2 Tutorial for the **AtlasRep** Package

2.4 Examples of Using the **AtlasRep** Package

2.4-1 Example: Class Representatives

2.4-2 Example: Permutation and Matrix Representations

2.4-3 Example: Outer Automorphisms

2.4-4 Example: Using Semi-presentations and Black Box Programs

2.4-5 Example: Using the**GAP** Library of Tables of Marks

2.4-6 Example: Index 770 Subgroups in M_22

2.4-7 Example: Index 462 Subgroups in M_22

2.4-1 Example: Class Representatives

2.4-2 Example: Permutation and Matrix Representations

2.4-3 Example: Outer Automorphisms

2.4-4 Example: Using Semi-presentations and Black Box Programs

2.4-5 Example: Using the

2.4-6 Example: Index 770 Subgroups in M_22

2.4-7 Example: Index 462 Subgroups in M_22

This chapter gives an overview of the basic functionality provided by the **AtlasRep** package. The main concepts and interface functions are presented in the first sections, and Section 2.4 shows a few small examples.

The **AtlasRep** package gives access to a database, the **ATLAS** of Group Representations [ATLAS], that contains generators and related data for several groups, mainly for extensions of simple groups (see Section 2.1-1) and for their maximal subgroups (see Section 2.1-2).

Note that the data are not part of the package. They are fetched from a web server as soon as they are needed for the first time, see Section 4.3-1.

First of all, we load the **AtlasRep** package. Some of the examples require also the **GAP** packages **CTblLib** and **TomLib**, so we load also these packages.

gap> LoadPackage( "AtlasRep" ); true gap> LoadPackage( "CTblLib" ); true gap> LoadPackage( "TomLib" ); true

Each group that occurs in this database is specified by a *name*, which is a string similar to the name used in the **ATLAS** of Finite Groups [CCNPW85]. For those groups whose character tables are contained in the **GAP** Character Table Library [Bre13], the names are equal to the `Identifier`

(Reference: Identifier (for character tables)) values of these character tables. Examples of such names are `"M24"`

for the Mathieu group M_24, `"2.A6"`

for the double cover of the alternating group A_6, and `"2.A6.2_1"`

for the double cover of the symmetric group S_6. The names that actually occur are listed in the first column of the overview table that is printed by the function `DisplayAtlasInfo`

(3.5-1), called without arguments, see below. The other columns of the table describe the data that are available in the database.

For example, `DisplayAtlasInfo`

(3.5-1) may print the following lines. Omissions are indicated with "`...`

".

gap> DisplayAtlasInfo(); group | # | maxes | cl | cyc | out | fnd | chk | prs -------------------------+----+-------+----+-----+-----+-----+-----+---- ... 2.A5 | 26 | 3 | | | | | + | + 2.A5.2 | 11 | 4 | | | | | + | + 2.A6 | 18 | 5 | | | | | | 2.A6.2_1 | 3 | 6 | | | | | | 2.A7 | 24 | | | | | | | 2.A7.2 | 7 | | | | | | | ... M22 | 58 | 8 | + | + | | + | + | + M22.2 | 46 | 7 | + | + | | + | + | + M23 | 66 | 7 | + | + | | + | + | + M24 | 62 | 9 | + | + | | + | + | + McL | 46 | 12 | + | + | | + | + | + McL.2 | 27 | 10 | | + | | + | + | + O7(3) | 28 | | | | | | | O7(3).2 | 3 | | | | | | | ...

Called with a group name as the only argument, the function `AtlasGroup`

(3.5-7) returns a group isomorphic to the group with the given name. If permutation generators are available in the database then a permutation group (of smallest available degree) is returned, otherwise a matrix group.

gap> g:= AtlasGroup( "M24" ); Group([ (1,4)(2,7)(3,17)(5,13)(6,9)(8,15)(10,19)(11,18)(12,21)(14,16) (20,24)(22,23), (1,4,6)(2,21,14)(3,9,15)(5,18,10)(13,17,16) (19,24,23) ]) gap> IsPermGroup( g ); NrMovedPoints( g ); Size( g ); true 24 244823040

Many maximal subgroups of extensions of simple groups can be constructed using the function `AtlasSubgroup`

(3.5-8). Given the name of the extension of the simple group and the number of the conjugacy class of maximal subgroups, this function returns a representative from this class.

gap> g:= AtlasSubgroup( "M24", 1 ); Group([ (2,10)(3,12)(4,14)(6,9)(8,16)(15,18)(20,22)(21,24), (1,7,2,9) (3,22,10,23)(4,19,8,12)(5,14)(6,18)(13,16,17,24) ]) gap> IsPermGroup( g ); NrMovedPoints( g ); Size( g ); true 23 10200960

The classes of maximal subgroups are ordered w. r. t. decreasing subgroup order. So the first class contains the largest maximal subgroups.

Note that groups obtained by `AtlasSubgroup`

(3.5-8) may be not very suitable for computations in the sense that much nicer representations exist. For example, the sporadic simple O'Nan group O'N contains a maximal subgroup S isomorphic with the Janko group J_1; the smallest permutation representation of O'N has degree 122760, so restricting this representation to S yields a representation of J_1 of that degree. However, J_1 has a faithful permutation representation of degree 266, which admits much more efficient computations. If you are just interested in J_1 and not in its embedding into O'N then one possibility to get a "nicer" faithful representation is to call `SmallerDegreePermutationRepresentation`

(Reference: SmallerDegreePermutationRepresentation). In the abovementioned example, this works quite well; note that in general, we cannot expect that we get a representation of smallest degree in this way.

gap> s:= AtlasSubgroup( "ON", 3 ); <permutation group of size 175560 with 2 generators> gap> NrMovedPoints( s ); Size( s ); 122760 175560 gap> hom:= SmallerDegreePermutationRepresentation( s );; gap> NrMovedPoints( Image( hom ) ); 1540

In this particular case, one could of course also ask directly for the group J_1.

gap> j1:= AtlasGroup( "J1" ); <permutation group of size 175560 with 2 generators> gap> NrMovedPoints( j1 ); 266

If you have a group G, say, and you are really interested in the embedding of a maximal subgroup of G into G then an easy way to get compatible generators is to create G with `AtlasGroup`

(3.5-7) and then to call `AtlasSubgroup`

(3.5-8) with first argument the group G.

gap> g:= AtlasGroup( "ON" ); <permutation group of size 460815505920 with 2 generators> gap> s:= AtlasSubgroup( g, 3 ); <permutation group of size 175560 with 2 generators> gap> IsSubset( g, s ); true gap> IsSubset( g, j1 ); false

The function `DisplayAtlasInfo`

(3.5-1), called with an admissible name of a group as the only argument, lists the **ATLAS** data available for this group.

gap> DisplayAtlasInfo( "A5" ); Representations for G = A5: (all refer to std. generators 1) --------------------------- 1: G <= Sym(5) 3-trans., on cosets of A4 (1st max.) 2: G <= Sym(6) 2-trans., on cosets of D10 (2nd max.) 3: G <= Sym(10) rank 3, on cosets of S3 (3rd max.) 4: G <= GL(4a,2) 5: G <= GL(4b,2) 6: G <= GL(4,3) 7: G <= GL(6,3) 8: G <= GL(2a,4) 9: G <= GL(2b,4) 10: G <= GL(3,5) 11: G <= GL(5,5) 12: G <= GL(3a,9) 13: G <= GL(3b,9) 14: G <= GL(4,Z) 15: G <= GL(5,Z) 16: G <= GL(6,Z) 17: G <= GL(3a,Field([Sqrt(5)])) 18: G <= GL(3b,Field([Sqrt(5)])) Programs for G = A5: (all refer to std. generators 1) -------------------- presentation std. gen. checker maxes (all 3): 1: A4 2: D10 3: S3

In order to fetch one of the listed permutation groups or matrix groups, you can call `AtlasGroup`

(3.5-7) with second argument the function `Position`

(Reference: Position) and third argument the position in the list.

gap> AtlasGroup( "A5", Position, 1 ); Group([ (1,2)(3,4), (1,3,5) ])

Note that this approach may yield a different group after an update of the database, if new data for the group become available.

Alternatively, you can describe the desired group by conditions, such as the degree in the case of a permutation group, and the dimension and the base ring in the case of a matrix group.

gap> AtlasGroup( "A5", NrMovedPoints, 10 ); Group([ (2,4)(3,5)(6,8)(7,10), (1,2,3)(4,6,7)(5,8,9) ]) gap> AtlasGroup( "A5", Dimension, 4, Ring, GF(2) ); <matrix group of size 60 with 2 generators>

The same holds for the restriction to maximal subgroups: Use `AtlasSubgroup`

(3.5-8) with the same arguments as `AtlasGroup`

(3.5-7), except that additionally the number of the class of maximal subgroups is entered as the last argument. Note that the conditions refer to the group, not to the subgroup; it may happen that the subgroup moves fewer points than the big group.

gap> AtlasSubgroup( "A5", Dimension, 4, Ring, GF(2), 1 ); <matrix group of size 12 with 2 generators> gap> g:= AtlasSubgroup( "A5", NrMovedPoints, 10, 3 ); Group([ (2,4)(3,5)(6,8)(7,10), (1,4)(3,8)(5,7)(6,10) ]) gap> Size( g ); NrMovedPoints( g ); 6 9

Up to now, we have talked only about groups and subgroups. The **AtlasRep** package provides access to *group generators*, and in fact these generators have the property that mapping one set of generators to another set of generators for the same group defines an isomorphism. These generators are called *standard generators*, see Section 3.3.

So instead of thinking about several generating sets of a group G, say, we can think about one abstract group G, with one fixed set of generators, and mapping these generators to any set of generators provided by **AtlasRep** defines a representation of G. This viewpoint motivates the name "**ATLAS** of Group Representations" for the database.

If you are interested in the generators provided by the database rather than in the groups they generate, you can use the function `OneAtlasGeneratingSetInfo`

(3.5-5) instead of `AtlasGroup`

(3.5-7), with the same arguments. This will yield a record that describes the representation in question. Calling the function `AtlasGenerators`

(3.5-2) with this record will then yield a record with the additional component `generators`

, which holds the list of generators.

gap> info:= OneAtlasGeneratingSetInfo( "A5", NrMovedPoints, 10 ); rec( groupname := "A5", id := "", identifier := [ "A5", [ "A5G1-p10B0.m1", "A5G1-p10B0.m2" ], 1, 10 ], isPrimitive := true, maxnr := 3, p := 10, rankAction := 3, repname := "A5G1-p10B0", repnr := 3, size := 60, stabilizer := "S3", standardization := 1, transitivity := 1, type := "perm" ) gap> info2:= AtlasGenerators( info ); rec( generators := [ (2,4)(3,5)(6,8)(7,10), (1,2,3)(4,6,7)(5,8,9) ], groupname := "A5", id := "", identifier := [ "A5", [ "A5G1-p10B0.m1", "A5G1-p10B0.m2" ], 1, 10 ], isPrimitive := true, maxnr := 3, p := 10, rankAction := 3, repname := "A5G1-p10B0", repnr := 3, size := 60, stabilizer := "S3", standardization := 1, transitivity := 1, type := "perm" ) gap> info2.generators; [ (2,4)(3,5)(6,8)(7,10), (1,2,3)(4,6,7)(5,8,9) ]

For computing certain group elements from standard generators, such as generators of a subgroup or class representatives, **AtlasRep** uses *straight line programs*, see Reference: Straight Line Programs. Essentially this means to evaluate words in the generators, similar to `MappedWord`

(Reference: MappedWord) but more efficiently.

It can be useful to deal with these straight line programs, see `AtlasProgram`

(3.5-3). For example, an automorphism α, say, of the group G, if available in **AtlasRep**, is given by a straight line program that defines the images of standard generators of G. This way, one can for example compute the image of a subgroup U of G under α by first applying the straight line program for α to standard generators of G, and then applying the straight line program for the restriction from G to U.

gap> prginfo:= AtlasProgramInfo( "A5", "maxes", 1 ); rec( groupname := "A5", identifier := [ "A5", "A5G1-max1W1", 1 ], size := 12, standardization := 1, subgroupname := "A4" ) gap> prg:= AtlasProgram( prginfo.identifier ); rec( groupname := "A5", identifier := [ "A5", "A5G1-max1W1", 1 ], program := <straight line program>, size := 12, standardization := 1, subgroupname := "A4" ) gap> Display( prg.program ); # input: r:= [ g1, g2 ]; # program: r[3]:= r[1]*r[2]; r[4]:= r[2]*r[1]; r[5]:= r[3]*r[3]; r[1]:= r[5]*r[4]; # return values: [ r[1], r[2] ] gap> ResultOfStraightLineProgram( prg.program, info2.generators ); [ (1,10)(2,3)(4,9)(7,8), (1,2,3)(4,6,7)(5,8,9) ]

First we show the computation of class representatives of the Mathieu group M_11, in a 2-modular matrix representation. We start with the ordinary and Brauer character tables of this group.

gap> tbl:= CharacterTable( "M11" );; gap> modtbl:= tbl mod 2;; gap> CharacterDegrees( modtbl ); [ [ 1, 1 ], [ 10, 1 ], [ 16, 2 ], [ 44, 1 ] ]

The output of `CharacterDegrees`

(Reference: CharacterDegrees) means that the 2-modular irreducibles of M_11 have degrees 1, 10, 16, 16, and 44.

Using `DisplayAtlasInfo`

(3.5-1), we find out that matrix generators for the irreducible 10-dimensional representation are available in the database.

gap> DisplayAtlasInfo( "M11", Characteristic, 2 ); Representations for G = M11: (all refer to std. generators 1) ---------------------------- 6: G <= GL(10,2) character 10a 7: G <= GL(32,2) character 16ab 8: G <= GL(44,2) character 44a 16: G <= GL(16a,4) character 16a 17: G <= GL(16b,4) character 16b

So we decide to work with this representation. We fetch the generators and compute the list of class representatives of M_11 in the representation. The ordering of class representatives is the same as that in the character table of the **ATLAS** of Finite Groups ([CCNPW85]), which coincides with the ordering of columns in the **GAP** table we have fetched above.

gap> info:= OneAtlasGeneratingSetInfo( "M11", Characteristic, 2, > Dimension, 10 );; gap> gens:= AtlasGenerators( info.identifier );; gap> ccls:= AtlasProgram( "M11", gens.standardization, "classes" ); rec( groupname := "M11", identifier := [ "M11", "M11G1-cclsW1", 1 ], outputs := [ "1A", "2A", "3A", "4A", "5A", "6A", "8A", "8B", "11A", "11B" ], program := <straight line program>, standardization := 1 ) gap> reps:= ResultOfStraightLineProgram( ccls.program, gens.generators );;

If we would need only a few class representatives, we could use the **GAP** library function `RestrictOutputsOfSLP`

(Reference: RestrictOutputsOfSLP) to create a straight line program that computes only specified outputs. Here is an example where only the class representatives of order eight are computed.

gap> ord8prg:= RestrictOutputsOfSLP( ccls.program, > Filtered( [ 1 .. 10 ], i -> ccls.outputs[i][1] = '8' ) ); <straight line program> gap> ord8reps:= ResultOfStraightLineProgram( ord8prg, gens.generators );; gap> List( ord8reps, m -> Position( reps, m ) ); [ 7, 8 ]

Let us check that the class representatives have the right orders.

gap> List( reps, Order ) = OrdersClassRepresentatives( tbl ); true

From the class representatives, we can compute the Brauer character we had started with. This Brauer character is defined on all classes of the 2-modular table. So we first pick only those representatives, using the **GAP** function `GetFusionMap`

(Reference: GetFusionMap); in this situation, it returns the class fusion from the Brauer table into the ordinary table.

gap> fus:= GetFusionMap( modtbl, tbl ); [ 1, 3, 5, 9, 10 ] gap> modreps:= reps{ fus };;

Then we call the **GAP** function `BrauerCharacterValue`

(Reference: BrauerCharacterValue), which computes the Brauer character value from the matrix given.

gap> char:= List( modreps, BrauerCharacterValue ); [ 10, 1, 0, -1, -1 ] gap> Position( Irr( modtbl ), char ); 2

The second example shows the computation of a permutation representation from a matrix representation. We work with the 10-dimensional representation used above, and consider the action on the 2^10 vectors of the underlying row space.

gap> grp:= Group( gens.generators );; gap> v:= GF(2)^10;; gap> orbs:= Orbits( grp, AsList( v ) );; gap> List( orbs, Length ); [ 1, 396, 55, 330, 66, 165, 11 ]

We see that there are six nontrivial orbits, and we can compute the permutation actions on these orbits directly using `Action`

(Reference: Action homomorphisms). However, for larger examples, one cannot write down all orbits on the row space, so one has to use another strategy if one is interested in a particular orbit.

Let us assume that we are interested in the orbit of length 11. The point stabilizer is the first maximal subgroup of M_11, thus the restriction of the representation to this subgroup has a nontrivial fixed point space. This restriction can be computed using the **AtlasRep** package.

gap> gens:= AtlasGenerators( "M11", 6, 1 );;

Now computing the fixed point space is standard linear algebra.

gap> id:= IdentityMat( 10, GF(2) );; gap> sub1:= Subspace( v, NullspaceMat( gens.generators[1] - id ) );; gap> sub2:= Subspace( v, NullspaceMat( gens.generators[2] - id ) );; gap> fix:= Intersection( sub1, sub2 ); <vector space of dimension 1 over GF(2)>

The final step is of course the computation of the permutation action on the orbit.

gap> orb:= Orbit( grp, Basis( fix )[1] );; gap> act:= Action( grp, orb );; Print( act, "\n" ); Group( [ ( 1, 2)( 4, 6)( 5, 8)( 7,10), ( 1, 3, 5, 9)( 2, 4, 7,11) ] )

Note that this group is *not* equal to the group obtained by fetching the permutation representation from the database. This is due to a different numbering of the points, so the groups are permutation isomorphic.

gap> permgrp:= Group( AtlasGenerators( "M11", 1 ).generators );; gap> Print( permgrp, "\n" ); Group( [ ( 2,10)( 4,11)( 5, 7)( 8, 9), ( 1, 4, 3, 8)( 2, 5, 6, 9) ] ) gap> permgrp = act; false gap> IsConjugate( SymmetricGroup(11), permgrp, act ); true

The straight line programs for applying outer automorphisms to standard generators can of course be used to define the automorphisms themselves as **GAP** mappings.

gap> DisplayAtlasInfo( "G2(3)", IsStraightLineProgram ); Programs for G = G2(3): (all refer to std. generators 1) ----------------------- class repres. presentation repr. cyc. subg. std. gen. checker automorphisms: 2 maxes (all 10): 1: U3(3).2 2: U3(3).2 3: (3^(1+2)+x3^2):2S4 4: (3^(1+2)+x3^2):2S4 5: L3(3).2 6: L3(3).2 7: L2(8).3 8: 2^3.L3(2) 9: L2(13) 10: 2^(1+4)+:3^2.2 gap> prog:= AtlasProgram( "G2(3)", "automorphism", "2" ).program;; gap> info:= OneAtlasGeneratingSetInfo( "G2(3)", Dimension, 7 );; gap> gens:= AtlasGenerators( info ).generators;; gap> imgs:= ResultOfStraightLineProgram( prog, gens );;

If we are not suspicious whether the script really describes an automorphism then we should tell this to **GAP**, in order to avoid the expensive checks of the properties of being a homomorphism and bijective (see Section Reference: Creating Group Homomorphisms). This looks as follows.

gap> g:= Group( gens );; gap> aut:= GroupHomomorphismByImagesNC( g, g, gens, imgs );; gap> SetIsBijective( aut, true );

If we are suspicious whether the script describes an automorphism then we might have the idea to check it with **GAP**, as follows.

gap> aut:= GroupHomomorphismByImages( g, g, gens, imgs );; gap> IsBijective( aut ); true

(Note that even for a comparatively small group such as G_2(3), this was a difficult task for **GAP** before version 4.3.)

Often one can form images under an automorphism α, say, without creating the homomorphism object. This is obvious for the standard generators of the group G themselves, but also for generators of a maximal subgroup M computed from standard generators of G, provided that the straight line programs in question refer to the same standard generators. Note that the generators of M are given by evaluating words in terms of standard generators of G, and their images under α can be obtained by evaluating the same words at the images under α of the standard generators of G.

gap> max1:= AtlasProgram( "G2(3)", 1 ).program;; gap> mgens:= ResultOfStraightLineProgram( max1, gens );; gap> comp:= CompositionOfStraightLinePrograms( max1, prog );; gap> mimgs:= ResultOfStraightLineProgram( comp, gens );;

The list `mgens`

is the list of generators of the first maximal subgroup of G_2(3), `mimgs`

is the list of images under the automorphism given by the straight line program `prog`

. Note that applying the program returned by `CompositionOfStraightLinePrograms`

(Reference: CompositionOfStraightLinePrograms) means to apply first `prog`

and then `max1`

. Since we have already constructed the **GAP** object representing the automorphism, we can check whether the results are equal.

gap> mimgs = List( mgens, x -> x^aut ); true

However, it should be emphasized that using `aut`

requires a huge machinery of computations behind the scenes, whereas applying the straight line programs `prog`

and `max1`

involves only elementary operations with the generators. The latter is feasible also for larger groups, for which constructing the **GAP** automorphism might be too hard.

Let us suppose that we want to restrict a representation of the Mathieu group M_12 to a non-maximal subgroup of the type L_2(11). The idea is that this subgroup can be found as a maximal subgroup of a maximal subgroup of the type M_11, which is itself maximal in M_12. For that, we fetch a representation of M_12 and use a straight line program for restricting it to the first maximal subgroup, which has the type M_11.

gap> info:= OneAtlasGeneratingSetInfo( "M12", NrMovedPoints, 12 ); rec( charactername := "1a+11a", groupname := "M12", id := "a", identifier := [ "M12", [ "M12G1-p12aB0.m1", "M12G1-p12aB0.m2" ], 1, 12 ], isPrimitive := true, maxnr := 1, p := 12, rankAction := 2, repname := "M12G1-p12aB0", repnr := 1, size := 95040, stabilizer := "M11", standardization := 1, transitivity := 5, type := "perm" ) gap> gensM12:= AtlasGenerators( info.identifier );; gap> restM11:= AtlasProgram( "M12", "maxes", 1 );; gap> gensM11:= ResultOfStraightLineProgram( restM11.program, > gensM12.generators ); [ (3,9)(4,12)(5,10)(6,8), (1,4,11,5)(2,10,8,3) ]

Now we *cannot* simply apply a straight line program for a group to some generators, since they are not necessarily *standard* generators of the group. We check this property using a semi-presentation for M_11, see 6.1-7.

gap> checkM11:= AtlasProgram( "M11", "check" ); rec( groupname := "M11", identifier := [ "M11", "M11G1-check1", 1, 1 ] , program := <straight line decision>, standardization := 1 ) gap> ResultOfStraightLineDecision( checkM11.program, gensM11 ); true

So we are lucky that applying the appropriate program for M_11 will give us the required generators for L_2(11).

gap> restL211:= AtlasProgram( "M11", "maxes", 2 );; gap> gensL211:= ResultOfStraightLineProgram( restL211.program, gensM11 ); [ (3,9)(4,12)(5,10)(6,8), (1,11,9)(2,12,8)(3,6,10) ] gap> G:= Group( gensL211 );; Size( G ); IsSimple( G ); 660 true

Usually representations are not given in terms of standard generators. For example, let us take the M_11 type group returned by the **GAP** function `MathieuGroup`

(Reference: MathieuGroup).

gap> G:= MathieuGroup( 11 );; gap> gens:= GeneratorsOfGroup( G ); [ (1,2,3,4,5,6,7,8,9,10,11), (3,7,11,8)(4,10,5,6) ] gap> ResultOfStraightLineDecision( checkM11.program, gens ); false

If we want to compute an L_2(11) type subgroup of this group, we can use a black box program for computing standard generators, and then apply the straight line program for computing the restriction.

gap> find:= AtlasProgram( "M11", "find" ); rec( groupname := "M11", identifier := [ "M11", "M11G1-find1", 1, 1 ], program := <black box program>, standardization := 1 ) gap> stdgens:= ResultOfBBoxProgram( find.program, Group( gens ) );; gap> List( stdgens, Order ); [ 2, 4 ] gap> ResultOfStraightLineDecision( checkM11.program, stdgens ); true gap> gensL211:= ResultOfStraightLineProgram( restL211.program, stdgens );; gap> List( gensL211, Order ); [ 2, 3 ] gap> G:= Group( gensL211 );; Size( G ); IsSimple( G ); 660 true

The **GAP** Library of Tables of Marks (the **GAP** package **TomLib**, [NMP13]) provides, for many almost simple groups, information for constructing representatives of all conjugacy classes of subgroups. If this information is compatible with the standard generators of the **ATLAS** of Group Representations then we can use it to restrict any representation from the **ATLAS** to prescribed subgroups. This is useful in particular for those subgroups for which the **ATLAS** of Group Representations itself does not contain a straight line program.

gap> tom:= TableOfMarks( "A5" ); TableOfMarks( "A5" ) gap> info:= StandardGeneratorsInfo( tom ); [ rec( ATLAS := true, description := "|a|=2, |b|=3, |ab|=5", generators := "a, b", script := [ [ 1, 2 ], [ 2, 3 ], [ 1, 1, 2, 1, 5 ] ], standardization := 1 ) ]

The `true`

value of the component `ATLAS`

indicates that the information stored on `tom`

refers to the standard generators of type 1 in the **ATLAS** of Group Representations.

We want to restrict a 4-dimensional integral representation of A_5 to a Sylow 2 subgroup of A_5, and use `RepresentativeTomByGeneratorsNC`

(Reference: RepresentativeTomByGeneratorsNC) for that.

gap> info:= OneAtlasGeneratingSetInfo( "A5", Ring, Integers, Dimension, 4 );; gap> stdgens:= AtlasGenerators( info.identifier ); rec( dim := 4, generators := [ [ [ 1, 0, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 1, 0, 0 ], [ -1, -1, -1, -1 ] ], [ [ 0, 1, 0, 0 ], [ 0, 0, 0, 1 ], [ 0, 0, 1, 0 ], [ 1, 0, 0, 0 ] ] ], groupname := "A5", id := "", identifier := [ "A5", "A5G1-Zr4B0.g", 1, 4 ], repname := "A5G1-Zr4B0", repnr := 14, ring := Integers, size := 60, standardization := 1, type := "matint" ) gap> orders:= OrdersTom( tom ); [ 1, 2, 3, 4, 5, 6, 10, 12, 60 ] gap> pos:= Position( orders, 4 ); 4 gap> sub:= RepresentativeTomByGeneratorsNC( tom, pos, stdgens.generators ); <matrix group of size 4 with 2 generators> gap> GeneratorsOfGroup( sub ); [ [ [ 1, 0, 0, 0 ], [ -1, -1, -1, -1 ], [ 0, 0, 0, 1 ], [ 0, 0, 1, 0 ] ], [ [ 1, 0, 0, 0 ], [ 0, 0, 1, 0 ], [ 0, 1, 0, 0 ], [ -1, -1, -1, -1 ] ] ]

The sporadic simple Mathieu group M_22 contains a unique class of subgroups of index 770 (and order 576). This can be seen for example using **GAP**'s Library of Tables of Marks.

gap> tom:= TableOfMarks( "M22" ); TableOfMarks( "M22" ) gap> subord:= Size( UnderlyingGroup( tom ) ) / 770; 576 gap> ord:= OrdersTom( tom );; gap> tomstabs:= Filtered( [ 1 .. Length( ord ) ], i -> ord[i] = subord ); [ 144 ]

The permutation representation of M_22 on the right cosets of such a subgroup S is contained in the **ATLAS** of Group Representations.

gap> DisplayAtlasInfo( "M22", NrMovedPoints, 770 ); Representations for G = M22: (all refer to std. generators 1) ---------------------------- 12: G <= Sym(770) rank 9, on cosets of (A4xA4):4 < 2^4:A6

We now verify the information shown about the point stabilizer and about the maximal overgroups of S in M_22.

gap> maxtom:= MaximalSubgroupsTom( tom ); [ [ 155, 154, 153, 152, 151, 150, 146, 145 ], [ 22, 77, 176, 176, 231, 330, 616, 672 ] ] gap> List( tomstabs, i -> List( maxtom[1], j -> ContainedTom( tom, i, j ) ) ); [ [ 0, 10, 0, 0, 0, 0, 0, 0 ] ]

We see that the only maximal subgroups of M_22 that contain S have index 77 in M_22. According to the **ATLAS** of Finite Groups, these maximal subgroups have the structure 2^4:A_6. From that and from the structure of A_6, we conclude that S has the structure 2^4:(3^2:4).

Alternatively, we look at the permutation representation of degree 770. We fetch it from the **ATLAS** of Group Representations. There is exactly one nontrivial block system for this representation, with 77 blocks of length 10.

gap> g:= AtlasGroup( "M22", NrMovedPoints, 770 ); <permutation group of size 443520 with 2 generators> gap> allbl:= AllBlocks( g );; gap> List( allbl, Length ); [ 10 ]

Furthermore, **GAP** computes that the point stabilizer S has the structure (A_4 × A_4):4.

gap> stab:= Stabilizer( g, 1 );; gap> StructureDescription( stab ); "(A4 x A4) : C4" gap> blocks:= Orbit( g, allbl[1], OnSets );; gap> act:= Action( g, blocks, OnSets );; gap> StructureDescription( Stabilizer( act, 1 ) ); "(C2 x C2 x C2 x C2) : A6"

The **ATLAS** of Group Representations contains three degree 462 permutation representations of the group M_22.

gap> DisplayAtlasInfo( "M22", NrMovedPoints, 462 ); Representations for G = M22: (all refer to std. generators 1) ---------------------------- 7: G <= Sym(462a) rank 5, on cosets of 2^4:A5 < 2^4:A6 8: G <= Sym(462b) rank 8, on cosets of 2^4:A5 < L3(4), 2^4:S5 9: G <= Sym(462c) rank 8, on cosets of 2^4:A5 < L3(4), 2^4:A6

The point stabilizers in these three representations have the structure 2^4:A_5. Using **GAP**'s Library of Tables of Marks, we can show that these stabilizers are exactly the three classes of subgroups of order 960 in M_22. For that, we first verify that the group generators stored in **GAP**'s table of marks coincide with the standard generators used by the **ATLAS** of Group Representations.

gap> tom:= TableOfMarks( "M22" ); TableOfMarks( "M22" ) gap> genstom:= GeneratorsOfGroup( UnderlyingGroup( tom ) );; gap> checkM22:= AtlasProgram( "M22", "check" ); rec( groupname := "M22", identifier := [ "M22", "M22G1-check1", 1, 1 ] , program := <straight line decision>, standardization := 1 ) gap> ResultOfStraightLineDecision( checkM22.program, genstom ); true

There are indeed three classes of subgroups of order 960 in M_22.

gap> ord:= OrdersTom( tom );; gap> tomstabs:= Filtered( [ 1 .. Length( ord ) ], i -> ord[i] = 960 ); [ 147, 148, 149 ]

Now we compute representatives of these three classes in the three representations `462a`

, `462b`

, and `462c`

. We see that each of the three classes occurs as a point stabilizer in exactly one of the three representations.

gap> atlasreps:= AllAtlasGeneratingSetInfos( "M22", NrMovedPoints, 462 ); [ rec( charactername := "1a+21a+55a+154a+231a", groupname := "M22", id := "a", identifier := [ "M22", [ "M22G1-p462aB0.m1", "M22G1-p462aB0.m2" ], 1, 462 ], isPrimitive := false, p := 462, rankAction := 5, repname := "M22G1-p462aB0", repnr := 7, size := 443520, stabilizer := "2^4:A5 < 2^4:A6", standardization := 1, transitivity := 1, type := "perm" ), rec( charactername := "1a+21a^2+55a+154a+210a", groupname := "M22", id := "b", identifier := [ "M22", [ "M22G1-p462bB0.m1", "M22G1-p462bB0.m2" ], 1, 462 ], isPrimitive := false, p := 462, rankAction := 8, repname := "M22G1-p462bB0", repnr := 8, size := 443520, stabilizer := "2^4:A5 < L3(4), 2^4:S5", standardization := 1, transitivity := 1, type := "perm" ), rec( charactername := "1a+21a^2+55a+154a+210a", groupname := "M22", id := "c", identifier := [ "M22", [ "M22G1-p462cB0.m1", "M22G1-p462cB0.m2" ], 1, 462 ], isPrimitive := false, p := 462, rankAction := 8, repname := "M22G1-p462cB0", repnr := 9, size := 443520, stabilizer := "2^4:A5 < L3(4), 2^4:A6", standardization := 1, transitivity := 1, type := "perm" ) ] gap> atlasreps:= List( atlasreps, AtlasGroup );; gap> tomstabreps:= List( atlasreps, G -> List( tomstabs, > i -> RepresentativeTomByGenerators( tom, i, GeneratorsOfGroup( G ) ) ) );; gap> List( tomstabreps, x -> List( x, NrMovedPoints ) ); [ [ 462, 462, 461 ], [ 460, 462, 462 ], [ 462, 461, 462 ] ]

More precisely, we see that the point stabilizers in the three representations `462a`

, `462b`

, `462c`

lie in the subgroup classes 149, 147, 148, respectively, of the table of marks.

The point stabilizers in the representations `462b`

and `462c`

are isomorphic, but not isomorphic with the point stabilizer in `462a`

.

gap> stabs:= List( atlasreps, G -> Stabilizer( G, 1 ) );; gap> List( stabs, IdGroup ); [ [ 960, 11358 ], [ 960, 11357 ], [ 960, 11357 ] ] gap> List( stabs, PerfectIdentification ); [ [ 960, 2 ], [ 960, 1 ], [ 960, 1 ] ]

The three representations are imprimitive. The containment of the point stabilizers in maximal subgroups of M_22 can be computed using the table of marks of M_22.

gap> maxtom:= MaximalSubgroupsTom( tom ); [ [ 155, 154, 153, 152, 151, 150, 146, 145 ], [ 22, 77, 176, 176, 231, 330, 616, 672 ] ] gap> List( tomstabs, i -> List( maxtom[1], j -> ContainedTom( tom, i, j ) ) ); [ [ 21, 0, 0, 0, 1, 0, 0, 0 ], [ 21, 6, 0, 0, 0, 0, 0, 0 ], [ 0, 6, 0, 0, 0, 0, 0, 0 ] ]

We see:

The point stabilizers in

`462a`

(subgroups in the class 149 of the table of marks) are contained only in maximal subgroups in class 154; these groups have the structure 2^4:A_6.The point stabilizers in

`462b`

(subgroups in the class 147) are contained in maximal subgroups in the classes 155 and 151; these groups have the structures L_3(4) and 2^4:S_5, respectively.The point stabilizers in

`462c`

(subgroups in the class 148) are contained in maximal subgroups in the classes 155 and 154.

We identify the supergroups of the point stabilizers by computing the block systems.

gap> bl:= List( atlasreps, AllBlocks );; gap> List( bl, Length ); [ 1, 3, 2 ] gap> List( bl, l -> List( l, Length ) ); [ [ 6 ], [ 21, 21, 2 ], [ 21, 6 ] ]

Note that the two block systems with blocks of length 21 for `462b`

belong to the same supergroups (of the type L_3(4)); each of these subgroups fixes two different subsets of 21 points.

The representation `462a`

is *multiplicity-free*, that is, it splits into a sum of pairwise nonisomorphic irreducible representations. This can be seen from the fact that the rank of this permutation representation (that is, the number of orbits of the point stabilizer) is five; each permutation representation with this property is multiplicity-free.

The other two representations have rank eight. We have seen the ranks in the overview that was shown by `DisplayAtlasInfo`

(3.5-1) in the beginning. Now we compute the ranks from the permutation groups.

gap> List( atlasreps, RankAction ); [ 5, 8, 8 ]

In fact the two representations `462b`

and `462c`

have the same permutation character. We check this by computing the possible permutation characters of degree 462 for M_22, and decomposing them into irreducible characters, using the character table from **GAP**'s Character Table Library.

gap> t:= CharacterTable( "M22" );; gap> perms:= PermChars( t, 462 ); [ Character( CharacterTable( "M22" ), [ 462, 30, 3, 2, 2, 2, 3, 0, 0, 0, 0, 0 ] ), Character( CharacterTable( "M22" ), [ 462, 30, 12, 2, 2, 2, 0, 0, 0, 0, 0, 0 ] ) ] gap> MatScalarProducts( t, Irr( t ), perms ); [ [ 1, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0 ], [ 1, 2, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0 ] ]

In particular, we see that the rank eight characters are not multiplicity-free.

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