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2. The User Interface of the AtlasRep Package
2.1 Accessing vs. Constructing Representations
2.2 Group Names Used in the AtlasRep Package
2.3 Standard Generators Used in the AtlasRep Package
2.4 Class Names Used in the AtlasRep Package
2.4-1 Definition of ATLAS Class Names
2.4-2 AtlasClassNames
2.5 Accessing Data of the AtlasRep Package
2.5-1 DisplayAtlasInfo
2.5-2 AtlasGenerators
2.5-3 AtlasProgram
2.5-4 OneAtlasGeneratingSetInfo
2.5-5 AllAtlasGeneratingSetInfos
2.6 Examples of Using the AtlasRep Package
2.6-1 Example: Class Representatives
2.6-2 Example: Permutation and Matrix Representations
2.6-3 Example: Outer Automorphisms
2.6-4 Example: Using Semi-presentations and Black Box Programs
2.6-5 Example: Using the GAP Library of Tables of Marks

### 2. The User Interface of the AtlasRep Package

The user interface is the part of the GAP interface that allows one to display information about the current contents of the database and to access individual data (perhaps from a remote server, see Section 1.7-1). The corresponding functions are described in this chapter. See Section 2.6 for some small examples how to use the functions of the interface.

Extensions of the AtlasRep package are regarded as another part of the GAP interface, they are described in Chapter 3.. Finally, the low level part of the interface are described in Chapter 5..

As stated in Section 1.2, the user interface is preliminary. It will be extended when the GAP version of the ATLAS of Group Representations is connected to other GAP databases such as the libraries of character tables and tables of marks.

For some of the examples in this chapter, the GAP packages CTblLib [Bre04] and TomLib are needed.

    gap> LoadPackage( "ctbllib" );
    true
    gap> LoadPackage( "tomlib" );
    true

#### 2.1 Accessing vs. Constructing Representations

Note that accessing the data means in particular that it is not the aim of this package to construct representations from known ones. For example, if at least one permutation representation for a group G is stored but no matrix representation in a positive characteristic p, say, then `OneAtlasGeneratingSetInfo` (2.5-4) returns `fail` when it is asked for a description of an available set of matrix generators for G in characteristic p, although such a representation can be obtained by reduction modulo p of an integral matrix representation, which in turn can be constructed from any permutation representation.

#### 2.2 Group Names Used in the AtlasRep Package

The AtlasRep package refers to data of the ATLAS of group representations by the name of the group in question plus additional information. Thus it is essential to know this name, which is called the GAP name of the group in the following.

For an almost simple group, the GAP name is equal to the `Identifier` (Reference: Identifier!for character tables) value of the character table of this group in the GAP library (see `Access to Library Character Tables` (CTblLib: Access to Library Character Tables)); this name is usually very similar to the name used in the ATLAS of Finite Groups [CCNPW85]. For example, `"M22"` is the GAP name of the Mathieu group M_22, and `"12_1.U4(3).2_1"` is the GAP name of 12_1.U_4(3).2_1.

Internally, for example as part of filenames (see Section 5.6), the package uses names that may differ from the GAP names; these names are called ATLAS-file names. For example, `A5`, `TE62`, and `F24` are possible values for groupname. Of these, only `A5` is also a GAP name, but the other two are not; the corresponding GAP names are `2E6(2)` and `Fi24'`, respectively.

#### 2.3 Standard Generators Used in the AtlasRep Package

For the general definition of standard generators of a group, see Section Reference: Standard Generators of Groups; details can be found in [Wil96].

Several different standard generators may be defined for a group, the definitions can be found at

http://brauer.maths.qmul.ac.uk/Atlas

When one specifies the standardization, the i-th set of standard generators is denoted by the number i. Note that when more than one set of standard generators is defined for a group, one must be careful to use compatible standardization. For example, the straight line programs, straight line decisions and black box programs in the database refer to a specific standardization of their inputs. That is, a straight line program for computing generators of a certain subgroup of a group G is defined only for a specific set of standard generators of G, and applying the program to matrix or permutation generators of G but w.r.t. a different standardization may yield unpredictable results. Therefore the results returned by the functions described in this chapter contain information about the standardizations they refer to.

#### 2.4 Class Names Used in the AtlasRep Package

For each straight line program (see `AtlasProgram` (2.5-3)) that is used to compute lists of class representatives, it is essential to describe the classes in which these elements lie. Therefore, in these cases the records returned by the function `AtlasProgram` (2.5-3) contain a component `outputs` with value a list of class names.

Currently we define these class names only for simple groups and automorphic extensions and central extensions of simple groups, see Section 2.4-1. The function `AtlasClassNames` (2.4-2) can be used to compute the list of class names from the character table in the GAP Library.

##### 2.4-1 Definition of ATLAS Class Names

For the definition of class names of an almost simple group, we assume that the ordinary character tables of all nontrivial normal subgroups are shown in the ATLAS of Finite Groups [CCNPW85].

Each class name is a string consisting of the element order of the class in question followed by a combination of capital letters, digits, and the characters `'` and `-` (starting with a capital letter). For example, `1A`, `12A1`, and `3B'` denote the class that contains the identity element, a class of element order 12, and a class of element order 3, respectively.

1. For the table of a simple group, the class names are the same as returned by the two argument version of the GAP function `ClassNames` (Reference: ClassNames), cf. [CCNPW85, Chapter 7, Section 5]: The classes are arranged w.r.t. increasing element order and for each element order w.r.t. decreasing centralizer order, the conjugacy classes that contain elements of order n are named n`A`, n`B`, n`C`, ...; the alphabet used here is potentially infinite, and reads `A`, `B`, `C`, ..., `Z`, `A1`, `B1`, ..., `A2`, `B2`, ....

   For example, the classes of the alternating group A_5 have the names `1A`, `2A`, `3A`, `5A`, and `5B`.

2. Next we consider the case of an upward extension G.A of a simple group G by a cyclic group of order A. The ATLAS defines class names for each element g of G.A only w.r.t. the group G.a, say, that is generated by G and g; namely, there is a power of g (with the exponent coprime to the order of g) for which the class has a name of the same form as the class names for simple groups, and the name of the class of g w.r.t. G.a is then obtained from this name by appending a suitable number of dashes `'`. So dashed class names refer exactly to those classes that are not printed in the ATLAS.

   For example, those classes of the symmetric group S_5 that do not lie in A_5 have the names `2B`, `4A`, and `6A`. The outer classes of the group L_2(8).3 have the names `3B`, `6A`, `9D`, and `3B'`, `6A'`, `9D'`. The outer elements of order 5 in the group Sz(32).5 lie in the classes with names `5B`, `5B'`, `5B''`, and `5B'''`.

   In the group G.A, the class of g may fuse with other classes. The name of the class of g in G.A is obtained from the names of the involved classes of G.a by concatenating their names after removing the element order part from all of them except the first one.

   For example, the elements of order 9 in the group L_2(27).6 are contained in the subgroup L_2(27).3 but not in L_2(27). In L_2(27).3, they lie in the classes `9A`, `9A'`, `9B`, and `9B'`; in L_2(27).6, these classes fuse to `9AB` and `9A'B'`.

3. Now we define class names for general upward extensions G.A of a simple group G. Each element g of such a group lies in an upward extension G.a by a cyclic group, and the class names w.r.t. G.a are already defined. The name of the class of g in G.A is obtained by concatenating the names of the classes in the orbit of G.A on the classes of cyclic upward extensions of G, after ordering the names lexicographically and removing the element order part from all of them except the first one. An exception is the situation where dashed and non-dashed class names appear in an orbit; in this case, the dashed names are omitted.

   For example, the classes `21A` and `21B` of the group U_3(5).3 fuse in U_3(5).S_3 to the class `21AB`, and the class `2B` of U_3(5).2 fuses with the involution classes `2B'`, `2B''` in the groups U_3(5).2^prime and U_3(5).2^primeprime to the class `2B` of U_3(5).S_3.

   It may happen that some names in the `outputs` component of a record returned by `AtlasProgram` (2.5-3) do not uniquely determine the classes of the corresponding elements. For example, the (algebraically conjugate) classes `39A` and `39B` of the group Co_1 have not been distinguished yet. In such cases, the names used contain a minus sign `-`, and mean "one of the classes in the range described by the name before and the name after the minus sign"; the element order part of the name does not appear after the minus sign. So the name `39A-B` for the group Co_1 means `39A` or `39B`, and the name `20A-B'''` for the group Sz(32).5 means one of the classes of element order 20 in this group (these classes lie outside the simple group Sz).

4. For a central downward extension m.G of a simple group G by a cyclic group of order m, let pi denote the natural epimorphism from m.G onto G. Each class name of m.G has the form `nX_0`, `nX_1` etc., where `nX` is the class name of the image under pi, and the indices `0`, `1` etc. are chosen according to the position of the class in the lifting order rows for G, see [CCNPW85, Chapter 7, Section 7, and the example in Section 8]).

   For example, if m = 6 then `1A_1` and `1A_5` denote the classes containing the generators of the kernel of pi, that is, central elements of order 6.

##### 2.4-2 AtlasClassNames

`> AtlasClassNames`( tbl ) | ( function )

Returns: a list of class names.

Let tbl be the ordinary character table of a group G that is simple or an automorphic or a central extension of a simple group and such that tbl is an ATLAS table from the GAP Character Table Library, according to its `InfoText` (Reference: InfoText) value. Then `AtlasClassNames` returns the list of class names for G, as defined in Section 2.4-1. The ordering of class names is the same as the ordering of the columns of tbl.

(The function may work also for character tables that are not ATLAS tables, but then clearly the class names returned are somewhat arbitrary.)

    gap> AtlasClassNames( CharacterTable( "L3(4).3" ) );
    [ "1A", "2A", "3A", "4ABC", "5A", "5B", "7A", "7B", "3B", "3B'", "3C", "3C'",
      "6B", "6B'", "15A", "15A'", "15B", "15B'", "21A", "21A'", "21B", "21B'" ]
    gap> AtlasClassNames( CharacterTable( "U3(5).2" ) );
    [ "1A", "2A", "3A", "4A", "5A", "5B", "5CD", "6A", "7AB", "8AB", "10A", "2B",
      "4B", "6D", "8C", "10B", "12B", "20A", "20B" ]
    gap> AtlasClassNames( CharacterTable( "L2(27).6" ) );
    [ "1A", "2A", "3AB", "7ABC", "13ABC", "13DEF", "14ABC", "2B", "4A", "26ABC",
      "26DEF", "28ABC", "28DEF", "3C", "3C'", "6A", "6A'", "9AB", "9A'B'", "6B",
      "6B'", "12A", "12A'" ]
    gap> AtlasClassNames( CharacterTable( "L3(4).3.2_2" ) );
    [ "1A", "2A", "3A", "4ABC", "5AB", "7A", "7B", "3B", "3C", "6B", "15A",
      "15B", "21A", "21B", "2C", "4E", "6E", "8D", "14A", "14B" ]
    gap> AtlasClassNames( CharacterTable( "3.A6" ) );
    [ "1A_0", "1A_1", "1A_2", "2A_0", "2A_1", "2A_2", "3A_0", "3B_0", "4A_0",
      "4A_1", "4A_2", "5A_0", "5A_1", "5A_2", "5B_0", "5B_1", "5B_2" ]

#### 2.5 Accessing Data of the AtlasRep Package

(Note that the output of the examples in this section refers to a perhaps outdated table of contents; the current version of the database may contain more information than is shown here.)

##### 2.5-1 DisplayAtlasInfo

`> DisplayAtlasInfo`( ) | ( function )

`> DisplayAtlasInfo`( listofnames ) | ( function )

`> DisplayAtlasInfo`( gapname[, std][, ...] ) | ( function )

This function lists the information available via the AtlasRep package, for the given input. Depending on whether remote access to data is enabled (see Section 1.7-1), all the data provided by the ATLAS of Group Representations or only those in the local installation are considered.

(An interactive alternative to `DisplayAtlasInfo` is the function `BrowseAtlasInfo` (Browse: BrowseAtlasInfo), see [BL07]; this function provides also the functionality of `AtlasGenerators` (2.5-2).)

Called without arguments, `DisplayAtlasInfo` prints an overview what information the ATLAS of Group Representations provides. One line is printed for each group G, with the following columns.

`group`

  the GAP name of G (see Section 2.2),

`#`

  the number of representations stored for G,

`maxes`

  the available straight line programs for computing generators of maximal subgroups of G,

`cl`

  a `+` sign if at least one program for computing representatives of conjugacy classes of elements of G is stored, and a `-` sign otherwise,

`cyc`

  a `+` sign if at least one program for computing representatives of classes of maximally cyclic subgroups of G is stored, and a `-` sign otherwise,

`out`

  descriptions of outer automorphisms of G for which at least one program is stored,

`check`

  a `+` sign if at least one program is available for checking whether a set of generators is a set of standard generators, and a `-` sign otherwise,

`pres`

  a `+` sign if at least one program is available that encodes a presentation, and a `-` sign otherwise,

`find`

  a `+` sign if at least one program is available for finding standard generators, and a `-` sign otherwise,

Called with a list listofnames of strings that are GAP names for a group from the ATLAS of Group Representations, `DisplayAtlasInfo` prints the overview described above but restricted to the groups in this list.

Called with a string gapname that is a GAP name for a group from the ATLAS of Group Representations, `DisplayAtlasInfo` prints an overview of the information that is available for this group. One line is printed for each representation, showing the number of this representation (which can be used in calls of `AtlasGenerators` (2.5-2)), and a string of one of the following forms; in both cases, id is a (possibly empty) string.

`G <= Sym(nid)`

  denotes a permutation representation of degree n, for example `G <= Sym(40a)` and `G <= Sym(40b)` denote two (nonequivalent) representations of degree 40.

`G <= GL(nid,descr)`

  denotes a matrix representation of dimension n over a coefficient ring described by descr, which can be a prime power, `Z` (denoting the ring of integers), a description of an algebraic extension field, `C` (denoting an unspecified algebraic extension field), or `Z/mZ` for an integer m (denoting the ring of residues mod m); for example, `G <= GL(2a,4)` and `G <= GL(2b,4)` denote two (nonequivalent) representations of dimension 2 over the field with four elements.

After the representations, the programs available for gapname are listed.

If the first argument is a string gapname, the following optional arguments can be used to restrict the overview.

std

  must be a positive integer or a list of positive integers; if it is given then only those representations are considered that refer to the std-th set of standard generators or the i-th set of standard generators, for i in std (see Section 2.3),

`IsPermGroup` and `true`

  restrict to permutation representations,

`NrMovedPoints` and n

  for a positive integer, a list of positive integers, or a property n, restrict to permutation representations of degree equal to n, or in the list n, or satisfying the function n,

`NrMovedPoints` and the string `"minimal"`

  restrict to faithful permutation representations of minimal degree (if this information is available),

`IsMatrixGroup` and `true`

  restrict to matrix representations,

`Characteristic` and p

  for a prime integer, a list of prime integers, or a property p, restrict to matrix representations over fields of characteristic equal to p, or in the list p, or satisfying the function p (representations over residue class rings that are not fields can be addressed by entering `fail` as the value of p),

`Dimension` and n

  for a positive integer, a list of positive integers, or a property n, restrict to matrix representations of dimension equal to n, or in the list n, or satisfying the function n,

`Characteristic`, p, `Dimension`, and the string `"minimal"`

  for a prime integer p, restrict to faithful matrix representations over fields of characteristic p that have minimal dimension (if this information is available),

`Ring` and R

  for a ring or a property R, restrict to matrix representations over this ring or satisfying this function (note that the representation might be defined over a proper subring of R), and

`Ring`, R, `Dimension`, and the string `"minimal"`

  for a ring R, restrict to faithful matrix representations over this ring that have minimal dimension (if this information is available),

`IsStraightLineProgram`

  restricts to straight line programs, straight line decisions (see Section 4.1), and black box programs (see Section 4.2).

If "minimality" information is requested and no available representation matches this condition then either no minimal representation is available or the information about the minimality is missing. See `MinimalRepresentationInfo` (4.3-1) for checking whether the minimality information is available for the group in question. Note that in the cases where the string `"minimal"` occurs as an argument, `MinimalRepresentationInfo` (4.3-1) is called with third argument `"lookup"`; this is because the stored information was computed just for the groups in the ATLAS of Group Representations, so trying to compute non-stored minimality information (using other available databases) will hardly be successful.

The representations are ordered as follows. Permutation representations come first (ordered according to their degrees), followed by matrix representations over finite fields (ordered first according to the field size and second according to the dimension), matrix representations over the integers, and then matrix representations over algebraic extension fields (both kinds ordered according to the dimension), the last representations are matrix representations over residue class rings (ordered first according to the modulus and second according to the dimension).

The maximal subgroups are ordered according to decreasing group order. For an extension G.p of a simple group G by an outer automorphism of prime order p, this means that G is the first maximal subgroup and then come the extensions of the maximal subgroups of G and the novelties; so the n-th maximal subgroup of G and the n-th maximal subgroup of G.p are in general not related. (This coincides with the numbering used for the `Maxes` (CTblLib: Maxes) attribute for character tables.)

    gap> DisplayAtlasInfo( [ "M11", "A5" ] );
    group     #  maxes  cl  cyc  out  find  check  pres
    ---------------------------------------------------
    M11      42      5   +    +          +      +     +
    A5       18      3   -    -          -      +     +

The above output means that the ATLAS of Group Representations contains 42 representations of the Mathieu group M_11, straight line programs for computing generators of representatives of all five classes of maximal subgroups, for computing representatives of the conjugacy classes of elements and of generators of maximally cyclic subgroups, contains no straight line program for applying outer automorphisms (well, in fact M_11 admits no nontrivial outer automorphism), and contains a straight line decision that checks generators for being standard generators. Analogously, 18 representations of the alternating group A_5 are available, straight line programs for computing generators of representatives of all three classes of maximal subgroups, and no straight line programs for computing representatives of the conjugacy classes of elements, of generators of maximally cyclic subgroups, and no for computing images under outer automorphisms; a straight line decision for checking the standardization of generators is contained.

    gap> DisplayAtlasInfo( "A5", IsPermGroup, true );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
    1: G <= Sym(5)
    2: G <= Sym(6)
    3: G <= Sym(10)
    gap> DisplayAtlasInfo( "A5", NrMovedPoints, [ 4 .. 9 ] );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
    1: G <= Sym(5)
    2: G <= Sym(6)

The first three representations stored for A_5 are (in fact primitive) permutation representations.

    gap> DisplayAtlasInfo( "A5", Dimension, [ 1 .. 3 ] );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
     8: G <= GL(2a,4)
     9: G <= GL(2b,4)
    10: G <= GL(3,5)
    12: G <= GL(3a,9)
    13: G <= GL(3b,9)
    17: G <= GL(3a,Field([Sqrt(5)]))
    18: G <= GL(3b,Field([Sqrt(5)]))
    gap> DisplayAtlasInfo( "A5", Characteristic, 0 );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
    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)]))

The representations with number between 4 and 13 are (in fact irreducible) matrix representations over various finite fields, those with numbers 14 to 16 are integral matrix representations, and the last two are matrix representations over the field generated by sqrt5 over the rational number field.

    gap> DisplayAtlasInfo( "A5", NrMovedPoints, IsPrimeInt );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
    1: G <= Sym(5)
    gap> DisplayAtlasInfo( "A5", Characteristic, IsOddInt );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
     6: G <= GL(4,3)
     7: G <= GL(6,3)
    10: G <= GL(3,5)
    11: G <= GL(5,5)
    12: G <= GL(3a,9)
    13: G <= GL(3b,9)
    gap> DisplayAtlasInfo( "A5", Dimension, IsPrimeInt );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
     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)
    15: G <= GL(5,Z)
    17: G <= GL(3a,Field([Sqrt(5)]))
    18: G <= GL(3b,Field([Sqrt(5)]))
    gap> DisplayAtlasInfo( "A5", Ring, IsFinite and IsPrimeField );
    Representations for G = A5:    (all refer to std. generators 1)
    ---------------------------
     4: G <= GL(4a,2)
     5: G <= GL(4b,2)
     6: G <= GL(4,3)
     7: G <= GL(6,3)
    10: G <= GL(3,5)
    11: G <= GL(5,5)

The above examples show how the output can be restricted using a property (a unary function that returns either `true` or `false`) that follows `NrMovedPoints`, `Characteristic`, `Dimension`, or `Ring` in the argument list of `DisplayAtlasInfo`.

    gap> DisplayAtlasInfo( "A5", IsStraightLineProgram, true );
    Programs for G = A5:    (all refer to std. generators 1)
    --------------------
    available maxes of G:  [ 1 .. 3 ]
    standard generators checker available
    presentation available

Straight line programs are available for computing generators of representatives of the three classes of maximal subgroups of A_5, and a straight line decision for checking whether given generators are in fact standard generators ia available as well as a presentation in terms of standard generators, see `AtlasProgram` (2.5-3).

##### 2.5-2 AtlasGenerators

`> AtlasGenerators`( gapname, repnr[, maxnr] ) | ( function )

`> AtlasGenerators`( identifier ) | ( function )

Returns: a record containing generators for a representation, or `fail`.

In the first form, gapname must be a string denoting a GAP name (see Section 2.2) of a group, and repnr a positive integer. If the ATLAS of Group Representations contains at least repnr representations for the group with GAP name gapname then `AtlasGenerators`, when called with gapname and repnr, returns an immutable record describing the repnr-th representation; otherwise `fail` is returned. If a third argument maxnr, a positive integer, is given then an immutable record describing the restriction of the repnr-th representation to the maxnr-th maximal subgroup is returned.

The result record has the following components.

`groupname`

  the GAP name of the group (see Section 2.2),

`generators`

  a list of generators for the group,

`standardization`

  the positive integer denoting the underlying standard generators,

`size` (only if known)

  the order of the group,

`identifier`

  a GAP object (a list of filenames plus additional information) that uniquely determines the representation; the value can be used as identifier argument of `AtlasGenerators` (see below).

`repnr`

  the number of the representation in the current session, equal to the argument repnr if this is given.

Additionally, there are describing components dependent on the data type of the representation: For permutation representations, these are `p` for the number of moved points and `id` for the distinguishing string as described for `DisplayAtlasInfo` (2.5-1); for matrix representations, these are `dim` for the dimension of the matrices, `ring` (if known) for the ring generated by the matrix entries, and `id` for the distinguishing string.

It should be noted that the number repnr refers to the number shown by `DisplayAtlasInfo` (2.5-1) in the current session; it may be that after the addition of new representations, repnr refers to another representation.

The alternative form of `AtlasGenerators`, with only argument identifier, can be used to fetch the result record with `identifier` value equal to identifier. The purpose of this variant is to access the same representation also in different GAP sessions.

    gap> gens1:= AtlasGenerators( "A5", 1 );
    rec( generators := [ (1,2)(3,4), (1,3,5) ], groupname := "A5",
      standardization := 1, repnr := 1,
      identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ], p := 5,
      id := "", size := 60 )
    gap> gens8:= AtlasGenerators( "A5", 8 );
    rec(
      generators := [ [ [ Z(2)^0, 0*Z(2) ], [ Z(2^2), Z(2)^0 ] ], [ [ 0*Z(2), Z(2
                     )^0 ], [ Z(2)^0, Z(2)^0 ] ] ], groupname := "A5",
      standardization := 1, repnr := 8,
      identifier := [ "A5", [ "A5G1-f4r2aB0.m1", "A5G1-f4r2aB0.m2" ], 1, 4 ],
      dim := 2, id := "a", ring := GF(2^2), size := 60 )
    gap> gens17:= AtlasGenerators( "A5", 17 );
    rec(
      generators := [ [ [ -1, 0, 0 ], [ 0, -1, 0 ], [ -E(5)-E(5)^4, -E(5)-E(5)^4,
                  1 ] ], [ [ 0, 1, 0 ], [ 0, 0, 1 ], [ 1, 0, 0 ] ] ],
      groupname := "A5", standardization := 1, repnr := 17,
      identifier := [ "A5", "A5G1-Ar3aB0.g", 1, 3 ], dim := 3, id := "a",
      ring := NF(5,[ 1, 4 ]), size := 60 )

Each of the above pairs of elements generates a group isomorphic to A_5.

    gap> gens1max2:= AtlasGenerators( "A5", 1, 2 );
    rec( generators := [ (1,2)(3,4), (2,3)(4,5) ], standardization := 1,
      repnr := 1, identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5,
          2 ], p := 5, id := "" )
    gap> id:= gens1max2.identifier;;
    gap> gens1max2 = AtlasGenerators( id );
    true
    gap> max2:= Group( gens1max2.generators );;
    gap> Size( max2 );
    10
    gap> IdGroup( max2 ) = IdGroup( DihedralGroup( 10 ) );
    true

The elements stored in `gens1max2.generators` describe the restriction of the first representation of A_5 to a group in the second class of maximal subgroups of A_5 according to the list in the ATLAS of Finite Groups [CCNPW85]; this subgroup is isomorphic to the dihedral group D_10.

##### 2.5-3 AtlasProgram

`> AtlasProgram`( gapname[, std], ... ) | ( function )

`> AtlasProgram`( identifier ) | ( function )

Returns: a record containing a program, or `fail`.

In the first form, gapname must be a string denoting a GAP name (see Section 2.2) of a group G, say. If the ATLAS of Group Representations contains a straight line program (see Section Reference: Straight Line Programs) or straight line decision (see Section 4.1) or black box program (see Section 4.2) as described by the remaining arguments (see below) then `AtlasProgram` returns an immutable record containing this program. Otherwise `fail` is returned.

If the optional argument std is given, only those straight line programs/decisions are considered that take generators from the std-th set of standard generators of G as input, see Section 2.3.

The result record has the following components.

`program`

  the required straight line program/decision, or black box program,

`standardization`

  the positive integer denoting the underlying standard generators of G,

`identifier`

  a GAP object (a list of filenames plus additional information) that uniquely determines the program; the value can be used as identifier argument of `AtlasProgram` (see below).

In the first form, the last arguments must be as follows.

(the string `"maxes"` and) a positive integer maxnr

  the required program computes generators of the maxnr-th maximal subgroup of the group with GAP name gapname.

one of the strings `"classes"` or `"cyclic"`

  the required program computes representatives of conjugacy classes of elements or representatives of generators of maximally cyclic subgroups of G, respectively.

  See [BSWW01] and [SWW00] for the background concerning these straight line programs. In these cases, the result record of `AtlasProgram` also contains a component `outputs`, whose value is a list of class names of the outputs, as described in Section 2.4.

the strings `"automorphism"` and autname

  the required program computes images of standard generators under the outer automorphism of G that is given by this string.

the string `"check"`

  the required result is a straight line decision that takes a list of generators for G and returns `true` if these generators are standard generators w.r.t. the standardization std, and `false` otherwise.

the string `"presentation"`

  the required result is a straight line decision that takes a list of group elements and returns `true` if these elements are standard generators of G w.r.t. the standardization std, and `false` otherwise.

the string `"find"`

  the required result is a black box program that takes G and returns a list of standard generators of G, w.r.t. the standardization std.

the string `"restandardize"` and an integer std2

  the required result is a straight line program that computes standard generators of G w.r.t. the std2-th set of standard generators of G; in this case, the argument std must be given.

the strings `"other"` and descr

  the required program is described by descr.

The second form of `AtlasProgram`, with only argument the list identifier, can be used to fetch the result record with `identifier` value equal to identifier.

    gap> prog:= AtlasProgram( "A5", 2 );
    rec( program := <straight line program>, standardization := 1,
      identifier := [ "A5", "A5G1-max2W1", 1 ], groupname := "A5" )
    gap> StringOfResultOfStraightLineProgram( prog.program, [ "a", "b" ] );
    "[ a, bbab ]"
    gap> gens1:= AtlasGenerators( "A5", 1 );
    rec( generators := [ (1,2)(3,4), (1,3,5) ], groupname := "A5",
      standardization := 1, repnr := 1,
      identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ], p := 5,
      id := "", size := 60 )
    gap> maxgens:= ResultOfStraightLineProgram( prog.program, gens1.generators );
    [ (1,2)(3,4), (2,3)(4,5) ]
    gap> maxgens = gens1max2.generators;
    true

The above example shows that for restricting representations given by standard generators to a maximal subgroup of A_5, we can also fetch and apply the appropriate straight line program. Such a program (see Reference: Straight Line Programs) takes standard generators of a group --in this example A_5-- as its input, and returns a list of elements in this group --in this example generators of the D_10 subgroup we had met above-- which are computed essentially by evaluating structured words in terms of the standard generators.

    gap> prog:= AtlasProgram( "J1", "cyclic" );
    rec( program := <straight line program>, standardization := 1,
      identifier := [ "J1", "J1G1-cycW1", 1 ],
      outputs := [ "6A", "7A", "10B", "11A", "15B", "19A" ], groupname := "J1" )
    gap> gens:= GeneratorsOfGroup( FreeGroup( "x", "y" ) );;
    gap> ResultOfStraightLineProgram( prog.program, gens );
    [ x*y*x*y^2*x*y*x*y^2*x*y*x*y*x*y^2*x*y^2, x*y, x*y*x*y^2*x*y*x*y*x*y^2*x*y^2,
      x*y*x*y*x*y^2*x*y^2*x*y*x*y^2*x*y*x*y*x*y^2*x*y^2*x*y*x*y^2*x*y*x*y*x*y^
        2*x*y^2, x*y*x*y*x*y^2*x*y^2, x*y*x*y^2 ]

The above example shows how to fetch and use straight line programs for computing generators of representatives of maximally cyclic subgroups of a given group.

##### 2.5-4 OneAtlasGeneratingSetInfo

`> OneAtlasGeneratingSetInfo`( [gapname][,][std][,][...] ) | ( function )

Returns: a record describing a representation that satisfies the conditions, or `fail`.

Let gapname be a string denoting a GAP name (see Section 2.2) of a group G, say. If the ATLAS of Group Representations contains at least one representation for G with the required properties then `OneAtlasGeneratingSetInfo` returns a record r whose components are the same as those of the records returned by `AtlasGenerators` (2.5-2), except that the component `generators` is not contained; the component `identifier` of r can be used as input for `AtlasGenerators` (2.5-2) in order to fetch the generators. If no representation satisfying the given conditions ia available then `fail` is returned.

If the argument std is given then it must be a positive integer or a list of positive integers, denoting the sets of standard generators w.r.t. which the representation shall be given (see Section 2.3).

The argument gapname can be missing (then all available groups are considered), or a list of group names can be given instead.

Further restrictions can be entered as arguments, with the same meaning as described for `DisplayAtlasInfo` (2.5-1). The result of `OneAtlasGeneratingSetInfo` describes the first generating set for G that matches the restrictions, in the ordering shown by `DisplayAtlasInfo` (2.5-1).

Note that even in the case that the user parameter "remote" has the value `true` (see Section 1.7-1), `OneAtlasGeneratingSetInfo` does not attempt to transfer remote data files, just the table of contents is evaluated. So this function (as well as `AllAtlasGeneratingSetInfos` (2.5-5)) can be used to check for the availability of certain representations, and afterwards one can call `AtlasGenerators` (2.5-2) for those representations one wants to work with.

In the following example, we try to access information about permutation representations for the alternating group A_5.

    gap> info:= OneAtlasGeneratingSetInfo( "A5" );
    rec( groupname := "A5", repnr := 1,
      identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ], p := 5,
      id := "", size := 60 )
    gap> gens:= AtlasGenerators( info.identifier );
    rec( generators := [ (1,2)(3,4), (1,3,5) ], groupname := "A5",
      standardization := 1, repnr := 1,
      identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ], p := 5,
      id := "", size := 60 )
    gap> info = OneAtlasGeneratingSetInfo( "A5", IsPermGroup, true );
    true
    gap> info = OneAtlasGeneratingSetInfo( "A5", NrMovedPoints, "minimal" );
    true
    gap> info = OneAtlasGeneratingSetInfo( "A5", NrMovedPoints, [ 1 .. 10 ] );
    true
    gap> OneAtlasGeneratingSetInfo( "A5", NrMovedPoints, 20 );
    fail

Note that a permutation representation of degree 20 could be obtained by taking twice the primitive representation on 10 points; however, the ATLAS of Group Representations does not store this imprimitive representation (cf. Section 2.1).

We continue this example a little. Next we access matrix representations of A_5.

    gap> info:= OneAtlasGeneratingSetInfo( "A5", IsMatrixGroup, true );
    rec( groupname := "A5", repnr := 4,
      identifier := [ "A5", [ "A5G1-f2r4aB0.m1", "A5G1-f2r4aB0.m2" ], 1, 2 ],
      dim := 4, id := "a", ring := GF(2), size := 60 )
    gap> gens:= AtlasGenerators( info.identifier );
    rec(
      generators := [ <an immutable 4x4 matrix over GF2>, <an immutable 4x4 matrix\
     over GF2> ], groupname := "A5", standardization := 1, repnr := 4,
      identifier := [ "A5", [ "A5G1-f2r4aB0.m1", "A5G1-f2r4aB0.m2" ], 1, 2 ],
      dim := 4, id := "a", ring := GF(2), size := 60 )
    gap> info = OneAtlasGeneratingSetInfo( "A5", Dimension, 4 );
    true
    gap> info = OneAtlasGeneratingSetInfo( "A5", Characteristic, 2 );
    true
    gap> info = OneAtlasGeneratingSetInfo( "A5", Ring, GF(2) );
    true
    gap> OneAtlasGeneratingSetInfo( "A5", Characteristic, [2,5], Dimension, 2 );
    rec( groupname := "A5", repnr := 8,
      identifier := [ "A5", [ "A5G1-f4r2aB0.m1", "A5G1-f4r2aB0.m2" ], 1, 4 ],
      dim := 2, id := "a", ring := GF(2^2), size := 60 )
    gap> OneAtlasGeneratingSetInfo( "A5", Characteristic, [2,5], Dimension, 1 );
    fail
    gap> info:= OneAtlasGeneratingSetInfo( "A5", Characteristic, 0, Dimension, 4 );
    rec( groupname := "A5", repnr := 14,
      identifier := [ "A5", "A5G1-Zr4B0.g", 1, 4 ], dim := 4, id := "",
      ring := Integers, size := 60 )
    gap> gens:= AtlasGenerators( info.identifier );
    rec(
      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", standardization := 1, repnr := 14,
      identifier := [ "A5", "A5G1-Zr4B0.g", 1, 4 ], dim := 4, id := "",
      ring := Integers, size := 60 )
    gap> info = OneAtlasGeneratingSetInfo( "A5", Ring, Integers );
    true
    gap> info = OneAtlasGeneratingSetInfo( "A5", Ring, CF(37) );
    true
    gap> OneAtlasGeneratingSetInfo( "A5", Ring, Integers mod 77 );
    fail
    gap> info:= OneAtlasGeneratingSetInfo( "A5", Ring, CF(5), Dimension, 3 );
    rec( groupname := "A5", repnr := 17,
      identifier := [ "A5", "A5G1-Ar3aB0.g", 1, 3 ], dim := 3, id := "a",
      ring := NF(5,[ 1, 4 ]), size := 60 )
    gap> gens:= AtlasGenerators( info.identifier );
    rec(
      generators := [ [ [ -1, 0, 0 ], [ 0, -1, 0 ], [ -E(5)-E(5)^4, -E(5)-E(5)^4,
                  1 ] ], [ [ 0, 1, 0 ], [ 0, 0, 1 ], [ 1, 0, 0 ] ] ],
      groupname := "A5", standardization := 1, repnr := 17,
      identifier := [ "A5", "A5G1-Ar3aB0.g", 1, 3 ], dim := 3, id := "a",
      ring := NF(5,[ 1, 4 ]), size := 60 )
    gap> OneAtlasGeneratingSetInfo( "A5", Ring, GF(17) );
    fail

##### 2.5-5 AllAtlasGeneratingSetInfos

`> AllAtlasGeneratingSetInfos`( [gapname][,][std][,][...] ) | ( function )

Returns: the list of all records describing representations that satisfy the conditions.

`AllAtlasGeneratingSetInfos` is similar to `OneAtlasGeneratingSetInfo` (2.5-4). The difference is that the list of all records describing the available representations with the given properties is returned instead of just one such component. In particular an empty list is returned if no such representation is available.

    gap> AllAtlasGeneratingSetInfos( "A5", IsPermGroup, true );
    [ rec( groupname := "A5", repnr := 1,
          identifier := [ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ],
          p := 5, id := "", size := 60 ),
      rec( groupname := "A5", repnr := 2, identifier :=
            [ "A5", [ "A5G1-p6B0.m1", "A5G1-p6B0.m2" ], 1, 6 ], p := 6, id := "",
          size := 60 ),
      rec( groupname := "A5", repnr := 3, identifier := [ "A5", [ "A5G1-p10B0.m1",
                  "A5G1-p10B0.m2" ], 1, 10 ], p := 10, id := "", size := 60 ) ]

Note that a matrix representation in any characteristic can be obtained by reducing a permutation representation or an integral matrix representation; however, the ATLAS of Group Representations does not store such a representation (cf. Section 2.1).

#### 2.6 Examples of Using the AtlasRep Package

##### 2.6-1 Example: Class Representatives

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` (2.5-1), 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)
     7: G <= GL(32,2)
     8: G <= GL(44,2)
    16: G <= GL(16a,4)
    17: G <= GL(16b,4)

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( program := <straight line program>, standardization := 1,
      identifier := [ "M11", "M11G1-cclsW1", 1 ],
      outputs := [ "1A", "2A", "3A", "4A", "5A", "6A", "8A", "8B", "11A", "11B" ],
      groupname := "M11" )
    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

##### 2.6-2 Example: Permutation and Matrix Representations

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`. 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

##### 2.6-3 Example: Outer Automorphisms

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)
    -----------------------
    available maxes of G:  [ 1 .. 10 ]
    class repres. of G available
    repres. of cyclic subgroups of G available
    available automorphisms:  [ "2" ]
    standard generators checker available
    presentation available
    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 alpha, 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 alpha can be obtained by evaluating the same words at the images under alpha 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 impossible.

##### 2.6-4 Example: Using Semi-presentations and Black Box Programs

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( groupname := "M12", repnr := 1,
      identifier := [ "M12", [ "M12G1-p12aB0.m1", "M12G1-p12aB0.m2" ], 1, 12 ],
      p := 12, id := "a", size := 95040 )
    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 M_11 to these generators of M_11, since they are not necessarily standard generators of M_11. We check this using a semi-presentation for M_11.

    gap> checkM11:= AtlasProgram( "M11", "check" );
    rec( program := <straight line decision>, standardization := 1,
      identifier := [ "M11", "M11G1-check1", 1, 1 ], groupname := "M11" )
    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( program := <black box program>, standardization := 1,
      identifier := [ "M11", "M11G1-find1", 1, 1 ], groupname := "M11" )
    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

##### 2.6-5 Example: Using the GAP Library of Tables of Marks

The GAP library of tables of marks 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( generators := "a, b", description := "|a|=2, |b|=3, |ab|=5",
          script := [ [ 1, 2 ], [ 2, 3 ], [ 1, 1, 2, 1, 5 ] ], ATLAS := true ) ]

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(
      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", standardization := 1, repnr := 14,
      identifier := [ "A5", "A5G1-Zr4B0.g", 1, 4 ], dim := 4, id := "",
      ring := Integers, size := 60 )
    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 ] ] ]

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