

The default PrintObj (see PrintObj) method for character tables does the same as ViewObj, except that the group is is Print-ed instead of View-ed.

The default Display (see Display) method for a character table tbl prepares the data contained in tbl for a pretty columnwise output. The number of columns printed at one time depends on the actual line length, which can be accessed and changed by the function SizeScreen (see SizeScreen).

Display shows certain characters (by default all irreducible characters) of tbl, together with the orders of the centralizers in factorized form and the available power maps (see ComputedPowerMaps). Each displayed character is given a name X.n.

The first lines of the output describe the order of the centralizer of an element of the class factorized into its prime divisors.

The next line gives the name of each class. If no class names are stored on tbl, ClassNames is called (see ClassNames).

Preceded by a name Pn, the next lines show the nth power maps of tbl in terms of the former shown class names.

Every ambiguous or unknown (see Chapter Unknowns) value of the table is displayed as a question mark ?.

Irrational character values are not printed explicitly because the lengths of their printed representation might disturb the layout. Instead of that every irrational value is indicated by a name, which is a string of at least one capital letter.

Once a name for an irrational value is found, it is used all over the printed table. Moreover the complex conjugate (see ComplexConjugate, GaloisCyc) and the star of an irrationality (see StarCyc) are represented by that very name preceded by a / and a *, respectively.

The printed character table is then followed by a legend, a list identifying the occurring symbols with their actual values. Occasionally this identification is supplemented by a quadratic representation of the irrationality together with the corresponding ATLAS-notation (see CCN85).

The optional second argument arec of Display can be used to change the default style (mentioned above) for displaying a character. arec must be a record, its relevant components are the following.

chars

an integer or a list of integers to select a sublist of the irreducible characters of tbl, or a list of characters of tbl (in this case the letter "X" is replaced by "Y"),

classes

an integer or a list of integers to select a sublist of the classes of tbl,

centralizers

suppresses the printing of the orders of the centralizers if false,

powermap

an integer or a list of integers to select a subset of the available power maps, or false to suppress the printing of power maps,

letter

a single capital letter (e. g. "P" for permutation characters) to replace "X",

indicator

true enables the printing of the second Frobenius Schur indicator, a list of integers enables the printing of the corresponding indicators (see Indicator),

StringEntry

a function that takes either a character value or a character value and the return value of StringEntryData (see below), and returns the string that is actually displayed; it is called for all character values to be displayed, and also for the displayed indicator values (see above); the default StringEntry function is CharacterTableDisplayStringEntryDefault,

StringEntryData

a unary function that is called once with argument tbl before the character values are displayed; it returns an object that is used as second argument of the function StringEntry; the default StringEntryData function is CharacterTableDisplayStringEntryDataDefault,

PrintLegend

a function that is called with the result of the StringEntryData call after the character table has been displayed; the default PrintLegend function is CharacterTableDisplayPrintLegendDefault.

There is no default method to compute a value, one can set a value with SetDisplayOptions.

gap> tbl:= CharacterTable( "A5" );;
gap> Display( tbl );
A5

     2  2  2  .  .  .
     3  1  .  1  .  .
     5  1  .  .  1  1

       1a 2a 3a 5a 5b
    2P 1a 1a 3a 5b 5a
    3P 1a 2a 1a 5b 5a
    5P 1a 2a 3a 1a 1a

X.1     1  1  1  1  1
X.2     3 -1  .  A *A
X.3     3 -1  . *A  A
X.4     4  .  1 -1 -1
X.5     5  1 -1  .  .

A = -E(5)-E(5)^4
  = (1-ER(5))/2 = -b5
gap> options:= rec( chars:= 4, classes:= [ tbl.3a .. tbl.5a ],
>                   centralizers:= false, indicator:= true,
>                   powermap:= [ 2 ] );;
gap> Indicator( tbl, 2 );;
gap> Display( tbl, options );
A5

          3a 5a
       2P 3a 5b
       2
X.4    +   1 -1
gap> SetDisplayOptions( tbl, options );  Display( tbl );
A5

          3a 5a
       2P 3a 5b
       2
X.4    +   1 -1

Let tbl be a nearly character table, and varname a string. PrintCharacterTable prints those values of the supported attributes (see SupportedCharacterTableInfo) that are known for tbl;

The output of PrintCharacterTable is GAP readable; actually reading it into GAP will bind the variable with name varname to a character table that coincides with tbl for all printed components.

This is used mainly for saving character tables to files. A more human readable form is produced by Display.

gap> PrintCharacterTable( CharacterTable( "Cyclic", 2 ), "tbl" );
tbl:= function()
local tbl;
tbl:=rec();
tbl.Irr:=
[ [ 1, 1 ], [ 1, -1 ] ];
tbl.NrConjugacyClasses:=
2;
tbl.Size:=
2;
tbl.OrdersClassRepresentatives:=
[ 1, 2 ];
tbl.SizesCentralizers:=
[ 2, 2 ];
tbl.UnderlyingCharacteristic:=
0;
tbl.ClassParameters:=
[ [ 1, 0 ], [ 1, 1 ] ];
tbl.CharacterParameters:=
[ [ 1, 0 ], [ 1, 1 ] ];
tbl.Identifier:=
"C2";
tbl.InfoText:=
"computed using generic character table for cyclic groups";
tbl.ComputedPowerMaps:=
[ , [ 1, 1 ] ];
ConvertToLibraryCharacterTableNC(tbl);
return tbl;
end;
tbl:= tbl();

69.12 Computing the Irreducible Characters of a Group

Several algorithms are available for computing the irreducible characters of a finite group G. The default method for arbitrary finite groups is to use the Dixon-Schneider algorithm (see IrrDixonSchneider). For supersolvable groups, Conlon's algorithm can be used (see IrrConlon). For abelian-by-supersolvable groups, the Baum-Clausen algorithm for computing the irreducible representations (see IrreducibleRepresentations) can be used to compute the irreducible characters (see IrrBaumClausen).

These functions are installed in methods for Irr (see Irr), but explicitly calling one of them will not set the Irr value of G.

computes the irreducible characters of the finite group G, using the Dixon-Schneider method (see The Dixon-Schneider Algorithm). It calls DixonInit and DixonSplit, and finally returns the list returned by DixontinI (see Advanced Methods for Dixon-Schneider Calculations, Components of a Dixon Record, An Example of Advanced Dixon-Schneider Calculations).

For a finite solvable group G, IrrConlon returns a list of certain irreducible characters of G, among those all irreducibles that have the supersolvable residuum of G in their kernels; so if G is supersolvable, all irreducible characters of G are returned. An error is signalled if G is not solvable.

The characters are computed using Conlon's algorithm (see Con90a and Con90b). For each irreducible character in the returned list, the monomiality information (see TestMonomial) is stored.

IrrBaumClausen returns the absolutely irreducible ordinary characters of the factor group of the finite solvable group G by the derived subgroup of its supersolvable residuum.

The characters are computed using the algorithm by Baum and Clausen (see BC94). An error is signalled if G is not solvable.

In the following example we temporarily increase the line length limit from its default value 80 to 87 in order to get a nicer output format.

gap> g:= SL(2,3);;
gap> SizeScreen([ 87, ]);;
gap> irr1:= IrrDixonSchneider( g );
[ Character( CharacterTable( SL(2,3) ), [ 1, 1, 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3), E(3)^2, E(3)^2, E(3), 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3)^2, E(3), E(3), E(3)^2, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 2, -2, 1, 1, -1, -1, 0 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 2, -2, E(3), E(3)^2, -E(3)^2, -E(3), 0 ] ),
  Character( CharacterTable( SL(2,3) ), [ 2, -2, E(3)^2, E(3), -E(3), -E(3)^2, 0 ] ),
  Character( CharacterTable( SL(2,3) ), [ 3, 3, 0, 0, 0, 0, -1 ] ) ]
gap> irr2:= IrrConlon( g );
[ Character( CharacterTable( SL(2,3) ), [ 1, 1, 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3), E(3)^2, E(3)^2, E(3), 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3)^2, E(3), E(3), E(3)^2, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 3, 3, 0, 0, 0, 0, -1 ] ) ]
gap> irr3:= IrrBaumClausen( g );
[ Character( CharacterTable( SL(2,3) ), [ 1, 1, 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3), E(3)^2, E(3)^2, E(3), 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 1, 1, E(3)^2, E(3), E(3), E(3)^2, 1 ] ), 
  Character( CharacterTable( SL(2,3) ), [ 3, 3, 0, 0, 0, 0, -1 ] ) ]
gap> SizeScreen([ 80, ]);;
gap> chi:= irr2[4];;  HasTestMonomial( chi );
true

Called with a finite group G and a field F, IrreducibleRepresentations returns a list of representatives of the irreducible matrix representations of G over F, up to equivalence.

If G is the only argument then IrreducibleRepresentations returns a list of representatives of the absolutely irreducible complex representations of G, up to equivalence.

At the moment, methods are available for the following cases: If G is abelian by supersolvable the method of BC94 is used.

Otherwise, if F and G are both finite, the regular module of G is split by MeatAxe methods which can make this an expensive operation.

Finally, if F is not given (i.e. it defaults to the cyclotomic numbers) and G is a finite group, the method of Dix93 (see IrreducibleRepresentationsDixon) is used.

For other cases no methods are implemented yet.

See also IrreducibleModules, which provides efficient methods for solvable groups.

gap> g:= AlternatingGroup( 4 );;
gap> repr:= IrreducibleRepresentations( g );
[ Pcgs([ (2,4,3), (1,3)(2,4), (1,2)(3,4) ]) -> 
    [ [ [ 1 ] ], [ [ 1 ] ], [ [ 1 ] ] ], 
  Pcgs([ (2,4,3), (1,3)(2,4), (1,2)(3,4) ]) -> 
    [ [ [ E(3) ] ], [ [ 1 ] ], [ [ 1 ] ] ], 
  Pcgs([ (2,4,3), (1,3)(2,4), (1,2)(3,4) ]) -> 
    [ [ [ E(3)^2 ] ], [ [ 1 ] ], [ [ 1 ] ] ], 
  Pcgs([ (2,4,3), (1,3)(2,4), (1,2)(3,4) ]) -> 
    [ [ [ 0, 0, 1 ], [ 1, 0, 0 ], [ 0, 1, 0 ] ], 
      [ [ -1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, -1 ] ], 
      [ [ 1, 0, 0 ], [ 0, -1, 0 ], [ 0, 0, -1 ] ] ] ]
gap> ForAll( repr, IsGroupHomomorphism );
true
gap> Length( repr );
4
gap> gens:= GeneratorsOfGroup( g );
[ (1,2,3), (2,3,4) ]
gap> List( gens, x -> x^repr[1] );
[ [ [ 1 ] ], [ [ 1 ] ] ]
gap>  List( gens, x -> x^repr[4] );
[ [ [ 0, 0, -1 ], [ 1, 0, 0 ], [ 0, -1, 0 ] ], 
  [ [ 0, 1, 0 ], [ 0, 0, 1 ], [ 1, 0, 0 ] ] ]

computes (reresentatives of) all irreducible complex representations for the finite group G, using the method of Dix93, which computes the character table and computes the representation as constituent of an induced monomial representation of a subgroup.

This method can be quite expensive for larger groups, for example it might involve calculation of the subgroup lattice of G.

If given, chi must be a character, in this case only a representation for chi is returned.

If given, chilist must be a list of characters, in this case only representations for characters in chilist are computed.

Note that this method might fail if for an irreducible representation there is no subgroup in which its reduction has a linear constituent with multiplicity one.

gap> a5:= AlternatingGroup( 5 );
Alt( [ 1 .. 5 ] )
gap> char:= First( Irr( a5 ), x -> x[1] = 4 );
Character( CharacterTable( Alt( [ 1 .. 5 ] ) ), [ 4, 0, 1, -1, -1 ] )
gap> hom:=IrreducibleRepresentationsDixon( a5, char );;
gap> Order( a5.1*a5.2 ) = Order( Image( hom, a5.1 )*Image( hom, a5.2 ) );
true
gap> reps:= List( ConjugacyClasses( a5 ), Representative );;
gap> List( reps, g -> TraceMat( Image( hom, g ) ) );
[ 4, 0, 1, -1, -1 ]

69.13 The Dixon-Schneider Algorithm

The GAP library implementation of the Dixon-Schneider algorithm first computes the linear characters, using the commutator factor group. If irreducible characters are missing afterwards, they are computed using the techniques described in Dix67, Sch90 and Hulpke93.

Called with a group G, the function CharacterTable (see CharacterTable) returns a character table object that stores already information such as class lengths, but not the irreducible characters. The routines that compute the irreducibles may use the information that is already contained in this table object. In particular the ordering of classes in the computed characters coincides with the ordering of classes in the character table of G (see The Interface between Character Tables and Groups). Thus it is possible to combine computations using the group with character theoretic computations (see Advanced Methods for Dixon-Schneider Calculations for details), for example one can enter known characters. Note that the user is responsible for the correctness of the characters. (There is little use in providing the trivial character to the routine.)

The computation of irreducible characters from the group needs to identify the classes of group elements very often, so it can be helpful to store a class list of all group elements. Since this is obviously limited by the group order, it is controlled by the global function IsDxLargeGroup (see IsDxLargeGroup).

The routines compute in a prime field of size p, such that the exponent of the group divides (p-1) and such that 2 Ö{|G|} < p. Currently prime fields of size smaller than 65 536 are handled more efficiently than larger prime fields, so the runtime of the character calculation depends on how large the chosen prime is.

The routine stores a Dixon record (see DixonRecord) in the group that helps routines that identify classes, for example FusionConjugacyClasses, to work much faster. Note that interrupting Dixon-Schneider calculations will prevent GAP from cleaning up the Dixon record; when the computation by IrrDixonSchneider is complete, the possibly large record is shrunk to an acceptable size.

69.14 Advanced Methods for Dixon-Schneider Calculations

The computation of irreducible characters of very large groups may take quite some time. On the other hand, for the expert only a few irreducible characters may be needed, since the other ones can be computed using character theoretic methods such as tensoring, induction, and restriction. Thus GAP provides also step-by-step routines for doing the calculations. These routines allow one to compute some characters and to stop before all are calculated. Note that there is no ``safety net'': The routines (being somehow internal) do no error checking, and assume the information given is correct.

When the info level of InfoCharacterTable if positive, information about the progress of splitting is printed. (The default value is zero.)

The DixonRecord of a group contains information used by the routines to compute the irreducible characters and related information via the Dixon-Schneider algorithm such as class arrangement and character spaces split obtained so far. Usually this record is passed as argument to all subfunctions to avoid a long argument list. It has a component .conjugacyClasses which contains the classes of G ordered as the algorithm needs them.

This function does all the initializations for the Dixon-Schneider algorithm. This includes calculation of conjugacy classes, power maps, linear characters and character morphisms. It returns a record (see DixonRecord, Components of a Dixon Record) that can be used when calculating the irreducible characters of G interactively.

This function ends a Dixon-Schneider calculation. It sorts the characters according to the degree and unbinds components in the Dixon record that are not of use any longer. It returns a list of irreducible characters.

This function performs one splitting step in the Dixon-Schneider algorithm. It selects a class, computes the (partial) class sum matrix, uses it to split character spaces and stores all the irreducible characters obtained that way.

The class to use for splitting is chosen via BestSplittingMatrix and the options described for this function apply here.

DixonSplit returns true if a split was performed and fail otherwise.

returns the number of the class sum matrix that is assumed to yield the best (cost/earning ration) split. This matrix then will be the next one computed and used.

The global option maxclasslen (defaulting to infinity) is recognized by BestSplittingMatrix: Only classes whose length is limited by the value of this option will be considered for splitting. If no usable class remains, fail is returned.

This function takes a list of irreducible characters new, each given as a list of values (corresponding to the class arrangement in D), and adds these to a partial computed list of irreducibles as maintained by the Dixon record D. This permits one to add characters in interactive use obtained from other sources and to continue the Dixon-Schneider calculation afterwards. If the optional argument newmod is given, it must be a list of reduced characters, corresponding to new. (Otherwise the function has to reduce the characters itself.)

The function closes the new characters under the action of Galois automorphisms and tensor products with linear characters.

This routine decomposes the characters given in list according to the character spaces found up to this point. By applying this routine to tensor products etc., it may result in characters with smaller norm, even irreducible ones. Since the recalculation of characters is only possible if the degree is small enough, the splitting process is applied only to characters of sufficiently small degree.

returns true if the order of the group G is smaller than the current value of the global variable DXLARGEGROUPORDER, and false otherwise. In Dixon-Schneider calculations, for small groups in the above sense a class map is stored, whereas for large groups, each occurring element is identified individually.

69.15 Components of a Dixon Record

The ``Dixon record'' D returned by DixonInit (see DixonInit) stores all the information that is used by the Dixon-Schneider routines while computing the irreducible characters of a group. Some entries, however, may be useful to know about when using the algorithm interactively (see An Example of Advanced Dixon-Schneider Calculations).

group:

the group G of which the character table is to be computed,

conjugacyClasses:

classes of G (all characters stored in the Dixon record correspond to this arrangement of classes),

irreducibles:

the already known irreducible characters (given as lists of their values on the conjugacy classes),

characterTable:

the CharacterTable value of G (whose characters are not yet known),

ClassElement( D, el ):

a function that returns the number of the class of G that contains the element el.

69.16 An Example of Advanced Dixon-Schneider Calculations

First, we set the appropriate info level higher

gap> SetInfoLevel( InfoCharacterTable, 1 );

gap> g:= PrimitiveGroup(40,5);
PSL(4,3)
gap> Size(g);
6065280
gap> d:= DixonInit( g );;
#I  29 classes
#I  choosing prime 28081
gap> c:= d.characterTable;;

gap> DixonSplit( d );
#I  Matrix 18,Representative of Order 3,Centralizer: 5832
#I  Dimensions: [ 1, 12, 2, 2, 4, 2, 1, 1, 1, 1, 1 ]
#I  Two-dim space split
#I  Two-dim space split
#I  Two-dim space split
18

We obtain several irreducible characters by tensor products and notify them to the Dixon record.

gap> asp:= AntiSymmetricParts( c, d.irreducibles, 2 );;
gap> ro:= ReducedOrdinary( c, d.irreducibles, asp );;
gap> Length( ro.irreducibles );
3
gap> DxIncludeIrreducibles( d, ro.irreducibles );

gap> nlc:= Filtered( d.irreducibles, i -> i[1] > 1 );;
gap> t:= Tensored( nlc, nlc );;
gap> ro:= ReducedOrdinary( c, d.irreducibles, t );;  ro.irreducibles;
[  ]
gap> List( ro.remainders, i -> ScalarProduct( c, i, i) );
[ 2, 2, 4, 4, 4, 4, 13, 13, 18, 18, 19, 21, 21, 36, 36, 29, 34, 34, 42, 34, 
  48, 54, 62, 68, 68, 78, 84, 84, 88, 90, 159, 169, 169, 172, 172, 266, 271, 
  271, 268, 274, 274, 280, 328, 373, 373, 456, 532, 576, 679, 683, 683, 754, 
  768, 768, 890, 912, 962, 1453, 1453, 1601, 1601, 1728, 1739, 1739, 1802, 
  2058, 2379, 2414, 2543, 2744, 2744, 2920, 3078, 3078, 4275, 4275, 4494, 
  4760, 5112, 5115, 5115, 5414, 6080, 6318, 7100, 7369, 7369, 7798, 8644, 
  10392, 12373, 12922, 14122, 14122, 18948, 21886, 24641, 24641, 25056, 
  38942, 44950, 78778 ]
gap> t:= SplitCharacters( d, ro.remainders );;
gap> List( t, i -> ScalarProduct( c, i, i ) );
[ 2, 2, 4, 2, 2, 4, 4, 3, 6, 5, 5, 9, 9, 4, 12, 13, 18, 18, 18, 26, 32, 32, 
  16, 42, 36, 84, 84, 88, 90, 159, 169, 169, 172, 172, 266, 271, 271, 268, 
  274, 274, 280, 328, 373, 373, 456, 532, 576, 679, 683, 683, 754, 768, 768, 
  890, 912, 962, 1453, 1453, 1601, 1601, 1728, 1739, 1739, 1802, 2058, 2379, 
  2414, 2543, 2744, 2744, 2920, 3078, 3078, 4275, 4275, 4494, 4760, 5112, 
  5115, 5115, 5414, 6080, 6318, 7100, 7369, 7369, 7798, 8644, 10392, 12373, 
  12922, 14122, 14122, 18948, 21886, 24641, 24641, 25056, 38942, 44950, 78778 
 ]

gap> ic:= InducedCyclic( c, "all" );;
gap> ro:= ReducedOrdinary( c, d.irreducibles, ic );;
gap> Length( ro.irreducibles );
0
gap> l:= LLL( c, ro.remainders );;
gap> Length( l.irreducibles );
13

gap> l.irreducibles[1];
Character( CharacterTable( PSL(4,3) ), [ 640, 0, 0, 0, 0, 0, 0, 0, 0, -8, 0, 
  0, E(13)^7+E(13)^8+E(13)^11, E(13)+E(13)^3+E(13)^9, E(13)^2+E(13)^5+E(13)^6,
  E(13)^4+E(13)^10+E(13)^12, 1, -8, 0, 0, -8, 0, 0, 0, 0, 0, 0, 1, 1 ] )
gap> l:=List(l.irreducibles,ValuesOfClassFunction);;
gap> DxIncludeIrreducibles( d, l );
gap> Length( d.irreducibles );
29
gap> Length( d.classes );
29

gap> irrs:= DixontinI( d );;
#I  Total:1 matrices,[ 18 ]
gap> SetIrr(g,irrs);
gap> Length(Irr(c));
29
gap> SetInfoLevel( InfoCharacterTable, 0 );

69.17 Constructing Character Tables from Others

The following operations take one or more character table arguments, and return a character table. This holds also for BrauerTable (see BrauerTable); note that the return value of BrauerTable will in general not know the irreducible Brauer characters, and GAP might be unable to compute these characters.

Note that whenever fusions between input and output tables occur in these operations, they are stored on the concerned tables, and the NamesOfFusionSources values are updated.

(The interactive construction of character tables using character theoretic methods and incomplete tables is not described here.) @Currently it is not supported and will be described in a chapter of its own when it becomes available@.

is the table of the direct product of the character tables tbl1 and tbl2.

The matrix of irreducibles of this table is the Kronecker product (see KroneckerProduct) of the irreducibles of tbl1 and tbl2.

Products of ordinary and Brauer character tables are supported.

In general, the result will not know an underlying group, so missing power maps (for prime divisors of the result) and irreducibles of tbl1 and tbl2 may be computed in order to construct the direct product.

The embeddings of tbl1 and tbl2 into the direct product are stored, they can be fetched with GetFusionMap (see GetFusionMap); if tbl1 is equal to tbl2 then the two embeddings are distinguished by their specification components "1" and "2", respectively.

Analogously, the projections from the direct product onto tbl1 and tbl2 are stored, and can be distinguished by the specification components.

The * operator for two character tables (see Operators for Character Tables) delegates to CharacterTableDirectProduct.

gap> c2:= CharacterTable( "Cyclic", 2 );;
gap> s3:= CharacterTable( "Symmetric", 3 );;
gap> Display( CharacterTableDirectProduct( c2, s3 ) );
C2xSym(3)

     2  2  2  1  2  2  1
     3  1  .  1  1  .  1

       1a 2a 3a 2b 2c 6a
    2P 1a 1a 3a 1a 1a 3a
    3P 1a 2a 1a 2b 2c 2b

X.1     1 -1  1  1 -1  1
X.2     2  . -1  2  . -1
X.3     1  1  1  1  1  1
X.4     1 -1  1 -1  1 -1
X.5     2  . -1 -2  .  1
X.6     1  1  1 -1 -1 -1

is the character table of the factor group of the ordinary character table tbl by the normal closure of the classes whose positions are contained in the list clases.

The / operator for a character table and a list of class positions (see Operators for Character Tables) delegates to CharacterTableFactorGroup.

gap> s4:= CharacterTable( "Symmetric", 4 );;
gap> ClassPositionsOfNormalSubgroups( s4 );
[ [ 1 ], [ 1, 3 ], [ 1, 3, 4 ], [ 1, 2, 3, 4, 5 ] ]
gap> f:= CharacterTableFactorGroup( s4, [ 3 ] );
CharacterTable( "Sym(4)/[ 1, 3 ]" )
gap> Display( f );
Sym(4)/[ 1, 3 ]

     2  1  1  .
     3  1  .  1

       1a 2a 3a
    2P 1a 1a 3a
    3P 1a 2a 1a

X.1     1 -1  1
X.2     2  . -1
X.3     1  1  1

If tbl is the character table of a group with structure 2.G.2 with a central subgroup Z of order 2 and a normal subgroup N of index 2 that contains Z then CharacterTableIsoclinic returns the character table of the isoclinic group in the sense of the ATLAS of Finite Groups CCN85, Chapter 6, Section 7. If N is not uniquely determined then the positions of the classes forming N must be entered as list classes. If Z is not unique in N then the position of the class consisting of the involution in Z must be entered as centre.

gap> d8:= CharacterTable( "Dihedral", 8 );;
gap> nsg:= ClassPositionsOfNormalSubgroups( d8 );
[ [ 1 ], [ 1, 3 ], [ 1, 2, 3 ], [ 1, 3, 4 ], [ 1, 3, 5 ], [ 1, 2, 3, 4, 5 ] ]
gap> Display( CharacterTableIsoclinic( d8, nsg[3] ) );
Isoclinic(D8)

     2  3  2  3  2  2

       1a 4a 2a 4b 4c
    2P 1a 2a 1a 2a 2a

X.1     1  1  1  1  1
X.2     1  1  1 -1 -1
X.3     1 -1  1  1 -1
X.4     1 -1  1 -1  1
X.5     2  . -2  .  .

returns the character table of the wreath product of a group G with the full symmetric group on n points, where tbl is the character table of G.

The result has values for ClassParameters and CharacterParameters (see ctbllib:ClassParameters, ctbllib:CharacterParameters in the manual for the GAP Character Table Library) stored, the entries in these lists are sequences of partitions. Note that this parametrization prevents the principal character from being the first one in the list of irreducibles.

gap> c3:= CharacterTable( "Cyclic", 3 );;
gap> wr:= CharacterTableWreathSymmetric( c3, 2 );;
gap> Display( wr );
C3wrS2

     2  1   .   .   1  .   1  1   1   1
     3  2   2   2   2  2   2  1   1   1

       1a  3a  3b  3c 3d  3e 2a  6a  6b
    2P 1a  3b  3a  3e 3d  3c 1a  3c  3e
    3P 1a  1a  1a  1a 1a  1a 2a  2a  2a

X.1     1   1   1   1  1   1 -1  -1  -1
X.2     2   A  /A   B -1  /B  .   .   .
X.3     2  /A   A  /B -1   B  .   .   .
X.4     1 -/A  -A  -A  1 -/A -1  /A   A
X.5     2  -1  -1   2 -1   2  .   .   .
X.6     1  -A -/A -/A  1  -A -1   A  /A
X.7     1   1   1   1  1   1  1   1   1
X.8     1 -/A  -A  -A  1 -/A  1 -/A  -A
X.9     1  -A -/A -/A  1  -A  1  -A -/A

A = -E(3)^2
  = (1+ER(-3))/2 = 1+b3
B = 2*E(3)
  = -1+ER(-3) = 2b3
gap> CharacterParameters( wr )[1];
[ [ 1, 1 ], [  ], [  ] ]

69.18 Sorted Character Tables

is a character table that differs from tbl only by the succession of its irreducible characters. This affects the values of the attributes Irr (see Irr) and CharacterParameters (see ctbllib:CharacterParameters in the manual for the GAP Character Table Library). Namely, these lists are permuted by the permutation perm.

If no second argument is given then a permutation is used that yields irreducible characters of increasing degree for the result. For the succession of characters in the result, see SortedCharacters.

The result has all those attributes and properties of tbl that are stored in SupportedCharacterTableInfo and do not depend on the ordering of characters (see SupportedCharacterTableInfo).

is a list containing the characters chars, ordered as specified by the other arguments.

There are three possibilities to sort characters: They can be sorted according to ascending norms (parameter "norm"), to ascending degree (parameter "degree"), or both (no third parameter), i.e., characters with same norm are sorted according to ascending degree, and characters with smaller norm precede those with bigger norm.

Rational characters in the result precede other ones with same norm and/or same degree.

The trivial character, if contained in chars, will always be sorted to the first position.

is a character table obtained by permutation of the classes of tbl. If the second argument is the string "centralizers" then the classes of the result are sorted according to descending centralizer orders. If the second argument is the string "representatives" then the classes of the result are sorted according to ascending representative orders. If no second argument is given then the classes of the result are sorted according to ascending representative orders, and classes with equal representative orders are sorted according to descending centralizer orders.

If the second argument is a permutation perm then the classes of the result are sorted by application of this permutation.

The result has all those attributes and properties of tbl that are stored in SupportedCharacterTableInfo and do not depend on the ordering of classes (see SupportedCharacterTableInfo).

is a character table obtained on permutation of the classes and the irreducibles characters of tbl.

The first form sorts the classes at positions contained in the list kernel to the beginning, and sorts all characters in Irr( tbl ) such that the first characters are those that contain kernel in their kernel.

The second form does the same successively for all kernels k_i in the list normalseries = [ k₁, k₂, ¼, k_n ] where k_i must be a sublist of k_i+1 for 1 £ i £ n-1.

The third form computes the table F of the factor group of tbl modulo the normal subgroup formed by the classes whose positions are contained in the list kernel; F must be permutation equivalent to the table facttbl, in the sense of TransformingPermutationsCharacterTables (see TransformingPermutationsCharacterTables), otherwise fail is returned. The classes of tbl are sorted such that the preimages of a class of F are consecutive, and that the succession of preimages is that of facttbl. Irr( tbl ) is sorted as with SortCharTable( tbl, kernel ).

(Note that the transformation is only unique up to table automorphisms of F, and this need not be unique up to table automorphisms of tbl.)

All rearrangements of classes and characters are stable, i.e., the relative positions of classes and characters that are not distinguished by any relevant property is not changed.

The result has all those attributes and properties of tbl that are stored in SupportedCharacterTableInfo and do not depend on the ordering of classes and characters (see SupportedCharacterTableInfo).

The ClassPermutation value of tbl is changed if necessary, see Conventions for Character Tables.

SortedCharacterTable uses CharacterTableWithSortedClasses and CharacterTableWithSortedCharacters (see CharacterTableWithSortedClasses, CharacterTableWithSortedCharacters).

is a permutation p of classes of the character table tbl. If it is stored then class fusions into tbl that are stored on other tables must be followed by p in order to describe the correct fusion.

This attribute value is bound only if tbl was obtained from another table by permuting the classes, using CharacterTableWithSortedClasses or SortedCharacterTable, (see CharacterTableWithSortedClasses, SortedCharacterTable).

It is necessary because the original table and the sorted table have the same identifier (and the same group if known), and hence the same fusions are valid for the two tables.

gap> tbl:= CharacterTable( "Symmetric", 4 );
CharacterTable( "Sym(4)" )
gap> Display( tbl );
Sym(4)

     2  3  2  3  .  2
     3  1  .  .  1  .

       1a 2a 2b 3a 4a
    2P 1a 1a 1a 3a 2b
    3P 1a 2a 2b 1a 4a

X.1     1 -1  1  1 -1
X.2     3 -1 -1  .  1
X.3     2  .  2 -1  .
X.4     3  1 -1  . -1
X.5     1  1  1  1  1

gap> srt1:= CharacterTableWithSortedCharacters( tbl );
CharacterTable( "Sym(4)" )
gap> List( Irr( srt1 ), Degree );
[ 1, 1, 2, 3, 3 ]
gap> srt2:= CharacterTableWithSortedClasses( tbl );
CharacterTable( "Sym(4)" )
gap> SizesCentralizers( tbl );
[ 24, 4, 8, 3, 4 ]
gap> SizesCentralizers( srt2 );
[ 24, 8, 4, 3, 4 ]
gap> nsg:= ClassPositionsOfNormalSubgroups( tbl );
[ [ 1 ], [ 1, 3 ], [ 1, 3, 4 ], [ 1, 2, 3, 4, 5 ] ]
gap> srt3:= SortedCharacterTable( tbl, nsg );
CharacterTable( "Sym(4)" )
gap> nsg:= ClassPositionsOfNormalSubgroups( srt3 );
[ [ 1 ], [ 1, 2 ], [ 1, 2, 3 ], [ 1, 2, 3, 4, 5 ] ]
gap> Display( srt3 );
Sym(4)

     2  3  3  .  2  2
     3  1  .  1  .  .

       1a 2a 3a 2b 4a
    2P 1a 1a 3a 1a 2a
    3P 1a 2a 1a 2b 4a

X.1     1  1  1  1  1
X.2     1  1  1 -1 -1
X.3     2  2 -1  .  .
X.4     3 -1  . -1  1
X.5     3 -1  .  1 -1

gap> ClassPermutation( srt3 );
(2,4,3)

69.19 Automorphisms and Equivalence of Character Tables

For a matrix mat, MatrixAutomorphisms returns the group of those permutations of the columns of mat that leave the set of rows of mat invariant.

If the arguments maps and subgroup are given, only the group of those permutations is constructed that additionally fix each list in the list maps under pointwise action OnTuples, and subgroup is a permutation group that is known to be a subgroup of this group of automorphisms.

Each entry in maps must be a list of same length as the rows of mat. For example, if mat is a list of irreducible characters of a group then the list of element orders of the conjugacy classes (see OrdersClassRepresentatives) may be an entry in maps.

TableAutomorphisms returns the permutation group of those matrix automorphisms (see MatrixAutomorphisms) of the list characters that leave the element orders (see OrdersClassRepresentatives) and all stored power maps (see ComputedPowerMaps) of the character table tbl.

If characters is closed under Galois conjugacy --this is always fulfilled for ordinary character tables-- the string "closed" may be entered as third argument.

The attribute AutomorphismsOfTable (see AutomorphismsOfTable) can be used to compute and store the table automorphisms for the case that characters equals Irr( tbl ).

gap> tbld8:= CharacterTable( "Dihedral", 8 );;
gap> irrd8:= Irr( tbld8 );
[ Character( CharacterTable( "D8" ), [ 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( "D8" ), [ 1, 1, 1, -1, -1 ] ), 
  Character( CharacterTable( "D8" ), [ 1, -1, 1, 1, -1 ] ), 
  Character( CharacterTable( "D8" ), [ 1, -1, 1, -1, 1 ] ), 
  Character( CharacterTable( "D8" ), [ 2, 0, -2, 0, 0 ] ) ]
gap> orders:= OrdersClassRepresentatives( tbld8 );
[ 1, 4, 2, 2, 2 ]
gap> MatrixAutomorphisms( irrd8 );
Group([ (4,5), (2,4) ])
gap> MatrixAutomorphisms( irrd8, [ orders ], Group( () ) );
Group([ (4,5) ])
gap> TableAutomorphisms( tbld8, irrd8 );
Group([ (4,5) ])

Let mat1 and mat2 be matrices. TransformingPermutations tries to construct a permutation p that transforms the set of rows of the matrix mat1 to the set of rows of the matrix mat2 by permuting the columns.

If such a permutation exists, a record with components columns, rows, and group is returned, otherwise fail. For TransformingPermutations( mat1 , mat2 ) = r ¹ fail, we have mat2 = Permuted( List( mat1, x -> Permuted( x, r.columns ) ),r.rows ).

r.group is the group of matrix automorphisms of mat2 (see MatrixAutomorphisms). This group stabilizes the transformation in the sense that applying any of its elements to the columns of mat2 preserves the set of rows of mat2.

Let tbl1 and tbl2 be character tables. TransformingPermutationsCharacterTables tries to construct a permutation p that transforms the set of rows of the matrix Irr( tbl1 ) to the set of rows of the matrix Irr( tbl2 ) by permuting the columns (see TransformingPermutations), such that p transforms also the power maps and the element orders.

If such a permutation p exists then a record with the components columns (p), rows (the permutation of Irr( tbl1 ) corresponding to p), and group (the permutation group of table automorphisms of tbl2, see AutomorphismsOfTable) is returned. If no such permutation exists, fail is returned.

gap> tblq8:= CharacterTable( "Quaternionic", 8 );;
gap> irrq8:= Irr( tblq8 );
[ Character( CharacterTable( "Q8" ), [ 1, 1, 1, 1, 1 ] ), 
  Character( CharacterTable( "Q8" ), [ 1, 1, 1, -1, -1 ] ), 
  Character( CharacterTable( "Q8" ), [ 1, -1, 1, 1, -1 ] ), 
  Character( CharacterTable( "Q8" ), [ 1, -1, 1, -1, 1 ] ), 
  Character( CharacterTable( "Q8" ), [ 2, 0, -2, 0, 0 ] ) ]
gap> OrdersClassRepresentatives( tblq8 );
[ 1, 4, 2, 4, 4 ]
gap> TransformingPermutations( irrd8, irrq8 );
rec( columns := (), rows := (), group := Group([ (4,5), (2,4) ]) )
gap> TransformingPermutationsCharacterTables( tbld8, tblq8 );
fail
gap> tbld6:= CharacterTable( "Dihedral", 6 );;
gap> tbls3:= CharacterTable( "Symmetric", 3 );;
gap> TransformingPermutationsCharacterTables( tbld6, tbls3 );
rec( columns := (2,3), rows := (1,3,2), group := Group(()) )

distributes the rows of the matrix mat into families as follows. Two rows of mat belong to the same family if there is a permutation of columns that maps one row to the other row. Each entry in the list maps is regarded to form a family of length 1.

FamiliesOfRows( mat, maps ) returns a record with components

famreps

the list of representatives for each family,

permutations

the list that contains at position i a list of permutations that map the members of the family with representative famreps[i] to that representative,

families

the list that contains at position i the list of positions of members of the family of representative famreps[i]; (for the element maps[i] the only member of the family will get the number Length( mat ) + i).

69.20 Interface to the CAS System

is a string that encodes the CAS library format of the character table tbl. This string can be printed to a file which then can be read into the CAS system using its get command (see NPP84).

The used line length is SizeScreen()[1] (see SizeScreen).

Only the known values of the following attributes are used. ClassParameters (for partitions only), ComputedClassFusions, ComputedPowerMaps, Identifier, InfoText, Irr, ComputedPrimeBlocks, ComputedIndicators, OrdersClassRepresentatives, Size, SizesCentralizers.

gap> Print( CASString( CharacterTable( "Cyclic", 2 ) ), "\n" );
'C2'
00/00/00. 00.00.00.
(2,2,0,2,-1,0)
text:
(#computed using generic character table for cyclic groups#),
order=2,
centralizers:(
2,2
),
reps:(
1,2
),
powermap:2(
1,1
),
characters:
(1,1
,0:0)
(1,-1
,0:0);
/// converted from GAP

69.21 Interface to the MOC System

Additionally it provides an alternative representation of (virtual) characters.

The MOC 3 code of a 5 digit number in MOC 2 code is given by the following list. (Note that the code must contain only lower case letters.)

ABCD    for  0ABCD
a       for  10000
b       for  10001          k       for  20001
c       for  10002          l       for  20002
d       for  10003          m       for  20003
e       for  10004          n       for  20004
f       for  10005          o       for  20005
g       for  10006          p       for  20006
h       for  10007          q       for  20007
i       for  10008          r       for  20008
j       for  10009          s       for  20009
tAB     for  100AB
uAB     for  200AB
vABCD   for  1ABCD
wABCD   for  2ABCD
yABC    for  30ABC
z       for  31000

Note that any long number in MOC 2 format is divided into packages of length 4, the beginning (!) filled with leading zeros if necessary. Such a number with decimals d₁, d₂, ¼, d_4n+k is the sequence

0d1d2d3d4 ¼0d4n-3d4n-2d4n-1d4n xd4n+1¼d4n+k

where 0 £ k £ 3, the first digit of x is 1 if the number is positive and 2 if the number is negative, and then follow (4-k) zeros. A brief description of the MOC system can be found in LP91.

MAKElb11 prints field information for all number fields with conductor n where the positive integer n is in the list listofns.

The output of MAKElb11 is used by the MOC system. MAKElb11( [ 3 .. 189 ] ) will print something very similar to Richard Parker's file lb11.

gap> MAKElb11( [ 3, 4 ] );
   3   2   0   1   0
   4   2   0   1   0

MOCTable returns the MOC table record of the GAP character table gaptbl.

The first form can be used only if gaptbl is an ordinary (G.0) table. For Brauer (G.p) tables one has to specify a basic set basicset of ordinary irreducibles. basicset must be a list of positions of the basic set characters in the Irr list of the ordinary table of gaptbl.

The result is a record that contains the information of gaptbl in a format similar to the MOC 3 format. This record can e.g. easily be printed out or be used to print out characters using MOCString (see MOCString).

The components of the result are

identifier

the string MOCTable(name) where name is the Identifier value of gaptbl,

GAPtbl

gaptbl,

prime

the characteristic of the field (label 30105 in MOC),

centralizers

centralizer orders for cyclic subgroups (label 30130)

orders

element orders for cyclic subgroups (label 30140)

fieldbases

at position i the Parker basis of the number field generated by the character values of the i-th cyclic subgroup. The length of fieldbases is equal to the value of label 30110 in MOC.

cycsubgps

cycsubgps[i] = j means that class i of the GAP table belongs to the j-th cyclic subgroup of the GAP table,

repcycsub

repcycsub[j] = i means that class i of the GAP table is the representative of the j-th cyclic subgroup of the GAP table. Note that the representatives of GAP table and MOC table need not agree!

galconjinfo

a list [ r₁, c₁, r₂, c₂, ¼, r_n, c_n ] which means that the i-th class of the GAP table is the c_i-th conjugate of the representative of the r_i-th cyclic subgroup on the MOC table. (This is used to translate back to GAP format, stored under label 30160)

30170

(power maps) for each cyclic subgroup (except the trivial one) and each prime divisor of the representative order store four values, namely the number of the subgroup, the power, the number of the cyclic subgroup containing the image, and the power to which the representative must be raised to yield the image class. (This is used only to construct the 30230 power map/embedding information.) In 30170 only a list of lists (one for each cyclic subgroup) of all these values is stored, it will not be used by GAP.

tensinfo

tensor product information, used to compute the coefficients of the Parker base for tensor products of characters (label 30210 in MOC). For a field with vector space basis (v₁,v₂,¼,v_n) the tensor product information of a cyclic subgroup in MOC (as computed by fct) is either 1 (for rational classes) or a sequence

n x1,1 y1,1 z1,1 x1,2 y1,2 z1,2 ¼x1,m1 y1,m1 z1,m1 0 x2,1 y2,1 z2,1 x2,2 y2,2 z2,2 ¼x2,m2 y2,m2 z2,m2 0 ¼zn,mn 0

which means that the coefficient of v_k in the product

æ è n å i=1  ai vi ö ø æ è n å j=1  bj vj ö ø

is equal to

mk å i=1  xk,i ayk,i bzk,i.

On a MOC table in GAP the tensinfo component is a list of lists, each containing exactly the sequence mentioned above.

invmap

inverse map to compute complex conjugate characters, label 30220 in MOC.

powerinfo

field embeddings for p-th symmetrizations, p prime in [ 2 .. 19 ]; note that the necessary power maps must be stored on gaptbl to compute this component. (label 30230 in MOC)

30900

basic set of restricted ordinary irreducibles in the case of nonzero characteristic, all ordinary irreducibles otherwise.

Let moctbl be a MOC table record as returned by MOCTable (see MOCTable). MOCString returns a string describing the MOC 3 format of moctbl.

If the second argument chars is specified, it must be a list of MOC format characters as returned by MOCChars (see MOCChars). In this case, these characters are stored under label 30900. If the second argument is missing then the basic set of ordinary irreducibles is stored under this label.

gap> moca5:= MOCTable( CharacterTable( "A5" ) );
rec( identifier := "MOCTable(A5)", prime := 0, fields := [  ], 
  GAPtbl := CharacterTable( "A5" ), cycsubgps := [ 1, 2, 3, 4, 4 ], 
  repcycsub := [ 1, 2, 3, 4 ], galconjinfo := [ 1, 1, 2, 1, 3, 1, 4, 1, 4, 2 ]
    , centralizers := [ 60, 4, 3, 5 ], orders := [ 1, 2, 3, 5 ], 
  fieldbases := [ CanonicalBasis( Rationals ), CanonicalBasis( Rationals ), 
      CanonicalBasis( Rationals ), 
      Basis( NF(5,[ 1, 4 ]), [ 1, E(5)+E(5)^4 ] ) ], 
  30170 := [ [  ], [ 2, 2, 1, 1 ], [ 3, 3, 1, 1 ], [ 4, 5, 1, 1 ] ], 
  tensinfo := 
    [ [ 1 ], [ 1 ], [ 1 ], [ 2, 1, 1, 1, 1, 2, 2, 0, 1, 1, 2, 1, 2, 1, -1, 2, 
          2, 0 ] ], 
  invmap := [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, 0, 1, 5, 0 ] ], 
  powerinfo := 
    [ , [ [ 1, 1, 0 ], [ 1, 1, 0 ], [ 1, 3, 0 ], [ 1, 4, -1, 5, 0, -1, 5, 0 ] 
         ], 
      [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 1, 0 ], [ 1, 4, -1, 5, 0, -1, 5, 0 ] ],
      , [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 1, 0, 0 ] ],, 
      [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, -1, 5, 0, -1, 5, 0 ] ],
      ,,, [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, 0, 1, 5, 0 ] ],, 
      [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, -1, 5, 0, -1, 5, 0 ] ],
      ,,, 
      [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, -1, 5, 0, -1, 5, 0 ] ],
      , [ [ 1, 1, 0 ], [ 1, 2, 0 ], [ 1, 3, 0 ], [ 1, 4, 0, 1, 5, 0 ] ] ], 
  30900 := [ [ 1, 1, 1, 1, 0 ], [ 3, -1, 0, 0, -1 ], [ 3, -1, 0, 1, 1 ], 
      [ 4, 0, 1, -1, 0 ], [ 5, 1, -1, 0, 0 ] ] )
gap> str:= MOCString( moca5 );;
gap> str{[1..70]};
"y100y105ay110fey130t60edfy140bcdfy150bbbfcabbey160bbcbdbebecy170ccbbdd"
gap> moca5mod3:= MOCTable( CharacterTable( "A5" ) mod 3, [ 1 .. 4 ] );;
gap> MOCString( moca5mod3 ){ [ 1 .. 70 ] };
"y100y105dy110edy130t60efy140bcfy150bbfcabbey160bbcbdbdcy170ccbbdfbby21"

returns a record containing the information encoded in the list list. The components of the result are the labels that occur in list. If list is in MOC 2 format (10000-format), the names of components are 30000-numbers; if it is in MOC 3 format the names of components have yABC-format.

Let tbl be a character table or a MOC table record, and mocchars either a list of MOC format characters (as returned by MOCChars (see MOCChars) or a list of positive integers such as a record component encoding characters, in a record produced by ScanMOC (see ScanMOC).

GAPChars returns translations of mocchars to GAP character values lists.

Let tbl be a character table or a MOC table record, and gapchars a list of (GAP format) characters. MOCChars returns translations of gapchars to MOC format.

gap> scan:= ScanMOC( str );
rec( y105 := [ 0 ], y110 := [ 5, 4 ], y130 := [ 60, 4, 3, 5 ], 
  y140 := [ 1, 2, 3, 5 ], y150 := [ 1, 1, 1, 5, 2, 0, 1, 1, 4 ], 
  y160 := [ 1, 1, 2, 1, 3, 1, 4, 1, 4, 2 ], 
  y170 := [ 2, 2, 1, 1, 3, 3, 1, 1, 4, 5, 1, 1 ], 
  y210 := [ 1, 1, 1, 2, 1, 1, 1, 1, 2, 2, 0, 1, 1, 2, 1, 2, 1, -1, 2, 2, 0 ], 
  y220 := [ 1, 1, 0, 1, 2, 0, 1, 3, 0, 1, 4, 0, 1, 5, 0 ], 
  y230 := [ 2, 1, 1, 0, 1, 1, 0, 1, 3, 0, 1, 4, -1, 5, 0, -1, 5, 0 ], 
  y050 := [ 19, 1, 1, 0, 1, 2, 0, 1, 3, 0, 1, 4, 0, 1, 5, 0 ], 
  y900 := [ 1, 1, 1, 1, 0, 3, -1, 0, 0, -1, 3, -1, 0, 1, 1, 4, 0, 1, -1, 0, 
      5, 1, -1, 0, 0 ] )
gap> gapchars:= GAPChars( moca5, scan.y900 );
[ [ 1, 1, 1, 1, 1 ], [ 3, -1, 0, -E(5)-E(5)^4, -E(5)^2-E(5)^3 ], 
  [ 3, -1, 0, -E(5)^2-E(5)^3, -E(5)-E(5)^4 ], [ 4, 0, 1, -1, -1 ], 
  [ 5, 1, -1, 0, 0 ] ]
gap> mocchars:= MOCChars( moca5, gapchars );
[ [ 1, 1, 1, 1, 0 ], [ 3, -1, 0, 0, -1 ], [ 3, -1, 0, 1, 1 ], 
  [ 4, 0, 1, -1, 0 ], [ 5, 1, -1, 0, 0 ] ]
gap> Concatenation( mocchars ) = scan.y900;
true

69.22 Storing Normal Subgroup Information

Let tbl be the ordinary character table of the group G. Many computations for group characters of G involve computations in normal subgroups or factor groups of G.

In some cases the character table tbl is sufficient; for example questions about a normal subgroup N of G can be answered if one knows the conjugacy classes that form N, e.g., the question whether a character of G restricts irreducibly to N. But other questions require the computation of N or even more information, like the character table of N.

In order to do these computations only once, one stores in the group a record with components to store normal subgroups, the corresponding lists of conjugacy classes, and (if necessary) the factor groups, namely

nsg:

list of normal subgroups of G, may be incomplete,

nsgclasses:

at position i, the list of positions of conjugacy classes of tbl forming the i-th entry of the nsg component,

nsgfactors:

at position i, if bound, the factor group modulo the i-th entry of the nsg component.

NormalSubgroupClasses, FactorGroupNormalSubgroupClasses, and ClassPositionsOfNormalSubgroup each use these components, and they are the only functions to do so.

So if you need information about a normal subgroup for that you know the conjugacy classes, you should get it using NormalSubgroupClasses. If the normal subgroup was already used it is just returned, with all the knowledge it contains. Otherwise the normal subgroup is added to the lists, and will be available for the next call.

For example, if you are dealing with kernels of characters using the KernelOfCharacter function you make use of this feature because KernelOfCharacter calls NormalSubgroupClasses.

is the list of positions of conjugacy classes of the character table tbl that are contained in the normal subgroup N of the underlying group of tbl.

returns the normal subgroup of the underlying group G of the ordinary character table tbl that consists of those conjugacy classes of tbl whose positions are in the list classes.

If NormalSubgroupClassesInfo( tbl ).nsg does not yet contain the required normal subgroup, and if NormalSubgroupClassesInfo( tbl ).normalSubgroups is bound then the result will be identical to the group in NormalSubgroupClassesInfo( tbl ).normalSubgroups.

is the factor group of the underlying group G of the ordinary character table tbl modulo the normal subgroup of G that consists of those conjugacy classes of tbl whose positions are in the list classes.

gap> g:= SymmetricGroup( 4 );
Sym( [ 1 .. 4 ] )
gap> SetName( g, "S4" );
gap> tbl:= CharacterTable( g );
CharacterTable( S4 )
gap> irr:= Irr( g );
[ Character( CharacterTable( S4 ), [ 1, -1, 1, 1, -1 ] ), 
  Character( CharacterTable( S4 ), [ 3, -1, -1, 0, 1 ] ), 
  Character( CharacterTable( S4 ), [ 2, 0, 2, -1, 0 ] ), 
  Character( CharacterTable( S4 ), [ 3, 1, -1, 0, -1 ] ), 
  Character( CharacterTable( S4 ), [ 1, 1, 1, 1, 1 ] ) ]
gap> kernel:= KernelOfCharacter( irr[3] );
Group([ (1,2)(3,4), (1,4)(2,3) ])
gap> HasNormalSubgroupClassesInfo( tbl );
true
gap> NormalSubgroupClassesInfo( tbl );
rec( nsg := [ Group([ (1,2)(3,4), (1,4)(2,3) ]) ], nsgclasses := [ [ 1, 3 ] ],
  nsgfactors := [  ] )
gap> ClassPositionsOfNormalSubgroup( tbl, kernel );
[ 1, 3 ]
gap> FactorGroupNormalSubgroupClasses( tbl, [ 1, 3 ] );
Group([ f1, f2 ])
gap> NormalSubgroupClassesInfo( tbl );
rec( nsg := [ Group([ (1,2)(3,4), (1,4)(2,3) ]) ], nsgclasses := [ [ 1, 3 ] ],
  nsgfactors := [ Group([ f1, f2 ]) ] )

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GAP 4 manual
May 2002