Multiplicity-Free Permutation Characters in GAP

THOMAS BREUER
Lehrstuhl D für Mathematik
RWTH, 52056 Aachen, Germany

October 6th, 2000

This note shows a few examples of GAP computations concerning multiplicity-free permutation characters, with an emphasis on the classification of the faithful multiplicity-free permutation characters of the sporadic simple groups and their automorphism groups given in [BL96].
For examples on GAP computations with permutation characters in general, see the note [Bre].
For further questions about GAP, consult its Reference Manual; in particular, for the description of the commands for character tables, see the chapter "Character Tables".
Section 1 of this note shows how to interpret the individual data available in the database. In Section 2, the main idea is to gather information from the database as a whole, by filtering items with suitable properties. Finally, Section 3 gives an impression how GAP can be used to obtain results such as the classification of described in [BL96].

Contents

1  The Database of Multiplicity-Free Characters
    1.1  The Faithful Multiplicity-Free Permutation Characters of M11
    1.2  The Faithful Multiplicity-Free Permutation Characters of M12.2
2  Using the Database
3  Using the Functions to Compute Multiplicity-Free Permutation Characters
    3.1  Using Tables of Marks
    3.2  Dealing with Possible Permutation Characters

1  The Database of Multiplicity-Free Characters

The database lists, for each group G that is either a sporadic simple group or an automorphism group of a sporadic simple group, a description of all conjugacy classes of subgroups H of G such that the action of G on the right cosets of H is a faithful and multiplicity-free permutation representation of G, plus the permutation character of this representation. The format how this information is stored is explained below, subtleties such as possibly equal characters for different classes of subgroups are discussed in Section 2.
(A GAP database providing more information about most of these representations is in preparation; this will cover, i.a., the character tables of the endomorphism rings of these representations and the permutation representations themselves.)
The data is stored in the file multfree.dat, which is part of the Character Table Library [Bre12] of the GAP system [GAP04] as well as the file you are currently reading. We load this GAP package and the data file into GAP 4. Afterwards the function MultFreePermChars is available.
    gap> LoadPackage( "ctbllib" );
    true
    gap> if not IsBound( MultFreePermChars ) then
    > ReadPackage( "ctbllib", "tst/multfree.dat" );
    --------------------------------------------------------------------
    Loading the Database of Multiplicity-Free Permutation Characters
    of the Sporadic Simple Groups and Their Automorphism Groups,
    by T. Breuer and K. Lux;
    call `MultFreePermChars( <name> )' for accessing the data
    for the group whose character table has identifier <name>.
    --------------------------------------------------------------------
    > fi;

1.1  The Faithful Multiplicity-Free Permutation Characters of M11

We start with the inspection of the Mathieu group M11, as an example of a simple group that is dealt with in the database.
    gap> info:= MultFreePermChars( "M11" );
    [ rec( ATLAS := "1a+10a", character := Character( CharacterTable( "M11" ),
            [ 11, 3, 2, 3, 1, 0, 1, 1, 0, 0 ] ), group := "$M_{11}$", rank := 2,
          subgroup := "$A_6.2_3$" ),
      rec( ATLAS := "1a+10a+11a",
          character := Character( CharacterTable( "M11" ),
            [ 22, 6, 4, 2, 2, 0, 0, 0, 0, 0 ] ), group := "$M_{11}$", rank := 3,
          subgroup := "$A_6 \\leq A_6.2_3$" ),
      rec( ATLAS := "1a+11a", character := Character( CharacterTable( "M11" ),
            [ 12, 4, 3, 0, 2, 1, 0, 0, 1, 1 ] ), group := "$M_{11}$", rank := 2,
          subgroup := "$L_2(11)$" ),
      rec( ATLAS := "1a+11a+16ab+45a+55a",
          character := Character( CharacterTable( "M11" ),
            [ 144, 0, 0, 0, 4, 0, 0, 0, 1, 1 ] ), group := "$M_{11}$", rank := 6,
          subgroup := "$11:5 \\leq L_2(11)$" ),
      rec( ATLAS := "1a+10a+44a",
          character := Character( CharacterTable( "M11" ),
            [ 55, 7, 1, 3, 0, 1, 1, 1, 0, 0 ] ), group := "$M_{11}$", rank := 3,
          subgroup := "$3^2:Q_8.2$" ),
      rec( ATLAS := "1a+10a+44a+55a",
          character := Character( CharacterTable( "M11" ),
            [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ), group := "$M_{11}$", rank := 4,
          subgroup := "$3^2:8 \\leq 3^2:Q_8.2$" ),
      rec( ATLAS := "1a+10a+11a+44a",
          character := Character( CharacterTable( "M11" ),
            [ 66, 10, 3, 2, 1, 1, 0, 0, 0, 0 ] ), group := "$M_{11}$", rank := 4,
          subgroup := "$A_5.2$" ) ]
    gap> List( info, x -> x.rank );
    [ 2, 3, 2, 6, 3, 4, 4 ]
    gap> chars:= List( info, x -> x.character );;
    gap> degrees:= List( chars, x -> x[1] );
    [ 11, 22, 12, 144, 55, 110, 66 ]

We see that M11 has seven multiplicity-free permutation characters, of the ranks and degrees listed above. (Note that for multiplicity-free permutation characters, the rank is equal to the number of irreducible constituents.) More precisely, there are exactly seven conjugacy classes of subgroups of M11 such that the permutation action on the cosets of these subgroups is faithful and multiplicity-free.
For displaying the characters compatibly with the character table of M11, we can use the Display operation. Note that the column and row ordering of character tables in GAP is compatible with that of the tables in the ATLAS of Finite Groups ([CCN+85]).
    gap> tbl:= CharacterTable( "M11" );
    CharacterTable( "M11" )
    gap> Display( tbl, rec( chars:= chars ) );
    M11
    
         2   4  4  1  3  .  1  3  3   .   .
         3   2  1  2  .  .  1  .  .   .   .
         5   1  .  .  .  1  .  .  .   .   .
        11   1  .  .  .  .  .  .  .   1   1
    
            1a 2a 3a 4a 5a 6a 8a 8b 11a 11b
        2P  1a 1a 3a 2a 5a 3a 4a 4a 11b 11a
        3P  1a 2a 1a 4a 5a 2a 8a 8b 11a 11b
        5P  1a 2a 3a 4a 1a 6a 8b 8a 11a 11b
       11P  1a 2a 3a 4a 5a 6a 8a 8b  1a  1a
    
    Y.1     11  3  2  3  1  .  1  1   .   .
    Y.2     22  6  4  2  2  .  .  .   .   .
    Y.3     12  4  3  .  2  1  .  .   1   1
    Y.4    144  .  .  .  4  .  .  .   1   1
    Y.5     55  7  1  3  .  1  1  1   .   .
    Y.6    110  6  2  2  .  .  2  2   .   .
    Y.7     66 10  3  2  1  1  .  .   .   .

The subgroup component of each record in info describes the isomorphism type of a subgroup U of M11 such that the value π of the character component is induced from the trivial character of U; in other words, U is a point stabilizer of the permutation representation of M11 with character π.
(Contrary to this example, in general it may happen that different classes of subgroups induce the same permutation character, and that these subgroups may also be nonisomorphic; see Section 2 for details.)
    gap> subgroups:= List( info, x -> x.subgroup );
    [ "$A_6.2_3$", "$A_6 \\leq A_6.2_3$", "$L_2(11)$", "$11:5 \\leq L_2(11)$", 
      "$3^2:Q_8.2$", "$3^2:8 \\leq 3^2:Q_8.2$", "$A_5.2$" ]

Each entry is a LATEX format string that is either a name of the point stabilizer or has the form <U> \leq <M> where <M> is the name of a maximal subgroup containing the point stabilizer <U> as a proper subgroup; in the former case, the point stabilizer is itself maximal.
Note that a backslash occurring in a subgroup string is escaped by another backslash; but only a single backslash is printed when the string is printed via the function Print.
    gap> Print( subgroups[2], "\n" );
    $A_6 \leq A_6.2_3$

Finally, the ATLAS component of each record in info describes the character value in terms of its irreducible constituents, as is computed by the function PermCharInfo. Examples can be found in Section 3; for details about the output format, see the documentation for this function in the GAP Reference Manual.

1.2  The Faithful Multiplicity-Free Permutation Characters of M12.2

The automorphism group of a sporadic simple group G is either equal to G or an upward extension of G by an outer automorphism of order 2. The nonsimple automorphism group M12.2 of the Mathieu group M12 serves as an example of the latter situation.
In addition to the aspects mentioned in Section 1.1, here we meet the situation that a permutation character either is induced from a permutation character of M12 or extends such a (not necessarily multiplicity-free) permutation character. The former case occurs exactly if the corresponding point stabilizer lies in M12.
    gap> info:= MultFreePermChars( "M12.2" );;
    gap> Length( info );
    13
    gap> info[1];
    rec( ATLAS := "1a^{\\pm}+11ab",
      character := Character( CharacterTable( "M12.2" ),
        [ 24, 0, 8, 6, 0, 4, 4, 0, 2, 2, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] ),
      group := "$M_{12}.2$", rank := 3, subgroup := "$M_{11}$" )
    gap> info[2];
    rec( ATLAS := "1a^++11ab+55a^++66a^+",
      character := Character( CharacterTable( "M12.2" ),
        [ 144, 0, 16, 9, 0, 0, 4, 0, 1, 0, 0, 1, 12, 4, 0, 0, 2, 2, 0, 1, 1 ] ),
      group := "$M_{12}.2$", rank := 4, subgroup := "$L_2(11).2$" )

The first character in the list info is induced from the trivial character of a subgroup of type M11 inside M12, the second character is induced from the trivial character of a L2(11).2 subgroup whose intersection with M12 is of type L2(11).
We can distinguish the two kinds of permutation characters by explicitly using the character tables; for example, a permutation character is induced from a subgroup of a normal subgroup if and only if it vanishes outside the classes forming this subgroup.
    gap> m12:= CharacterTable( "M12" );;
    gap> m122:= UnderlyingCharacterTable( info[1].character );;
    gap> fus:= GetFusionMap( m12, m122 );
    [ 1, 2, 3, 4, 5, 6, 6, 7, 8, 9, 10, 10, 11, 12, 12 ]
    gap> outer:= Difference( [ 1 .. NrConjugacyClasses( m122 ) ], fus );
    [ 13, 14, 15, 16, 17, 18, 19, 20, 21 ]
    gap> info[1].character{ outer };
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ]
    gap> info[2].character{ outer };
    [ 12, 4, 0, 0, 2, 2, 0, 1, 1 ]

A perhaps easier way is to look at the ATLAS components of the info records. Namely, the characters induced from subgroups of M12 have both linear characters of M12.2 as constituents, which is expressed by the substring "1a^{\\pm}".
More generally, the ATLAS component lists the irreducible constituents of the restriction to M12, where the two extensions of a character to M12.2 are distinguished by a superscript +, −, or ±; the latter means that both extensions occur.
The ATLAS components describing the constituents relative to a subgroup of index 2 can be computed using the GAP function PermCharInfoRelative, see Section 3.
It should be noted that the \leq substrings in the subgroup component cannot be used to distinguish the two kinds of permutation characters, since these substrings refer only to maximal subgroups different from M12. Examples are the first entry in info (see above), the fourth entry (containing a character that is induced from a subgroup of type A6.22 which lies in a A6.22 subgroup that is maximal in M11), and the nineth entry (containing a character induced from a subgroup of index 2 in a (22 ×A5).2 subgroup that is maximal in M12.2.
    gap> info[4];
    rec( ATLAS := "1a^{\\pm}+11ab+54a^{\\pm}+66a^{\\pm}",
      character := Character( CharacterTable( "M12.2" ),
        [ 264, 24, 24, 12, 0, 4, 4, 0, 0, 2, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] ),
      group := "$M_{12}.2$", rank := 7, subgroup := "$A_6.2_2 \\leq A_6.2^2$" )
    gap> info[9];
    rec(
      ATLAS := "1a^++16ab+45a^++54a^{\\pm}+55a^-+66a^{\\pm}+99a^-+144a^++176a^-",
      character := Character( CharacterTable( "M12.2" ),
        [ 792, 32, 24, 0, 6, 0, 2, 2, 0, 0, 2, 0, 0, 0, 8, 0, 0, 0, 2, 0, 0 ] ),
      group := "$M_{12}.2$", rank := 11,
      subgroup := "$(2 \\times A_5).2 \\leq (2^2 \\times A_5).2$" )

2  Using the Database

In this section, we study the complete list of multiplicity-free permutation characters of the sporadic simple groups and their automorphism groups as a whole.
    gap> info:= MultFreePermChars( "all" );;
    gap> Length( info );
    267
    gap> Length( Set( info ) );
    262
    gap> chars:= List( info, x -> x.character );;
    gap> Length( Set( chars ) );
    261

We see that there are exactly 267 conjugacy classes of subgroups such that the permutation representation on the cosets is multiplicity-free. Only 262 of the info records are different, and there is exactly one case where two different info records belong to the same permutation character.
Let us look where these multiple entries arise.
    gap> distrib:= List( info, x -> Position( chars, x.character ) );;
    gap> ambiguous:= Filtered( InverseMap( distrib ), IsList );
    [ [ 12, 15 ], [ 40, 41 ], [ 83, 84 ], [ 88, 90 ], [ 132, 133 ], [ 202, 203 ] ]
    gap> except:= Filtered( ambiguous, x -> info[ x[1] ] <> info[ x[2] ] );
    [ [ 83, 84 ] ]
    gap> ambiguous:= Difference( ambiguous, except );;
    gap> info{ except[1] };
    [ rec( ATLAS := "1a+22a+230a", 
          character := Character( CharacterTable( "M23" ), 
            [ 253, 29, 10, 5, 3, 2, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0 ] ), 
          group := "$M_{23}$", rank := 3, subgroup := "$L_3(4).2_2$" ), 
      rec( ATLAS := "1a+22a+230a", 
          character := Character( CharacterTable( "M23" ), 
            [ 253, 29, 10, 5, 3, 2, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0 ] ), 
          group := "$M_{23}$", rank := 3, subgroup := "$2^4:A_7$" ) ]

So the Mathieu group M23 contains two classes of maximal subgroups, of the structures L3(4).22 and 24:A7, respectively, such that the characters of the permutation representations on the cosets of these subgroups are equal.
Furthermore, it is a consequence of the classification in [BL96] that in all cases except this one, the isomorphism types of the point stabilizers are uniquely determined by the permutation characters.
    gap> ambiginfo:= info{ List( ambiguous, x -> x[1] ) };;
    gap> for pair in ambiginfo do
    >      Print( pair.group, ", ", pair.subgroup, ", ", pair.ATLAS, "\n" );
    > od;
    $M_{12}$, $A_6.2_1 \leq A_6.2^2$, 1a+11ab+54a+55a
    $M_{22}$, $A_7$, 1a+21a+154a
    $HS$, $U_3(5).2$, 1a+175a
    $McL$, $M_{22}$, 1a+22a+252a+1750a
    $Fi_{22}$, $O_7(3)$, 1a+429a+13650a

In the other five cases of ambiguities, the whole info records are equal, and from the above list we conclude that for each pair, the point stabilizers are isomorphic. In fact the subgroups are conjugate in the outer automorphism groups of the simple groups involved.
Next let us look at the distribution of ranks.
    gap> Collected( List( info, x -> x.rank ) );
    [ [ 2, 11 ], [ 3, 31 ], [ 4, 25 ], [ 5, 43 ], [ 6, 24 ], [ 7, 21 ], 
      [ 8, 26 ], [ 9, 16 ], [ 10, 17 ], [ 11, 9 ], [ 12, 9 ], [ 13, 8 ], 
      [ 14, 4 ], [ 15, 3 ], [ 16, 3 ], [ 17, 5 ], [ 18, 5 ], [ 19, 2 ], 
      [ 20, 2 ], [ 23, 1 ], [ 26, 1 ], [ 34, 1 ] ]
    gap> max:= Filtered( info, x -> x.rank = 34 );;
    gap> max[1].group;  max[1].subgroup;  max[1].character[1]; 
    "$F_{3+}.2$"
    "$O_{10}^-(2) \\leq O_{10}^-(2).2$"
    100354720284

The maximal rank, 34, is attained for a degree 100 354 720 284 character of F3+.2 = Fi24.
For the nonsimple automorphism groups of sporadic simple groups, the simple group G involved is of index 2, and each permutation characters either is induced from a character of G or extends a permutation character of G.
    gap> nonsimple:= Filtered( info,
    >        x -> not IsSimple( UnderlyingCharacterTable( x.character ) ) );;
    gap> Length( nonsimple );
    120
    gap> ind:= Filtered( nonsimple, x -> ScalarProduct( x.character,
    >              Irr( UnderlyingCharacterTable( x.character ) )[2] ) = 1 );;
    gap> Length( ind );
    48

There are exactly 120 multiplicity-free permutation characters of nonsimple automorphism groups of sporadic simple groups, and 48 of them are induced from characters of the simple groups. (Note that the second irreducible character of the GAP character tables in question is the unique nontrivial linear character.)
    gap> ind[1];
    rec( ATLAS := "1a^{\\pm}+11ab", 
      character := Character( CharacterTable( "M12.2" ), 
        [ 24, 0, 8, 6, 0, 4, 4, 0, 2, 2, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] ), 
      group := "$M_{12}.2$", rank := 3, subgroup := "$M_{11}$" )
    gap> ForAll( ind, x -> x.ATLAS{ [ 1 .. 8 ] } = "1a^{\\pm}" );
    true

Another possibility to select the induced characters is to check whether the initial part of the ATLAS component is the string "1a^{\\pm}".

3  Using the Functions to Compute Multiplicity-Free Permutation Characters

The functions MultFreeFromTomAndTable and MultFree will be used later on.
(The functions can also be found in the file multfree.g, which can be downloaded from the same webpage where also this file can be found.)
For a character table tbl for which the table of marks is available in the GAP library, the function MultFreeFromTomAndTable returns the list of all multiplicity-free permutation characters of tbl.
    gap> BindGlobal( "MultFreeFromTomAndTable", function( tbl )
    >     local tom,     # the table of marks
    >           fus,     # fusion map from `t' to `tom'
    >           perms;   # perm. characters of `t'
    > 
    >     if HasFusionToTom( tbl ) or HasUnderlyingGroup( tbl ) then
    >       tom:= TableOfMarks( tbl );
    >     else
    >       Error( "no table of marks for character table <tbl> available" );
    >     fi;
    >     fus:= FusionCharTableTom( tbl, tom );
    >     if fus = fail then
    >       Error( "no unique fusion from <tbl> to the table of marks" );
    >     fi;
    >     perms:= PermCharsTom( tbl, tom );
    >     return Filtered( perms,
    >                x -> ForAll( Irr( tbl ),
    >                             y -> ScalarProduct( tbl, x, y ) <= 1 ) );
    >     end );

TestPerm calls the GAP library functions TestPerm1, TestPerm2, and TestPerm3; the return value is true if the argument pi is a possible permutation character of the character table tbl, and false otherwise.
    gap> BindGlobal( "TestPerm", function( tbl, pi )
    >     return     TestPerm1( tbl, pi ) = 0
    >            and TestPerm2( tbl, pi ) = 0
    >            and not IsEmpty( TestPerm3( tbl, [ pi ] ) );
    >     end );

Let H be a character table, S be a list of characters of H, psi a character of H, scprS a matrix, the i-th entry being the coefficients of the decomposition of the induced character of S[i] to a supergroup G, say, of H, scprpsi the decomposition of psi induced to G, and k a positive integer.
CharactersInducingWithBoundedMultiplicity returns the list C( S, psi, k ); this is the list of all those characters psi + ϑ of multiplicity at most k such that all constituents of ϑ are contained in S.
    gap> DeclareGlobalFunction( "CharactersInducingWithBoundedMultiplicity" );
    
    gap> InstallGlobalFunction( CharactersInducingWithBoundedMultiplicity,
    >     function( H, S, psi, scprS, scprpsi, k )
    >     local result,       # the list $S( .. )$
    >           chi,          # $\chi$
    >           scprchi,      # decomposition of $\chi^G$
    >           i,            # loop from `1' to `k'
    >           allowed,      # indices of possible constituents
    >           Sprime,       # $S^{\prime}_i$
    >           scprSprime;   # decomposition of characters in $S^{\prime}_i$,
    >                         # induced to $G$
    > 
    >     if IsEmpty( S ) then
    > 
    >       # Test whether `psi' is a possible permutation character.
    >       if TestPerm( H, psi ) then
    >         result:= [ psi ];
    >       else
    >         result:= [];
    >       fi;
    > 
    >     else
    > 
    >       # Fix a character $\chi$.
    >       chi     := S[1];
    >       scprchi := scprS[1];
    > 
    >       # Form the union.
    >       result:= CharactersInducingWithBoundedMultiplicity( H,
    >                    S{ [ 2 .. Length( S ) ] }, psi,
    >                    scprS{ [ 2 .. Length( S ) ] }, scprpsi, k );
    >       for i in [ 1 .. k ] do
    >         allowed    := Filtered( [ 2 .. Length( S ) ],
    >                           j -> Maximum( i * scprchi + scprS[j] ) <= k );
    >         Sprime     := S{ allowed };
    >         scprSprime := scprS{ allowed };
    > 
    >         Append( result, CharactersInducingWithBoundedMultiplicity( H,
    >                             Sprime, psi + i * chi,
    >                             scprSprime, scprpsi + i * scprchi, k ) );
    >       od;
    > 
    >     fi;
    > 
    >     return result;
    >     end );

Let G and H be character tables of groups G and H, respectively, such that H is a subgroup of G and the class fusion from H to G is stored on H. MultAtMost returns the list of all characters ϕG of G of multiplicity at most k such that ϕ is a possible permutation character of H.
    gap> BindGlobal( "MultAtMost", function( G, H, k )
    >     local triv,     # $1_H$
    >           permch,   # $(1_H)^G$
    >           scpr1H,   # decomposition of $(1_H)^G$
    >           rat,      # rational irreducible characters of $H$
    >           ind,      # induced rational irreducible characters
    >           mat,      # decomposition of `ind'
    >           allowed,  # indices of possible constituents
    >           S0,       # $S_0$
    >           scprS0,   # decomposition of characters in $S_0$,
    >                     # induced to $G$, with $Irr(G)$
    >           cand;     # list of multiplicity-free candidates, result
    > 
    >     # Compute $(1_H)^G$ and its decomposition into irreducibles of $G$.
    >     triv   := TrivialCharacter( H );
    >     permch := Induced( H, G, [ triv ] );
    >     scpr1H := MatScalarProducts( G, Irr( G ), permch )[1];
    > 
    >     # If $(1_H)^G$ has multiplicity larger than `k' then we are done.
    >     if Maximum( scpr1H ) > k then
    >       return [];
    >     fi;
    > 
    >     # Compute the set $S_0$ of all possible nontrivial
    >     # rational constituents of a candidate of multiplicity at most `k',
    >     # that is, all those rational irreducible characters of
    >     # $H$ that induce to $G$ with multiplicity at most `k'.
    >     rat:= RationalizedMat( Irr( H ) );
    >     ind:= Induced( H, G, rat );
    >     mat:= MatScalarProducts( G, Irr( G ), ind );
    >     allowed:= Filtered( [ 1.. Length( mat ) ],
    >                         x -> Maximum( mat[x] + scpr1H ) <= k );
    >     S0     := rat{ allowed };
    >     scprS0 := mat{ allowed };
    > 
    >     # Compute $C( S_0, 1_H, k )$.
    >     cand:= CharactersInducingWithBoundedMultiplicity( H,
    >                S0, triv, scprS0, scpr1H, k );
    > 
    >     # Induce the candidates to $G$, and return the sorted list.
    >     cand:= Induced( H, G, cand );
    >     Sort( cand );
    >     return cand;
    >     end );

MultFree returns MultAtMost( G, H, 1 ).
    gap> BindGlobal( "MultFree", function( G, H )
    >     return MultAtMost( G, H, 1 );
    >     end );

Let tbl be a character table with known Maxes value, and k a positive integer. The function PossiblePermutationCharactersWithBoundedMultiplicity returns a record with the following components.
    gap> BindGlobal( "PossiblePermutationCharactersWithBoundedMultiplicity",
    >     function( tbl, k )
    >     local permcand, # list of all mult. free perm. character candidates
    >           maxname,  # loop over tables of maximal subgroups
    >           max;      # one table of a maximal subgroup
    > 
    >     if not HasMaxes( tbl ) then
    >       return fail;
    >     fi;
    > 
    >     permcand:= [];
    > 
    >     # Loop over the tables of maximal subgroups.
    >     for maxname in Maxes( tbl ) do
    > 
    >       max:= CharacterTable( maxname );
    >       if max = fail or GetFusionMap( max, tbl ) = fail then
    > 
    >         Print( "#E  no fusion `", maxname, "' -> `", Identifier( tbl ),
    >                "' stored\n" );
    >         Add( permcand, Unknown() );
    > 
    >       else
    > 
    >         # Compute the possible perm. characters inducing through `max'.
    >         Add( permcand, MultAtMost( tbl, max, k ) );
    > 
    >       fi;
    >     od;
    > 
    >     # Return the result record.
    >     return rec( identifier := Identifier( tbl ),
    >                 maxnames   := Maxes( tbl ),
    >                 permcand   := permcand,
    >                 k          := k );
    >     end );

3.1  Using Tables of Marks

As a small example for the computation of multiplicity-free permutation characters from the table of marks of a group, we consider the alternating group A5. Its character table as well as its table of marks are accessible from the respective GAP library, via the identifier A5.
    gap> tbl:= CharacterTable( "A5" );;
    gap> chars:= MultFreeFromTomAndTable( tbl );
    [ Character( CharacterTable( "A5" ), [ 12, 0, 0, 2, 2 ] ), 
      Character( CharacterTable( "A5" ), [ 10, 2, 1, 0, 0 ] ), 
      Character( CharacterTable( "A5" ), [ 6, 2, 0, 1, 1 ] ), 
      Character( CharacterTable( "A5" ), [ 5, 1, 2, 0, 0 ] ), 
      Character( CharacterTable( "A5" ), [ 1, 1, 1, 1, 1 ] ) ]

As the GAP databases do not provide information about the isomorphism types of arbitrary subgroups, there is no way to compute automatically the subgroup strings as contained in the database of multiplicity-free permutation characters (cf. Section 1). Of course it is easy to see that the above characters of A5 are induced from the trivial characters of the cyclic group of order 5, the dihedral groups of orders 6 and 10, the alternating group A4, and the group A5 itself, respectively.
The ATLAS information used in the database records can be computed using the GAP function PermCharInfo.
    gap> PermCharInfo( tbl, chars ).ATLAS;
    [ "1a+3ab+5a", "1a+4a+5a", "1a+5a", "1a+4a", "1a" ]

As an example for a nonsimple group, we repeat the computation of all multiplicity-free permutation characters of M12.2, using the GAP table of marks.
    gap> tbl:= CharacterTable( "M12.2" );;
    gap> chars:= MultFreeFromTomAndTable( tbl );;
    gap> lib:= MultFreePermChars( "M12.2" );;
    gap> Length( lib );  Length( chars );
    13
    15
    gap> Difference( chars, List( lib, x -> x.character ) );
    [ Character( CharacterTable( "M12.2" ), [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 
          1, 1, 1, 1, 1, 1, 1, 1, 1 ] ), Character( CharacterTable( "M12.2" ), 
        [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] ) ]

This confirms the classification for M12.2, since the additional characters found from the table of marks are not faithful.
The corresponding ATLAS information is computed using the GAP function PermCharInfoRelative, since the constituents shall be listed relative to the simple group M12.
    gap> tblsimple:= CharacterTable( "M12" );;
    gap> PermCharInfoRelative( tblsimple, tbl, chars ).ATLAS;
    [ "1a^++16ab+45a^-+54a^{\\pm}+55a^{\\pm}bc+66a^++99a^{\\pm}+144a^++176a^+", 
      "1a^++11ab+45a^-+54a^{\\pm}+55a^++66a^{\\pm}+99a^-+120a^{\\pm}+144a^{\\pm}",
      "1a^{\\pm}+11ab+45a^{\\pm}+54a^{\\pm}+55a^{\\pm}bc+99a^{\\pm}+120a^{\\pm}", 
      "1a^++16ab+45a^++54a^{\\pm}+55a^-+66a^{\\pm}+99a^-+144a^++176a^-", 
      "1a^++16ab+45a^-+54a^{\\pm}+66a^++99a^-+144a^+", 
      "1a^++11ab+54a^{\\pm}+55a^++66a^++99a^-+144a^+", 
      "1a^{\\pm}+11ab+54a^{\\pm}+55a^{\\pm}+99a^{\\pm}", 
      "1a^++16ab+45a^++54a^{\\pm}+66a^++144a^+", 
      "1a^{\\pm}+11ab+54a^{\\pm}+66a^{\\pm}", "1a^++16ab+45a^++66a^+", 
      "1a^++11ab+55a^++66a^+", "1a^{\\pm}+11ab+54a^{\\pm}", "1a^{\\pm}+11ab", 
      "1a^{\\pm}", "1a^+" ]

For more information about tables of marks, see [Pfe97].

3.2  Dealing with Possible Permutation Characters

In this section, we deal with possible permutation characters, that is, characters that have certain properties of permutation characters but for which no subgroups need to exist from whose trivial characters they are induced. For more information about such characters, see the section "Possible Permutation Characters" in the GAP Reference Manual, the paper [BP98], and the note [Bre].
We can compute possible permutation characters from the character table of the group in question, the table of marks need not be available. The problem is of course that for classifying the permutation characters, we have to decide which of the candidates are in fact permutation characters.
Here we show only two small examples that could also be handled via tables of marks. (The GAP code shown uses only standard functions lists, such as List, Filtered, and ForAll, and functions for character tables, such as Irr and ScalarProduct; if you are not familiar with these functions, consult the corresponding sections in the GAP Reference Manual.)
The first example is the Mathieu group M11 that has been inspected already in Section 1.1. This group is small enough for the computation of all possible permutation characters, and then filtering out the multiplicity-free ones.
    gap> tbl:= CharacterTable( "M11" );;
    gap> perms:= PermChars( tbl );;
    gap> multfree:= Filtered( perms,
    >        x -> ForAll( Irr( tbl ), chi -> ScalarProduct( chi, x ) <= 1 ) );
    [ Character( CharacterTable( "M11" ), [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ] ), 
      Character( CharacterTable( "M11" ), [ 11, 3, 2, 3, 1, 0, 1, 1, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 12, 4, 3, 0, 2, 1, 0, 0, 1, 1 ] ), 
      Character( CharacterTable( "M11" ), [ 22, 6, 4, 2, 2, 0, 0, 0, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 55, 7, 1, 3, 0, 1, 1, 1, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 66, 10, 3, 2, 1, 1, 0, 0, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 144, 0, 0, 0, 4, 0, 0, 0, 1, 1 ] ) ]
    gap> Length( multfree );
    8

Comparing this list with the seven faithful multiplicity-free permutation characters of M11 shown in Section 1.1, we see that all candidates are in fact permutation characters. Without this information, we have to show, for each candidate, the existence of a subgroup that serves as the point stabilizer.
Additionally, if we are interested in the subgroup information contained in the database (cf. the subgroup components of the info records in Section 1), we want to relate the point stabilizers to the maximal subgroups of M11.
In the case of the sporadic simple groups and their automorphism groups, we can use the fact that for many of these groups, the character tables of all maximal subgroups and the class fusions of these tables are known. Since each multiplicity-free permutation character of a group is either trivial or induced from a multiplicity-free permutation character of a maximal subgroup, we can thus reduce our problem to the computation of multiplicity-free possible permutation characters of all maximal subgroups. (That this really is a reduction can be read in [BL96].) This approach is implemented in the function MultFree.
    gap> tbl:= CharacterTable( "M11" );
    CharacterTable( "M11" )
    gap> maxes:= Maxes( tbl );
    [ "A6.2_3", "L2(11)", "3^2:Q8.2", "A5.2", "2.S4" ]
    gap> name:= maxes[1];;
    gap> MultFree( tbl, CharacterTable( name ) );
    [ Character( CharacterTable( "M11" ), [ 11, 3, 2, 3, 1, 0, 1, 1, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 22, 6, 4, 2, 2, 0, 0, 0, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ) ]

The function MultFree computes all multiplicity-free characters of the given character table that are induced from possible permutation characters of the given character table of a subgroup. (Note that these characters need not necessarily be faithful.) If we loop over all classes of maximal subgroups then we get all candidates for M11, together with the information in which maximal subgroup the hypothetical point stabilizer lies.
    gap> cand:= [];;
    gap> for name in maxes do
    >      max:= CharacterTable( name );
    >      Append( cand, List( MultFree( tbl, max ),
    >                     chi -> [ name, Size( tbl ) / Size( max ), chi ] ) );
    > od;
    gap> cand;
    [ [ "A6.2_3", 11, Character( CharacterTable( "M11" ), 
            [ 11, 3, 2, 3, 1, 0, 1, 1, 0, 0 ] ) ], 
      [ "A6.2_3", 11, Character( CharacterTable( "M11" ), 
            [ 22, 6, 4, 2, 2, 0, 0, 0, 0, 0 ] ) ], 
      [ "A6.2_3", 11, Character( CharacterTable( "M11" ), 
            [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ) ], 
      [ "L2(11)", 12, Character( CharacterTable( "M11" ), 
            [ 12, 4, 3, 0, 2, 1, 0, 0, 1, 1 ] ) ], 
      [ "L2(11)", 12, Character( CharacterTable( "M11" ), 
            [ 144, 0, 0, 0, 4, 0, 0, 0, 1, 1 ] ) ], 
      [ "3^2:Q8.2", 55, Character( CharacterTable( "M11" ), 
            [ 55, 7, 1, 3, 0, 1, 1, 1, 0, 0 ] ) ], 
      [ "3^2:Q8.2", 55, Character( CharacterTable( "M11" ), 
            [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ) ], 
      [ "A5.2", 66, Character( CharacterTable( "M11" ), 
            [ 66, 10, 3, 2, 1, 1, 0, 0, 0, 0 ] ) ] ]
    gap> Length( cand );  Length( Set( cand, x -> x[3] ) );
    8
    7

We immediately see that the candidates of degrees 11, 12, 55, and 66 are permutation characters, since they are obtained by inducing the trivial characters of the maximal subgroups. The permutation characters of degrees 22 and 144 can be established in two steps. First we note that the group A6.23 contains the subgroup A6 of index 2, and the group L2(11) contains a class of subgroups of index 12, of isomorphism type 11:5. Second the possible permutation characters of degrees 2 and 12 of these maximal subgroups of M11 are uniquely determined, and inducing these characters to M11 yields in fact multiplicity-free characters.
    gap> max1:= CharacterTable( maxes[1] );;
    gap> perms1:= PermChars( max1, [ 2 ] );
    [ Character( CharacterTable( "A6.2_3" ), [ 2, 2, 2, 2, 2, 0, 0, 0 ] ) ]
    gap> perms1[1]^tbl = cand[2][3];
    true
    gap> max2:= CharacterTable( maxes[2] );;
    gap> perms2:= PermChars( max2, [ 12 ] );
    [ Character( CharacterTable( "L2(11)" ), [ 12, 0, 0, 2, 2, 0, 1, 1 ] ) ]
    gap> perms2[1]^tbl = cand[5][3];
    true

The last candidate to deal with is the degree 110 character, which might be induced from a subgroup of A6.23 or 32:Q8.2 or both. Let us first look at the possible permutation characters of degree 10 of A6.23.
    gap> PermChars( max1, [ 10 ] );
    [ Character( CharacterTable( "A6.2_3" ), [ 10, 2, 1, 2, 0, 0, 2, 2 ] ), 
      Character( CharacterTable( "A6.2_3" ), [ 10, 2, 1, 2, 0, 2, 0, 0 ] ) ]
    gap> OrdersClassRepresentatives( max1 );
    [ 1, 2, 3, 4, 5, 4, 8, 8 ]

There are two possibilities, and only the first induces the candidate of degree 110. The latter follows from the fact that the nonzero character value of the candidate on classes of element order 8 means that the hypothetical point stabilizer contains elements of order 8, cf. the Display call in Section 1.1.
The group A6.23 has a unique class of subgroups of index 10, which are the Sylow 3 normalizers, of type 32:Q8. Since Q8 has no elements of order 8, the first candidate is not a permutation character.
The remaining subgroup from which the degree 110 character can be induced is 32:Q8.2; this group has three index 2 subgroups, and the candidate is in fact induced from the trivial character of one of these subgroups.
    gap> max3:= CharacterTable( maxes[3] );;
    gap> classes:= SizesConjugacyClasses( max3 );;
    gap> Filtered( ClassPositionsOfNormalSubgroups( max3 ),
    >              x -> Sum( classes{ x } ) = Size( max3 ) / 2 );
    [ [ 1, 2, 4, 5, 6 ], [ 1, 2, 3, 4, 5, 7 ], [ 1, 2, 4, 5, 8, 9 ] ]
    gap> perms3:= PermChars( max3, [ 2 ] );
    [ Character( CharacterTable( "3^2:Q8.2" ), [ 2, 2, 0, 2, 2, 0, 0, 2, 2 ] ), 
      Character( CharacterTable( "3^2:Q8.2" ), [ 2, 2, 0, 2, 2, 2, 0, 0, 0 ] ), 
      Character( CharacterTable( "3^2:Q8.2" ), [ 2, 2, 2, 2, 2, 0, 2, 0, 0 ] ) ]
    gap> induced:= List( perms3, x -> x^tbl );
    [ Character( CharacterTable( "M11" ), [ 110, 6, 2, 2, 0, 0, 2, 2, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 110, 6, 2, 6, 0, 0, 0, 0, 0, 0 ] ), 
      Character( CharacterTable( "M11" ), [ 110, 14, 2, 2, 0, 2, 0, 0, 0, 0 ] ) ]
    gap> Position( induced, cand[3][3] );
    1

Putting these considerations together, we thus get a confirmation of the classification for M11.
As a second example, we look at the group M12.2. The database contains 13 characters, and the approach using MultFree yields 17 different characters. We are interested in disproving the candidates that are not permutation characters.
    gap> info:= MultFreePermChars( "M12.2" );;
    gap> perms:= Set( List( info, x -> x.character ) );;
    gap> Length( info );  Length( perms );
    13
    13
    gap> tbl:= CharacterTable( "M12.2" );;
    gap> maxes:= Maxes( tbl );
    [ "M12", "L2(11).2", "M12.2M3", "(2^2xA5):2", "D8.(S4x2)", "4^2:D12.2", 
      "3^(1+2):D8", "S4xS3", "A5.2" ]
    gap> cand:= [];;
    gap> for name in maxes do
    >      max:= CharacterTable( name );
    >      Append( cand, List( MultFree( tbl, max ),
    >                     chi -> [ name, Size( tbl ) / Size( max ), chi ] ) );
    > od;
    gap> Length( cand );  Length( Set( List( cand, x -> x[3] ) ) );
    25
    17
    gap> toexclude:= Set( Filtered( cand, x -> not x[3] in perms ) );
    [ [ "M12", 2, Character( CharacterTable( "M12.2" ), 
            [ 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] ) ], 
      [ "M12", 2, Character( CharacterTable( "M12.2" ), 
            [ 440, 0, 24, 8, 8, 8, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 
             ] ) ], 
      [ "M12", 2, Character( CharacterTable( "M12.2" ),
            [ 1320, 0, 8, 6, 0, 8, 0, 0, 2, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
             ] ) ],
      [ "M12", 2, Character( CharacterTable( "M12.2" ),
            [ 1320, 0, 24, 6, 0, 4, 0, 0, 6, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
             ] ) ] ]

Clearly the degree 2 character is a permutation character, but as it is not faithful, it is not contained in the database.
The other three characters are all induced from candidates of the maximal subgroup M12, and we may use the same approach for M12 in order to find out whether they can be permutation characters.
    gap> m12:= CharacterTable( "M12" );;
    gap> subcand:= [];;
    gap> submaxes:= Maxes( m12 );
    [ "M11", "M12M2", "A6.2^2", "M12M4", "L2(11)", "3^2.2.S4", "M12M7", "2xS5", 
      "M8.S4", "4^2:D12", "A4xS3" ]
    gap> for name in submaxes do
    >      max:= CharacterTable( name );
    >      Append( subcand, MultFree( m12, max ) );
    > od;
    gap> induced:= List( subcand, x -> x^tbl );;
    gap> Intersection( induced, List( toexclude, x -> x[3] ) );
    [  ]

Thus none of the candidates in the list toexclude is a permutation character.

References

[BL96]
T. Breuer and K. Lux, The multiplicity-free permutation characters of the sporadic simple groups and their automorphism groups, Comm. Algebra 24 (1996), no. 7, 2293-2316. MR 1390375 (97c:20020)
[BP98]
T. Breuer and G. Pfeiffer, Finding possible permutation characters, J. Symbolic Comput. 26 (1998), no. 3, 343-354. MR 1633876 (99e:20005)
[Bre]
T. Breuer, Permutation Characters in GAP, http://www.math.rwth-aachen.de/ ~Thomas.Breuer/ ctbllib/doc/ ctblpope.pdf.
[Bre12]


, The GAP Character Table Library, Version 1.2, http://www.math.rwth-aachen.de/ ~Thomas.Breuer/ ctbllib, Mar 2012, GAP package.
[CCN+85]
J. H. Conway, R. T. Curtis, S. P. Norton, R. A. Parker, and R. A. Wilson, Atlas of finite groups, Oxford University Press, Eynsham, 1985, Maximal subgroups and ordinary characters for simple groups, With computational assistance from J. G. Thackray. MR 827219 (88g:20025)
[GAP04]
The GAP Group, GAP-Groups, Algorithms, and Programming, Version 4.4, 2004, http://www.gap-system.org.
[Pfe97]
G. Pfeiffer, The subgroups of M24, or how to compute the table of marks of a finite group, Experiment. Math. 6 (1997), no. 3, 247-270. MR 1481593 (98h:20032)



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