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# 48 Group Libraries

### Sections

1. Basic Groups
2. Classical Groups
3. Conjugacy Classes in Classical Groups
4. Constructors for Basic Groups
5. Selection Functions
6. Transitive Permutation Groups
7. Small Groups
8. Finite Perfect Groups
9. Primitive Permutation Groups
10. Index numbers of primitive groups
11. Irreducible Solvable Matrix Groups
12. Irreducible Maximal Finite Integral Matrix Groups

When you start GAP, it already knows several groups. Currently GAP initially knows the following groups:

- some basic groups, such as cyclic groups or symmetric groups (see Basic Groups),
- Classical matrix groups (see Classical Groups),
- the transitive permutation groups of degree at most 30 (see Transitive permutation groups),
- a library of groups of small order (see Small Groups),
- the finite perfect groups of size at most 106 (excluding 11 sizes) (see Finite perfect groups).
- the primitive permutation groups of degree < 2499 (see Primitive permutation groups),
- the irreducible solvable subgroups of GL(n,p) for n > 1 and pn < 256 (see Irreducible Solvable Matrix Groups),
- the irreducible maximal finite integral matrix groups of dimension at most 31 (see Irreducible Maximal Finite Integral Matrix Groups),

There is usually no relation between the groups in the different libraries and a group may occur in different libraries in different incarnations.

Note that a system administrator may choose to install all, or only a few, or even none of the libraries. So some of the libraries mentioned below may not be available on your installation.

## 48.1 Basic Groups

There are several infinite families of groups which are parametrized by numbers. GAP provides various functions to construct these groups. The functions always permit (but do not require) one to indicate a filter (see Filters), for example `IsPermGroup`, `IsMatrixGroup` or `IsPcGroup`, in which the group shall be constructed. There always is a default filter corresponding to a ``natural'' way to describe the group in question. Note that not every group can be constructed in every filter, there may be theoretical restrictions (`IsPcGroup` only works for solvable groups) or methods may be available only for a few filters.

Certain filters may admit additional hints. For example, groups constructed in `IsMatrixGroup` may be constructed over a specified field, which can be given as second argument of the function that constructs the group; The default field is `Rationals`.

- `TrivialGroup( [`filter`] ) F`

  constructs a trivial group in the category given by the filter filter. If filter is not given it defaults to `IsPcGroup`.

      gap> TrivialGroup();
      <pc group of size 1 with 0 generators>
      gap> TrivialGroup( IsPermGroup );
      Group(())

- `CyclicGroup( [`filt`, ]`n`) F`

  constructs the cyclic group of size n in the category given by the filter filt. If filt is not given it defaults to `IsPcGroup`.

      gap> CyclicGroup(12);
      <pc group of size 12 with 3 generators>
      gap> CyclicGroup(IsPermGroup,12);
      Group([ (1,2,3,4,5,6,7,8,9,10,11,12) ])
      gap> matgrp1:= CyclicGroup( IsMatrixGroup, 12 );
      <matrix group of size 12 with 1 generators>
      gap> FieldOfMatrixGroup( matgrp1 );
      Rationals
      gap> matgrp2:= CyclicGroup( IsMatrixGroup, GF(2), 12 );
      <matrix group of size 12 with 1 generators>
      gap> FieldOfMatrixGroup( matgrp2 );
      GF(2)

- `AbelianGroup( [`filt`, ]`ints`) F`

  constructs an abelian group in the category given by the filter filt which is of isomorphism type Cints[1] * Cints[2] * …* Cints[n]. ints must be a list of positive integers. If filt is not given it defaults to `IsPcGroup`. The generators of the group returned are the elements corresponding to the integers in ints.

      gap> AbelianGroup([1,2,3]);
      <pc group of size 6 with 3 generators>

- `ElementaryAbelianGroup( [`filt`, ]`n`) F`

  constructs the elementary abelian group of size n in the category given by the filter filt. If filt is not given it defaults to `IsPcGroup`.

      gap> ElementaryAbelianGroup(8192);
      <pc group of size 8192 with 13 generators>

- `DihedralGroup( [`filt`, ]`n`) F`

  constructs the dihedral group of size n in the category given by the filter filt. If filt is not given it defaults to `IsPcGroup`.

      gap> DihedralGroup(10);
      <pc group of size 10 with 2 generators>

- `ExtraspecialGroup( [`filt`, ]`order`,` exp`) F`

  Let order be of the form p2n+1, for a prime integer p and a positive integer n. `ExtraspecialGroup` returns the extraspecial group of order order that is determined by exp, in the category given by the filter filt.

  If p is odd then admissible values of exp are the exponent of the group (either p or p2) or one of `'+'`, `"+"`, `'-'`, `"-"`. For p = 2, only the above plus or minus signs are admissible.

  If filt is not given it defaults to `IsPcGroup`.

      gap> ExtraspecialGroup( 27, 3 );
      <pc group of size 27 with 3 generators>
      gap> ExtraspecialGroup( 27, '+' );
      <pc group of size 27 with 3 generators>
      gap> ExtraspecialGroup( 8, "-" );
      <pc group of size 8 with 3 generators>

- `AlternatingGroup( [`filt`, ]`deg`) F`
- `AlternatingGroup( [`filt`, ]`dom`) F`

  constructs the alternating group of degree deg in the category given by the filter filt. If filt is not given it defaults to `IsPermGroup`. In the second version, the function constructs the alternating group on the points given in the set dom which must be a set of positive integers.

      gap> AlternatingGroup(5);
      Alt( [ 1 .. 5 ] )

- `SymmetricGroup( [`filt`, ]`deg`) F`
- `SymmetricGroup( [`filt`, ]`dom`) F`

  constructs the symmetric group of degree deg in the category given by the filter filt. If filt is not given it defaults to `IsPermGroup`. In the second version, the function constructs the symmetric group on the points given in the set dom which must be a set of positive integers.

      gap> SymmetricGroup(10);
      Sym( [ 1 .. 10 ] )

  Note that permutation groups provide special treatment of symmetric and alternating groups, see Symmetric and Alternating Groups.

- `MathieuGroup( [`filt`, ]`degree`) F`

  constructs the Mathieu group of degree degree in the category given by the filter filt, where degree must be in { 9, 10, 11, 12, 21, 22, 23, 24 }. If filt is not given it defaults to `IsPermGroup`.

      gap> MathieuGroup( 11 );
      Group([ (1,2,3,4,5,6,7,8,9,10,11), (3,7,11,8)(4,10,5,6) ])

- `SuzukiGroup( [`filt`, ]` q`) F`
- `Sz( [`filt`, ]` q`) F`

  Constructs a group isomorphic to the Suzuki group Sz( q ) over the field with q elements, where q is a non-square power of 2.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the Suzuki group itself.

      gap> SuzukiGroup( 32 );
      Sz(32)

- `ReeGroup( [`filt`, ]` q`) F`
- `Ree( [`filt`, ]` q`) F`

  Constructs a group isomorphic to the Ree group 2G2(q) where q=31+2m for m a non-negative integer.

  If filt is not given it defaults to `IsMatrixGroup` and the generating matrices are based on KLM01. (No particular choice of a generating set is guaranteed.)

      gap> ReeGroup( 27 );
      Ree(27)

  ## 48.2 Classical Groups

  The following functions return classical groups. For the linear, symplectic, and unitary groups (the latter in dimension at least 3), the generators are taken from Tay87; for the unitary groups in dimension 2, the isomorphism of SU(2,q) and SL(2,q) is used, see for example Hup67. The generators of the orthogonal groups are taken from IshibashiEarnest94 and KleidmanLiebeck90, except that the generators of the orthogonal groups in odd dimension in even characteristic are constructed via the isomorphism to a symplectic group, see for example Car72a.

  For symplectic and orthogonal matrix groups returned by the functions described below, the invariant bilinear form is stored as the value of the attribute `InvariantBilinearForm` (see InvariantBilinearForm). Analogously, the invariant sesquilinear form defining the unitary groups is stored as the value of the attribute `InvariantSesquilinearForm` (see InvariantSesquilinearForm). The defining quadratic form of orthogonal groups is stored as the value of the attribute `InvariantQuadraticForm` (see InvariantQuadraticForm).

- `GeneralLinearGroup( [`filt`, ]`d`,` R`) F`
- `GL( [`filt`, ]`d`,` R`) F`
- `GeneralLinearGroup( [`filt`, ]`d`,` q`) F`
- `GL( [`filt`, ]`d`,` q`) F`

  The first two forms construct a group isomorphic to the general linear group GL( d, R ) of all d ×d matrices that are invertible over the ring R, in the category given by the filter filt.

  The third and the fourth form construct the general linear group over the finite field with q elements.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the general linear group as a matrix group in its natural action (see also IsNaturalGL, IsNaturalGLnZ).

  Currently supported rings R are finite fields, the ring `Integers`, and residue class rings `Integers mod` m``.

      gap> GL(4,3);
      GL(4,3)
      gap> GL(2,Integers);
      GL(2,Integers)
      gap> GL(3,Integers mod 12);
      GL(3,Z/12Z)

- `SpecialLinearGroup( [`filt`, ]`d`,` R`) F`
- `SL( [`filt`, ]`d`,` R`) F`
- `SpecialLinearGroup( [`filt`, ]`d`,` q`) F`
- `SL( [`filt`, ]`d`,` q`) F`

  The first two forms construct a group isomorphic to the special linear group SL( d, R ) of all those d ×d matrices over the ring R whose determinant is the identity of R, in the category given by the filter filt.

  The third and the fourth form construct the special linear group over the finite field with q elements.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the special linear group as a matrix group in its natural action (see also IsNaturalSL, IsNaturalSLnZ).

  Currently supported rings R are finite fields, the ring `Integers`, and residue class rings `Integers mod` m``.

      gap> SpecialLinearGroup(2,2);
      SL(2,2)
      gap> SL(3,Integers);
      SL(3,Integers)
      gap> SL(4,Integers mod 4);
      SL(4,Z/4Z)

  Using the `OnLines` operation it is possible to obtain the corresponding projective groups in a permutation action:

      gap> g:=GL(4,3);;Size(g);
      24261120
      gap> pgl:=Action(g,Orbit(g,Z(3)^0*[1,0,0,0],OnLines),OnLines);;
      gap> Size(pgl);
      12130560

- `GeneralUnitaryGroup( [`filt`, ]`d`,` q`) F`
- `GU( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the general unitary group GU( d, q ) of those d ×d matrices over the field with q 2 elements that respect a fixed nondegenerate sesquilinear form, in the category given by the filter filt.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the general unitary group itself.

      gap> GeneralUnitaryGroup( 3, 5 );
      GU(3,5)

- `SpecialUnitaryGroup( [`filt`, ]`d`,` q`) F`
- `SU( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the special unitary group `GU(`d`,` q`)` of those d ×d matrices over the field with q 2 elements whose determinant is the identity of the field and that respect a fixed nondegenerate sesquilinear form, in the category given by the filter filt.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the special unitary group itself.

      gap> SpecialUnitaryGroup( 3, 5 );
      SU(3,5)

- `SymplecticGroup( [`filt`, ]`d`,` q`) F`
- `Sp( [`filt`, ]`d`,` q`) F`
- `SP( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the symplectic group Sp( d, q ) of those d ×d matrices over the field with q elements that respect a fixed nondegenerate symplectic form, in the category given by the filter filt.

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the symplectic group itself.

      gap> SymplecticGroup( 4, 2 );
      Sp(4,2)

- `GeneralOrthogonalGroup( [`filt`, ][`e`, ]`d`,` q`) F`
- `GO( [`filt`, ][`e`, ]`d`,` q`) F`

  constructs a group isomorphic to the general orthogonal group GO( e, d, q ) of those d ×d matrices over the field with q elements that respect a non-singular quadratic form (see InvariantQuadraticForm) specified by e, in the category given by the filter filt.

  The value of e must be 0 for odd d (and can optionally be omitted in this case), respectively one of 1 or −1 for even d. If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the general orthogonal group itself.

  Note that in KleidmanLiebeck90, GO is defined as the stabilizer ∆(V,F,κ) of the quadratic form, up to scalars, whereas our GO is called I(V,F,κ) there.

- `SpecialOrthogonalGroup( [`filt`, ][`e`, ]`d`,` q`) F`
- `SO( [`filt`, ][`e`, ]`d`,` q`) F`

  `SpecialOrthogonalGroup` returns a group isomorphic to the special orthogonal group SO( e, d, q ), which is the subgroup of all those matrices in the general orthogonal group (see GeneralOrthogonalGroup) that have determinant one, in the category given by the filter filt. (The index of SO( e, d, q ) in GO( e, d, q ) is 2 if q is odd, and 1 if q is even.)

  If filt is not given it defaults to `IsMatrixGroup`, and the returned group is the special orthogonal group itself.

      gap> GeneralOrthogonalGroup( 3, 7 );
      GO(0,3,7)
      gap> GeneralOrthogonalGroup( -1, 4, 3 );
      GO(-1,4,3)
      gap> SpecialOrthogonalGroup( 1, 4, 4 );
      GO(+1,4,4)

- `ProjectiveGeneralLinearGroup( [`filt`, ]`d`,` q`) F`
- `PGL( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the projective general linear group PGL( d, q ) of those d ×d matrices over the field with q elements, modulo the centre, in the category given by the filter filt.

  If filt is not given it defaults to `IsPermGroup`, and the returned group is the action on lines of the underlying vector space.

- `ProjectiveSpecialLinearGroup( [`filt`, ]`d`,` q`) F`
- `PSL( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the projective special linear group PSL( d, q ) of those d ×d matrices over the field with q elements whose determinant is the identity of the field, modulo the centre, in the category given by the filter filt.

  If filt is not given it defaults to `IsPermGroup`, and the returned group is the action on lines of the underlying vector space.

- `ProjectiveGeneralUnitaryGroup( [`filt`, ]`d`,` q`) F`
- `PGU( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the projective general unitary group PGU( d, q ) of those d ×d matrices over the field with q 2 elements that respect a fixed nondegenerate sesquilinear form, modulo the centre, in the category given by the filter filt.

  If filt is not given it defaults to `IsPermGroup`, and the returned group is the action on lines of the underlying vector space.

- `ProjectiveSpecialUnitaryGroup( [`filt`, ]`d`,` q`) F`
- `PSU( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the projective special unitary group PSU( d, q ) of those d ×d matrices over the field with q 2 elements that respect a fixed nondegenerate sesquilinear form and have determinant 1, modulo the centre, in the category given by the filter filt.

  If filt is not given it defaults to `IsPermGroup`, and the returned group is the action on lines of the underlying vector space.

- `ProjectiveSymplecticGroup( [`filt`, ]`d`,` q`) F`
- `PSP( [`filt`, ]`d`,` q`) F`
- `PSp( [`filt`, ]`d`,` q`) F`

  constructs a group isomorphic to the projective symplectic group PSp(d,q) of those d ×d matrices over the field with q elements that respect a fixed nondegenerate symplectic form, modulo the centre, in the category given by the filter filt.

  If filt is not given it defaults to `IsPermGroup`, and the returned group is the action on lines of the underlying vector space.

  ## 48.3 Conjugacy Classes in Classical Groups

  For general and special linear groups (see GeneralLinearGroup and SpecialLinearGroup) GAP has an efficient method to generate representatives of the conjugacy classes. This uses results from linear algebra on normal forms of matrices. If you know how to do this for other types of classical groups, please, tell us.

      gap> g := SL(4,9);
      SL(4,9)
      gap> NrConjugacyClasses(g);
      861
      gap> cl := ConjugacyClasses(g);;
      gap> Length(cl);
      861

- `NrConjugacyClassesGL(` n`,` q`) F`
- `NrConjugacyClassesGU(` n`,` q`) F`
- `NrConjugacyClassesSL(` n`,` q`) F`
- `NrConjugacyClassesSU(` n`,` q`) F`
- `NrConjugacyClassesPGL(` n`,` q`) F`
- `NrConjugacyClassesPGU(` n`,` q`) F`
- `NrConjugacyClassesPSL(` n`,` q`) F`
- `NrConjugacyClassesPSU(` n`,` q`) F`
- `NrConjugacyClassesSLIsogeneous(` n`,` q`,` f`) F`
- `NrConjugacyClassesSUIsogeneous(` n`,` q`,` f`) F`

  The first of these functions compute for given n ∈ N and prime power q the number of conjugacy classes in the classical groups GL( n , q ), GU( n , q ), SL( n , q ), SU( n , q ), PGL( n , q ), PGU( n , q ), PSL( n , q ), PSL( n , q ), respectively. (See also ConjugacyClasses!attribute and Section Classical Groups.)

  For each divisor f of n there is a group of Lie type with the same order as SL( n , q ), such that its derived subgroup modulo its center is isomorphic to PSL( n , q ). The various such groups with fixed n and q are called isogeneous. (Depending on congruence conditions on q and n several of these groups may actually be isomorphic.) The function `NrConjugacyClassesSLIsogeneous` computes the number of conjugacy classes in this group. The extreme cases f = 1 and f = n lead to the groups SL( n , q ) and PGL( n , q ), respectively.

  The function `NrConjugacyClassesSUIsogeneous` is the analogous one for the corresponding unitary groups.

  The formulae for the number of conjugacy classes are taken from Mac81.

      gap> NrConjugacyClassesGL(24,27);
      22528399544939174406067288580609952
      gap> NrConjugacyClassesPSU(19,17);
      15052300411163848367708
      gap> NrConjugacyClasses(SL(16,16));
      1229782938228219920

  ## 48.4 Constructors for Basic Groups

  All functions described in the previous sections call constructor operations to do the work. The names of the constructors are obtained from the names of the functions by appending `Cons`, so for example `CyclicGroup` calls the constructor

  `CyclicGroupCons(` cat`,` n`) O`

  The first argument cat for each method of this constructor must be the category for which the method is installed. For example the method for constructing a cyclic permutation group is installed as follows (see InstallMethod in ``Programming in GAP'' for the meaning of the arguments of `InstallMethod`).

      InstallMethod( CyclicGroupCons,
          "regular perm group",
          true,
          [ IsPermGroup and IsRegularProp and IsFinite, IsInt and IsPosRat ], 0,
          function( filter, n )

          ...

          end );

  ## 48.5 Selection Functions

- `All` Library `Groups(` fun1`,` val1`, ... )`

  For a number of the group libraries two selection functions are provided. Each `All` Library `Groups` selection function permits one to select all groups from the library Library that have a given set of properties. Currently, the library selection functions provided, of this type, are `AllSmallGroups`, `AllIrreducibleSolvableGroups`, `AllTransitiveGroups`, and `AllPrimitiveGroups`. Corresponding to each of these there is a `One` Library `Group` function (see OneLibraryGroup) which returns at most one group.

  These functions take an arbitrary number of pairs (but at least one pair) of arguments. The first argument in such a pair is a function that can be applied to the groups in the library, and the second argument is either a single value that this function must return in order to have this group included in the selection, or a list of such values. For the function `AllSmallGroups` the first such function must be `Size`, and, unlike the other library selection functions, it supports an alternative syntax where `Size` is omitted (see AllSmallGroups). Also, see AllIrreducibleSolvableGroups, for details pertaining to `AllIrreducibleSolvableGroups`.

  For an example, let us consider the selection function for the library of transitive groups (also see Transitive permutation groups). The command,

      gap> AllTransitiveGroups(NrMovedPoints,[10..15],
      >                        Size,         [1..100],
      >                        IsAbelian,    false    );

  returns a list of all transitive groups with degree between 10 and 15 and size less than 100 that are not abelian.

  Thus the `AllTransitiveGroups` behaves as if it was implemented by a function similar to the one defined below, where `TransitiveGroupsList` is a list of all transitive groups. (Note that in the definition below we assume for simplicity that `AllTransitiveGroups` accepts exactly 4 arguments. It is of course obvious how to change this definition so that the function would accept a variable number of arguments.)

      AllTransitiveGroups := function( fun1, val1, fun2, val2 )
      local    groups, g, i;
        groups := [];
        for i  in [ 1 .. Length( TransitiveGroupsList ) ] do
          g := TransitiveGroupsList[i];
          if      fun1(g) = val1  or IsList(val1) and fun1(g) in val1
              and fun2(g) = val2  or IsList(val2) and fun2(g) in val2
           then
            Add( groups, g );
          fi;
        od;
        return groups;
      end;

  Note that the real selection functions are considerably more difficult, to improve the efficiency. Most important, each recognizes a certain set of properties which are precomputed for the library without having to compute them anew for each group. This will substantially speed up the selection process. In the description of each library we will list the properties that are stored for this library.

- `One` Library `Group(` fun1`,` val1`, ... )`

  For each `All` Library `Groups` function (see AllLibraryGroups) there is a corresponding function `One` Library `Group` on exactly the same arguments, i.e. there are `OneSmallGroup`, `OneIrreducibleSolvableGroup`, `OneTransitiveGroup`, and `OnePrimitiveGroup`. Each function simply returns the first group in the library (in the stored order) that has the prescribed properties, instead of all such groups. It returns `fail` if no such group exists in the library.

  ## 48.6 Transitive Permutation Groups

  The transitive groups library currently contains representatives for all transitive permutation groups of degree at most 30. Two permutations groups of the same degree are considered to be equivalent, if there is a renumbering of points, which maps one group into the other one. In other words, if they lie in the same conjugacy class under operation of the full symmetric group by conjugation.

- `TransitiveGroup(` deg`,` nr`) F`

  returns the nr-th transitive group of degree deg. Both deg and nr must be positive integers. The transitive groups of equal degree are sorted with respect to their size, so for example `TransitiveGroup(` deg`, 1 )` is a transitive group of degree and size deg, e.g, the cyclic group of size deg, if deg is a prime.

- `NrTransitiveGroups(` deg`) F`

  returns the number of transitive groups of degree deg stored in the library of transitive groups. The function returns `fail` if deg is beyond the range of the library.

  The selection functions (see Selection functions) for the transitive groups library are `AllTransitiveGroups` and `OneTransitiveGroup`. They obtain the following properties from the database without having to compute them anew:

  `NrMovedPoints`, `Size`, `Transitivity`, and `IsPrimitive`.

  This library was computed by Gregory Butler, John McKay, Gordon Royle and Alexander Hulpke. The list of transitive groups up to degree 11 was published in BM83, the list of degree 12 was published in Roy87, degree 14 and 15 were published in Butler93 and degrees 16--30 were published in Hulpke96 and HulpkeTG. (Groups of prime degree of course are primitive and were known long before.)

  The arrangement and the names of the groups of degree up to 15 is the same as given in ConwayHulpkeMcKay98. With the exception of the symmetric and alternating group (which are represented as `SymmetricGroup` and `AlternatingGroup`) the generators for these groups also conform to this paper with the only difference that 0 (which is not permitted in GAP for permutations to act on) is always replaced by the degree.

      gap> TransitiveGroup(10,22);
      S(5)[x]2
      gap> l:=AllTransitiveGroups(NrMovedPoints,12,Size,1440,IsSolvable,false);
      [ S(6)[x]2, M_10.2(12)=A_6.E_4(12)=[S_6[1/720]{M_10}S_6]2 ]
      gap> List(l,IsSolvable);
      [ false, false ]

- `TransitiveIdentification(` G`) A`

  Let G be a permutation group, acting transitively on a set of up to 30 points. Then `TransitiveIdentification` will return the position of this group in the transitive groups library. This means, if G acts on m points and `TransitiveIdentification` returns n, then G is permutation isomorphic to the group `TransitiveGroup(m,n)`.

  Note: The points moved do not need to be [1..n], the group 〈(2,3,4),(2,3)〉 is considered to be transitive on 3 points. If the group has several orbits on the points moved by it the result of `TransitiveIdentification` is undefined.

      gap> TransitiveIdentification(Group((1,2),(1,2,3)));
      2

  ## 48.7 Small Groups

  The Small Groups library gives access to all groups of certain ``small'' orders. The groups are sorted by their orders and they are listed up to isomorphism; that is, for each of the available orders a complete and irredundant list of isomorphism type representatives of groups is given. Currently, the library contains the following groups:

  - those of order at most 2000 except 1024 (423 164 062 groups);
  - those of cubefree order at most 50 000 (395 703 groups);
  - those of order pn for n ≤ 6 and all primes p
  - those of order qn ·p for qn dividing 28, 36, 55 or 74 and all primes p with p ≠ q;
  - those of squarefree order;
  - those whose order factorises into at most 3 primes.
  The first two items in this list cover an explicit range of orders; the last four provide access to infinite families of groups having orders of certain types.

  The library also has an identification function: it returns the library number of a given group. This function determines library numbers using invariants of groups. The function is available for all orders in the library except 512, 1536, p6 for p > 3 and p5 for p > 5.

  The library is organised in 10 layers. Each layer contains the groups of certain orders and their corresponding group identification routines. It is possible to install the first n layers of the group library and the first m layers of the group identification for each 1 ≤ m ≤ n ≤ 10. This might be useful to save disk space. There is an extensive `README` file for the Small Groups library available in the `small` directory of the GAP distribution containing detailed information on the layers. A brief description of the layers is given here:

  1. the groups whose order factorises into at most 3 primes.
  2. the remaining groups of order at most 1000 except 512 and 768.
  3. the remaining groups of order 2n * p with n ≤ 8 and p an odd prime.
  4. the remaining groups of order 55, 74 and of order qn ·p for qn dividing 36, 55 or 74 and p ≠ q a prime.
  5. the remaining groups of order at most 2000 except 1024, 1152, 1536 and 1920.
  6. the groups of orders 1152 and 1920.
  7. the groups of order 512.
  8. the groups of order 1536.
  9. the remaining groups of order pn for 4 ≤ n ≤ 6.
  10. the remaining groups of cubefree order at most 50 000 and of squarefree order.

  The data in this library has been carefully checked and cross-checked. It is believed to be reliable. However, no absolute guarantees are given and users should, as always, make their own checks in critical cases.

  The data occupies about 30 MB (storing over 400 million groups in about 200 megabits). The group identification occupies about 47 MB of which 18 MB is used for the groups in layer (6). More information on the Small Groups library can be found on http://www.tu-bs.de/~hubesche/small.html

  This library has been constructed by Hans Ulrich Besche, Bettina Eick and E. A. O'Brien. A survey on this topic and an account of the history of group constructions can be found in BEO01. Further detailed information on the construction of this library is available in New77, OBr90, OBr91, BescheEick98, BescheEick1000, BescheEick768, BEO00, EOB99, EOB98, NOV04, Gir03, DEi05.

  The Small Groups library incorporates the GAP 3 libraries `TwoGroup` and `ThreeGroup`. The data from these libraries was directly included into the Small Groups library, and the ordering there was preserved. The Small Groups library replaces the Gap 3 library of solvable groups of order at most 100. However, both the organisation and data descriptions of these groups has changed in the Small Groups library.

- `SmallGroup(` order`,` i`) F`
- `SmallGroup( [`order`,` i`] ) F`

  returns the i-th group of order order in the catalogue. If the group is solvable, it will be given as a PcGroup; otherwise it will be given as a permutation group. If the groups of order order are not installed, the function reports an error and enters a break loop.

- `AllSmallGroups(` arg`) F`

  returns all groups with certain properties as specified by arg. If arg is a number n, then this function returns all groups of order n. However, the function can also take several arguments which then must be organized in pairs `function` and `value`. In this case the first function must be `Size` and the first value an order or a range of orders. If value is a list then it is considered a list of possible function values to include. The function returns those groups of the specified orders having those properties specified by the remaining functions and their values.

  Precomputed information is stored for the properties `IsAbelian`, `IsNilpotentGroup`, `IsSupersolvableGroup`, `IsSolvableGroup`, `RankPGroup`, `PClassPGroup`, `LGLength`, `FrattinifactorSize` and `FrattinifactorId` for the groups of order at most 2000 which have more than three prime factors, except those of order 512, 768, 1024, 1152, 1536, 1920 and those of order pn ·q > 1000 with n > 2.

- `OneSmallGroup(` arg`) F`

  returns one group with certain properties as specified by arg. The permitted arguments are those supported by `AllSmallGroups`.

- `NumberSmallGroups(` order`) F`

  returns the number of groups of order order.

- `IdSmallGroup(` G`) A`
- `IdGroup(` G`) A`

  returns the library number of G; that is, the function returns a pair `[`order`,` i`]` where G is isomorphic to `SmallGroup(` order`,` i`)`.

- `IdsOfAllSmallGroups(` arg`) F`

  similar to `AllSmallGroups` but returns ids instead of groups. This may prevent workspace overflows, if a large number of groups are expected in the output.

- `IdGap3SolvableGroup(` G`) A`
- `Gap3CatalogueIdGroup(` G`) A`

  returns the catalogue number of G in the GAP 3 catalogue of solvable groups; that is, the function returns a pair `[`order`,` i`]` meaning that G is isomorphic to the group `SolvableGroup(` order`,` i`)` in GAP 3.

- `SmallGroupsInformation(` order`) F`

  prints information on the groups of the specified order.

- `UnloadSmallGroupsData( ) F`

  GAP loads all necessary data from the library automatically, but it does not delete the data from the workspace again. Usually, this will be not necessary, since the data is stored in a compressed format. However, if a large number of groups from the library have been loaded, then the user might wish to remove the data from the workspace and this can be done by the above function call.

      gap> G := SmallGroup( 768, 1000000 );
      <pc group of size 768 with 9 generators>
      gap> G := SmallGroup( [768, 1000000] );
      <pc group of size 768 with 9 generators>

      gap> AllSmallGroups( 6 );
      [ <pc group of size 6 with 2 generators>,
        <pc group of size 6 with 2 generators> ]
      gap> AllSmallGroups( Size, 120, IsSolvableGroup, false );
      [ Group([ (1,2,4,8)(3,6,9,5)(7,12,13,17)(10,14,11,15)(16,20,21,24)(18,22,19,
              23), (1,3,7)(2,5,10)(4,9,13)(6,11,8)(12,16,20)(14,18,22)(15,19,23)(17,
              21,24) ]), Group([ (1,2,3,4,5), (1,2) ]),
        Group([ (1,2,3,5,4), (1,3)(2,4)(6,7) ]) ]

      gap> G := OneSmallGroup( 120, IsNilpotentGroup, false );
      <pc group of size 120 with 5 generators>
      gap> IdSmallGroup(G);
      [ 120, 1 ]
      gap> G := OneSmallGroup( Size, [1..1000], IsSolvableGroup, false );
      Group([ (1,2,3,4,5), (1,2,3) ])
      gap> IdSmallGroup(G);
      [ 60, 5 ]
      gap> UnloadSmallGroupsData();

      gap> IdSmallGroup( GL( 2,3 ) );
      [ 48, 29 ]
      gap> IdSmallGroup( Group( (1,2,3,4),(4,5) ) );
      [ 120, 34 ]
      gap> IdsOfAllSmallGroups( Size, 60, IsSupersolvableGroup, true );
      [ [ 60, 1 ], [ 60, 2 ], [ 60, 3 ], [ 60, 4 ], [ 60, 6 ], [ 60, 7 ],
        [ 60, 8 ], [ 60, 10 ], [ 60, 11 ], [ 60, 12 ], [ 60, 13 ] ]

      gap> NumberSmallGroups( 512 );
      10494213
      gap> NumberSmallGroups( 2^8 * 23 );
      1083472

      gap> NumberSmallGroups( 2^9 * 23 );
      Error, the library of groups of size 11776 is not available called from
      <function>( <arguments> ) called from read-eval-loop
      Entering break read-eval-print loop ...
      you can 'quit;' to quit to outer loop, or
      you can 'return;' to continue
      brk> quit;
      gap>

      gap> SmallGroupsInformation( 32 );

        There are 51 groups of order 32.
        They are sorted by their ranks.
           1 is cyclic.
           2 - 20 have rank 2.
           21 - 44 have rank 3.
           45 - 50 have rank 4.
           51 is elementary abelian.

        For the selection functions the values of the following attributes
        are precomputed and stored:
           IsAbelian, PClassPGroup, RankPGroup, FrattinifactorSize and
           FrattinifactorId.

        This size belongs to layer 2 of the SmallGroups library.
        IdSmallGroup is available for this size.

  ## 48.8 Finite Perfect Groups

  The GAP library of finite perfect groups provides, up to isomorphism, a list of all perfect groups whose sizes are less than 106 excluding the following sizes:

  - For n = 61440, 122880, 172032, 245760, 344064, 491520, 688128, or 983040, the perfect groups of size n have not completely been determined yet. The library neither provides the number of these groups nor the groups themselves.
  - For n = 86016, 368640, or 737280, the library does not yet contain the perfect groups of size n, it only provides their numbers which are 52, 46, and 54, respectively.

  Except for these eleven sizes, the list of altogether 1097 perfect groups in the library is complete. It relies on results of Derek F. Holt and Wilhelm Plesken which are published in their book ``Perfect Groups'' HP89. Moreover, they have supplied us with files with presentations of 488 of the groups. In terms of these, the remaining 607 nontrivial groups in the library can be described as 276 direct products, 107 central products, and 224 subdirect products. They are computed automatically by suitable GAP functions whenever they are needed. Two additional groups omitted from the book ``Perfect Groups'' have also been included.

  We are grateful to Derek Holt and Wilhelm Plesken for making their groups available to the GAP community by contributing their files. It should be noted that their book contains a lot of further information for many of the library groups. So we would like to recommend it to any GAP user who is interested in the groups.

  The library has been brought into GAP format by Volkmar Felsch.

- `SizesPerfectGroups( ) F`

- `PerfectGroup( [`filt`, ]`size`[,` n`] ) F`
- `PerfectGroup( [`filt`, ]`sizenumberpair`) F`

  returns a group which is isomorphic to the library group specified by the size number `[` size`,` n`]` or by the two separate arguments size and n, assuming a default value of n = 1. The optional argument filt defines the filter in which the group is returned. Possible filters so far are `IsPermGroup` and `IsSubgroupFpGroup`. In the latter case, the generators and relators used coincide with those given in HP89.

      gap> G := PerfectGroup(IsPermGroup,6048,1);
      U3(3)

  As all groups are stored by presentations, a permutation representation is obtained by coset enumeration. Note that some of the library groups do not have a faithful permutation representation of small degree. Computations in these groups may be rather time consuming.

      gap> G:=PerfectGroup(IsPermGroup,823080,2);
      A5 2^1 19^2 C 19^1
      gap> NrMovedPoints(G);
      6859

- `PerfectIdentification(` G`) A`

  This attribute is set for all groups obtained from the perfect groups library and has the value `[`size`,`nr`]` if the group is obtained with these parameters from the library.

- `NumberPerfectGroups(` size`) F`

  returns the number of non-isomorphic perfect groups of size size for each positive integer size up to 106 except for the eight sizes listed at the beginning of this section for which the number is not yet known. For these values as well as for any argument out of range it returns `fail`.

- `NumberPerfectLibraryGroups(` size`) F`

  returns the number of perfect groups of size size which are available in the library of finite perfect groups. (The purpose of the function is to provide a simple way to formulate a loop over all library groups of a given size.)

- `SizeNumbersPerfectGroups(` factor1`,` factor2`, ... ) F`

  `SizeNumbersPerfectGroups` returns a list of pairs, each entry consisting of a group order and the number of those groups in the library of perfect groups that contain the specified factors factor1, factor2, ... among their composition factors.

  Each argument must either be the name of a simple group or an integer which stands for the product of the sizes of one or more cyclic factors. (In fact, the function replaces all integers among the arguments by their product.)

  The following text strings are accepted as simple group names.

  - `A` n`` or `A(`n`)` for the alternating groups An, 5 ≤ n ≤ 9, for example `A5` or `A(6)`.
  - `L` n`(`q`)` or `L(`n`,`q`)` for PSL(n,q), where n ∈ {2,3} and q a prime power, ranging
    - for n=2 from 4 to 125
    - for n=3 from 2 to 5
  - `U` n`(`q`)` or `U(`n`,`q`)` for PSU(n,q), where n ∈ {3,4} and q a prime power, ranging
    - for n=3 from 3 to 5
    - for n=4 from 2 to 2
  - `Sp4(4)` or `S(4,4)` for the symplectic group S(4,4),
  - `Sz(8)` for the Suzuki group Sz(8),
  - `M` n`` or `M(`n`)` for the Mathieu groups M11, M12, and M22, and
  - `J` n`` or `J(`n`)` for the Janko groups J1 and J2.

  Note that, for most of the groups, the preceding list offers two different names in order to be consistent with the notation used in HP89 as well as with the notation used in the `DisplayCompositionSeries` command of GAP. However, as the names are compared as text strings, you are restricted to the above choice. Even expressions like `L2(2^5)` are not accepted.

  As the use of the term PSU(n,q) is not unique in the literature, we mention that in this library it denotes the factor group of SU(n,q) by its centre, where SU(n,q) is the group of all n ×n unitary matrices with entries in GF(q2) and determinant 1.

  The purpose of the function is to provide a simple way to formulate a loop over all library groups which contain certain composition factors.

- `DisplayInformationPerfectGroups(` size`) F`
- `DisplayInformationPerfectGroups(` size`,` n`) F`
- `DisplayInformationPerfectGroups( [` size`,` n`] ) F`

  `DisplayInformationPerfectGroups` displays some invariants of the n-th group of order size from the perfect groups library.

  If no value of n has been specified, the invariants will be displayed for all groups of size size available in the library. The information provided for G includes the following items:

  - a headline containing the size number `[` size`,` n`]` of G in the form ``size`.`n`` (the suffix `.`n`` will be suppressed if, up to isomorphism, G is the only perfect group of order size),
  - a message if G is simple or quasisimple, i.e., if the factor group of G by its centre is simple,
  - the ``description'' of the structure of G as it is given by Holt and Plesken in HP89 (see below),
  - the size of the centre of G (suppressed, if G is simple),
  - the prime decomposition of the size of G,
  - orbit sizes for a faithful permutation representation of G which is provided by the library (see below),
  - a reference to each occurrence of G in the tables of section 5.3 of HP89. Each of these references consists of a class number and an internal number (i,j) under which G is listed in that class. For some groups, there is more than one reference because these groups belong to more than one of the classes in the book.

      gap> DisplayInformationPerfectGroups( 30720, 3 );
      #I Perfect group 30720:  A5 ( 2^4 E N 2^1 E 2^4 ) A
      #I   size = 2^11*3*5  orbit size = 240
      #I   Holt-Plesken class 1 (9,3)
      gap> DisplayInformationPerfectGroups( 30720, 6 );
      #I Perfect group 30720:  A5 ( 2^4 x 2^4 ) C N 2^1
      #I   centre = 2  size = 2^11*3*5  orbit size = 384
      #I   Holt-Plesken class 1 (9,6)
      gap> DisplayInformationPerfectGroups( Factorial( 8 ) / 2 );
      #I Perfect group 20160.1:  A5 x L3(2) 2^1
      #I   centre = 2  size = 2^6*3^2*5*7  orbit sizes = 5 + 16
      #I   Holt-Plesken class 31 (1,1) (occurs also in class 32)
      #I Perfect group 20160.2:  A5 2^1 x L3(2)
      #I   centre = 2  size = 2^6*3^2*5*7  orbit sizes = 7 + 24
      #I   Holt-Plesken class 31 (1,2) (occurs also in class 32)
      #I Perfect group 20160.3:  ( A5 x L3(2) ) 2^1
      #I   centre = 2  size = 2^6*3^2*5*7  orbit size = 192
      #I   Holt-Plesken class 31 (1,3)
      #I Perfect group 20160.4:  simple group  A8
      #I   size = 2^6*3^2*5*7  orbit size = 8
      #I   Holt-Plesken class 26 (0,1)
      #I Perfect group 20160.5:  simple group  L3(4)
      #I   size = 2^6*3^2*5*7  orbit size = 21
      #I   Holt-Plesken class 27 (0,1)

  For any library group G, the library files do not only provide a presentation, but, in addition, a list of one or more subgroups S1, …, Sr of G such that there is a faithful permutation representation of G of degree ∑i=1r [G:Si] on the set { Si g | 1 ≤ i ≤ r, g ∈ G } of the cosets of the Si. This allows one to construct the groups as permutation groups. The `DisplayInformationPerfectGroups` function displays only the available degree. The message

      orbit size = 8

  in the above example means that the available permutation representation is transitive and of degree 8, whereas the message

      orbit sizes = 5 + 16

  means that a nontransitive permutation representation is available which acts on two orbits of size 5 and 16 respectively.

  The notation used in the ``description'' of a group is explained in section 5.1.2 of HP89. We quote the respective page from there:

  Within a class Q # p, an isomorphism type of groups will be denoted by an ordered pair of integers (r,n), where r ≥ 0 and n > 0. More precisely, the isomorphism types in Q # p of order pr |Q| will be denoted by (r,1), (r,2), (r,3), … . Thus Q will always get the size number (0,1).

  In addition to the symbol (r,n), the groups in Q # p will also be given a more descriptive name. The purpose of this is to provide a very rough idea of the structure of the group. The names are derived in the following manner. First of all, the isomorphism classes of irreducible FpQ-modules M with |Q|·|M| ≤ 106, where Fp is the field of order p, are assigned symbols. These will either be simply px, where x is the dimension of the module, or, if there is more than one isomorphism class of irreducible modules having the same dimension, they will be denoted by px, px′, etc. The one-dimensional module with trivial Q-action will therefore be denoted by p1. These symbols will be listed under the description of Q. The group name consists essentially of a list of the composition factors working from the top of the group downwards; hence it always starts with the name of Q itself. (This convention is the most convenient in our context, but it is different from that adopted in the ATLAS CCN85, for example, where composition factors are listed in the reverse order. For example, we denote a group isomorphic to SL(2,5) by A5 21 rather than 2·A5.)

  Some other symbols are used in the name, in order to give some idea of the relationship between these composition factors, and splitting properties. We shall now list these additional symbols.

  ×
    between two factors denotes a direct product of FpQ-modules or groups.

  C
    (for ``commutator'') between two factors means that the second lies in the commutator subgroup of the first. Similarly, a segment of the form (f1 × f2) C f3 would mean that the factors f1 and f2 commute modulo f3 and f3 lies in [f1,f2].

  A
    (for ``abelian'') between two factors indicates that the second is in the pth power (but not the commutator subgroup) of the first. ``A'' may also follow the factors, if bracketed.

  E
    (for ``elementary abelian'') between two factors indicates that together they generate an elementary abelian group (modulo subsequent factors), but that the resulting FpQ-module extension does not split.

  N
    (for ``nonsplit'') before a factor indicates that Q (or possibly its covering group) splits down as far at this factor but not over the factor itself. So ``Q f1 N f2'' means that the normal subgroup f1f2 of the group has no complement but, modulo f2, f1, does have a complement.

  Brackets have their obvious meaning. Summarizing, we have:

  ×
    = direct product;

  C
    = commutator subgroup;

  A
    = abelian;

  E
    = elementary abelian; and

  N
    = nonsplit.

  Here are some examples.

  1. A5 (24 E 21 E 24) A means that the pairs 24 E 21 and 21 E 24 are both elementary abelian of exponent 4.

  2. A5 (24 E 21 A) C 21 means that O2(G) is of symplectic type 21+5, with Frattini factor group of type 24 E 21. The ``A'' after the 21 indicates that G has a central cyclic subgroup 21 A 21 of order 4.

  3. L3(2) ((21 E) × (N 23 E 23′ A) C) 23′ means that the 23′ factor at the bottom lies in the commutator subgroup of the pair 23 E 23′ in the middle, but the lower pair 23′ A 23′ is abelian of exponent 4. There is also a submodule 21 E 23′, and the covering group L3(2) 21 of L3(2) does not split over the 23 factor. (Since G is perfect, it goes without saying that the extension L3(2) 21 cannot split itself.)

  We must stress that this notation does not always succeed in being precise or even unambiguous, and the reader is free to ignore it if it does not seem helpful.

  If such a group description has been given in the book for G (and, in fact, this is the case for most of the library groups), it is displayed by the `DisplayInformationPerfectGroups` function. Otherwise the function provides a less explicit description of the (in these cases unique) Holt-Plesken class to which G belongs, together with a serial number if this is necessary to make it unique.

  ## 48.9 Primitive Permutation Groups

  GAP contains a library of primitive permutation groups which includes the following permutation groups up to permutation isomorphism (i.e., up to conjugacy in the corresponding symmetric group)

  - all primitive permutation groups of degree < 2500, calculated in RoneyDougal05 in particular,
    - the primitive permutation groups up to degree 50, calculated by C. Sims,
    - the primitive groups with insoluble socles of degree < 1000 as calculated in DixonMortimer88,
    - the solvable (hence affine) primitive permutation groups of degree < 256 as calculated by M. Short Sho92,
    - some insolvable affine primitive permutation groups of degree < 256 as calculated in Theissen97.
    - The solvable primitive groups of degree up to 999 as calculated in EickHoefling02.
    - The primitive groups of affine type of degree up to 999 as calculated in RoneyDougal02.

  Not all groups are named, those which do have names use ATLAS notation. Not all names are necessary unique!

  The list given in RoneyDougal05 is believed to be complete, correcting various omissions in DixonMortimer88, Sho92 and Theissen97.

  In detail, we guarantee the following properties for this and further versions (but not versions which came before GAP 4.2) of the library:

  - All groups in the library are primitive permutation groups of the indicated degree.
  - The positions of the groups in the library are stable. That is `PrimitiveGroup(`n`,`nr`)` will always give you a permutation isomorphic group. Note however that we do not guarantee to keep the chosen Sn-representative, the generating set or the name for eternity.
  - Different groups in the library are not conjugate in Sn.
  - If a group in the library has a primitive subgroup with the same socle, this group is in the library as well.

  (Note that the arrangement of groups is not guaranteed to be in increasing size, though it holds for many degrees.)

- `PrimitiveGroup(` deg`,` nr`) F`

  returns the primitive permutation group of degree deg with number nr from the list.

  The arrangement of the groups differs from the arrangement of primitive groups in the list of C. Sims, which was used in GAP 3. See `SimsNo` (SimsNo).

- `NrPrimitiveGroups(` deg`) F`

  returns the number of primitive permutation groups of degree deg in the library.

      gap> NrPrimitiveGroups(25);
      28
      gap> PrimitiveGroup(25,19);
      5^2:((Q(8):3)'4)
      gap> PrimitiveGroup(25,20);
      ASL(2, 5)
      gap> PrimitiveGroup(25,22);
      AGL(2, 5)
      gap> PrimitiveGroup(25,23);
      (A(5) x A(5)):2

  The selection functions (see Selection functions) for the primitive groups library are `AllPrimitiveGroups` and `OnePrimitiveGroup`. They obtain the following properties from the database without having to compute them anew:

  `NrMovedPoints`, `Size`, `Transitivity`, `ONanScottType`, `IsSimpleGroup`, `IsSolvableGroup`, and `SocleTypePrimitiveGroup`.

  (Note, that for groups of degree up to 2499, O'Nan-Scott types 4a, 4b and 5 cannot occur.)

- `PrimitiveGroupsIterator(` attr1`,` val1`,` attr2`,` val2`, ... ) F`

  returns an iterator through `AllPrimitiveGroups(`attr1`,`val1`,`attr2`,`val2`,...)` without creating all these groups at the same time.

- `COHORTS_PRIMITIVE_GROUPS V`

  In DixonMortimer88 the primitive groups are sorted in ``cohorts'' according to their socle. For each degree, the variable `COHORTS_PRIMITIVE_GROUPS` contains a list of the cohorts for the primitive groups of this degree. Each cohort is represented by a list of length 2, the first entry specifies the socle type (see `SocleTypePrimitiveGroup`, section SocleTypePrimitiveGroup), the second entry listing the index numbers of the groups in this degree.

  For example in degree 49, we have four cohorts with socles F72, L2(7)2, A72 and A49 respectively. the group `PrimitiveGroup(49,36)`, which is isomorphic to (A7×A7):22, lies in the third cohort with socle (A7×A7).

      gap> COHORTS_PRIMITIVE_GROUPS[49];
      [ [ rec( series := "Z", parameter := 7, width := 2 ),
            [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
                20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 ] ],
        [ rec( series := "L", parameter := [ 2, 7 ], width := 2 ), [ 34 ] ],
        [ rec( series := "A", parameter := 7, width := 2 ), [ 35, 36, 37, 38 ] ],
        [ rec( series := "A", parameter := 49, width := 1 ), [ 39, 40 ] ] ]

  ## 48.10 Index numbers of primitive groups

- `PrimitiveIdentification(` G`) A`

  For a primitive permutation group for which an Sn-conjugate exists in the library of primitive permutation groups (see Primitive Permutation Groups), this attribute returns the index position. That is G is conjugate to `PrimitiveGroup(NrMovedPoints(`G`),PrimitiveIdentification(`G`))`.

  Methods only exist if the primitive groups library is installed.

  Note: As this function uses the primitive groups library, the result is only guaranteed to the same extent as this library. If it is incomplete, `PrimitiveIdentification` might return an existing index number for a group not in the library.

      gap> PrimitiveIdentification(Group((1,2),(1,2,3)));
      2

- `SimsNo(` G`) A`

  If G is a primitive group obtained by `PrimitiveGroup` (respectively one of the selection functions) this attribute contains the number of the isomorphic group in the original list of C. Sims. (this is the arrangement as it was used in GAP 3.

      gap> g:=PrimitiveGroup(25,2);
      5^2:S(3)
      gap> SimsNo(g);
      3

  As mentioned in the previous section, the index numbers of primitive groups in GAP are guaranteed to remain stable. (Thus, missing groups will be added to the library at the end of each degree.) In particular, it is safe to refer to a primitive group of type deg,nr in the GAP library.

  The system `Magma` also provides a list of primitive groups (see RoneyDougal02). For historical reasons, its indexing up to degree 999 differs from the one used by GAP. The variable

- `PRIMITIVE_INDICES_MAGMA V`

  can be used to obtain this correspondence. The magma index number of the GAP group `PrimitiveGroup(`deg`,`nr`)` is stored in the entry `PRIMITIVE_INDICES_MAGMA[`deg`][`nr`]`, for degree at most 999.

  Vice versa, the group of degree deg with `Magma` index number nr has the GAP index

  `Position(PRIMITIVE_INDICES_MAGMA[`deg`],`nr`)`, in particular it can be obtained by the GAP command

  `PrimitiveGroup(`deg`,Position(PRIMITIVE_INDICES_MAGMA[`deg`],`nr`));`

  ## 48.11 Irreducible Solvable Matrix Groups

- `IrreducibleSolvableGroupMS(` n`,` p`,` i`) F`

  This function returns a representative of the i-th conjugacy class of irreducible solvable subgroup of GL(n,p), where n is an integer > 1, p is a prime, and p n < 256.

  The numbering of the representatives should be considered arbitrary. However, it is guaranteed that the i-th group on this list will lie in the same conjugacy class in all future versions of GAP, unless two (or more) groups on the list are discovered to be duplicates, in which case `IrreducibleSolvableMatrixGroup` will return `fail` for all but one of the duplicates.

  For values of n, p, and i admissible to `IrreducibleSolvableGroup`, `IrreducibleSolvableMatrixGroup` returns a representative of the same conjugacy class of subgroups of GL(n,p) as `IrreducibleSolvableGroup`. Note that it currently adds two more groups (missing from the original list by Mark Short) for n = 2, p = 13.

- `NumberIrreducibleSolvableGroups(` n`,` p`) F`

  This function returns the number of conjugacy classes of irreducible solvable subgroup of GL(n,p).

- `AllIrreducibleSolvableGroups(` func_1`,` val_1`,` func_2`,` val_2`, ... ) F`

  This function returns a list of conjugacy class representatives G of matrix groups over a prime field such that funci (G) = vali or funci (G) ∈ vali . The following possibilities for func_i are particularly efficient, because the values can be read off the information in the data base: `DegreeOfMatrixGroup` (or `Dimension` or `DimensionOfMatrixGroup`) for the linear degree, `Characteristic` for the field characteristic, `Size`, `IsPrimitiveMatrixGroup` (or `IsLinearlyPrimitive`), and `MinimalBlockDimension`.

- `OneIrreducibleSolvableGroup(` func1`,` val1`,` func2`,` val2`, ... ) F`

  This function returns one solvable subgroup G of a matrix group over a prime field such that funci (G) = vali or funci (G) ∈ vali for all i. The following possibilities for func_i are particularly efficient, because the values can be read off the information in the data base: `DegreeOfMatrixGroup` (or `Dimension` or `DimensionOfMatrixGroup`) for the linear degree, `Characteristic` for the field characteristic, `Size`, `IsPrimitiveMatrixGroup` (or `IsLinearlyPrimitive`), and `MinimalBlockDimension`.

- `PrimitiveIndexIrreducibleSolvableGroup V`

  This variable provides a way to get from irreducible solvable groups to primitive groups and vice versa. For the group G=`IrreducibleSolvableGroup(` n`,` p`,` k`)` and d=pn, the entry `PrimitiveIndexIrreducibleSolvableGroup[d][i]` gives the index number of the semidirect product pn:G in the library of primitive groups.

  Searching for an index `Position` in this list gives the translation in the other direction.

- `IrreducibleSolvableGroup(` n`,` p`,` i`) F`

  This function is obsolete, because for n = 2, p = 13, two groups were missing from the underlying database. It has been replaced by the function `IrreducibleSolvableGroupMS` (see IrreducibleSolvableGroupMS). Please note that the latter function does not guarantee any ordering of the groups in the database. However, for values of n, p, and i admissible to `IrreducibleSolvableGroup`, `IrreducibleSolvableGroupMS` returns a representative of the same conjugacy class of subgroups of GL(n,p) as `IrreducibleSolvableGroup` did before.

  ## 48.12 Irreducible Maximal Finite Integral Matrix Groups

  A library of irreducible maximal finite integral matrix groups is provided with GAP. It contains Q-class representatives for all of these groups of dimension at most 31, and Z-class representatives for those of dimension at most 11 or of dimension 13, 17, 19, or 23.

  The groups provided in this library have been determined by Wilhelm Plesken, partially as joint work with Michael Pohst, or by members of his institute (Lehrstuhl B für Mathematik, RWTH Aachen). In particular, the data for the groups of dimensions 2 to 9 have been taken from the output of computer calculations which they performed in 1979 (see PP77, PP80). The Z-class representatives of the groups of dimension 10 have been determined and computed by Bernd Souvignier (Sou94), and those of dimensions 11, 13, and 17 have been recomputed for this library from the circulant Gram matrices given in Ple85, using the stand-alone programs for the computation of short vectors and Bravais groups which have been developed in Plesken's institute. The Z-class representatives of the groups of dimensions 19 and 23 had already been determined in Ple85. Gabriele Nebe has recomputed them for us. Her main contribution to this library, however, is that she has determined and computed the Q-class representatives of the groups of non-prime dimensions between 12 and 24 and the groups of dimensions 25 to 31 (see PN95, NP95, Neb95, Neb96).

  The library has been brought into GAP format by Volkmar Felsch. He has applied several GAP routines to check certain consistency of the data. However, the credit and responsibility for the lists remain with the authors. We are grateful to Wilhelm Plesken, Gabriele Nebe, and Bernd Souvignier for supplying their results to GAP.

  In the preceding acknowledgement, we used some notations that will also be needed in the sequel. We first define these.

  Any integral matrix group G of dimension n is a subgroup of GLn(Z) as well as of GLn(Q) and hence lies in some conjugacy class of integral matrix groups under GLn(Z) and also in some conjugacy class of rational matrix groups under GLn(Q). As usual, we call these classes the Z-class and the Q-class of G, respectively. Note that any conjugacy class of subgroups of GLn(Q) contains at least one Z-class of subgroups of GLn(Z) and hence can be considered as the Q-class of some integral matrix group.

  In the context of this library we are only concerned with Z-classes and Q-classes of subgroups of GLn(Z) which are irreducible and maximal finite in GLn(Z) (we will call them i.m.f. subgroups of GLn(Z)). We can distinguish two types of these groups:

  First, there are those i.m.f. subgroups of GLn(Z) which are also maximal finite subgroups of GLn(Q). Let us denote the set of their Q-classes by Q1(n). It is clear from the above remark that Q1(n) just consists of the Q-classes of i.m.f. subgroups of GLn(Q).

  Secondly, there is the set Q2(n) of the Q-classes of the remaining i.m.f. subgroups of GLn(Z), i.e., of those which are not maximal finite subgroups of GLn(Q). For any such group G, say, there is at least one class C ∈ Q1(n) such that G is conjugate under Q to a proper subgroup of some group H ∈ C. In fact, the class C is uniquely determined for any group G occurring in the library (though there seems to be no reason to assume that this property should hold in general). Hence we may call C the rational i.m.f. class of G. Finally, we will denote the number of classes in Q1(n) and Q2(n) by q1(n) and q2(n), respectively.

  As an example, let us consider the case n = 4. There are 6 Z-classes of i.m.f. subgroups of GL4(Z) with representative subgroups G1, …, G6 of isomorphism types G1 ≅ W(F4), G2 ≅ D12 wr C2, G3 ≅ G4 ≅ C2 ×S5, G5 ≅ W(B4), and G6 ≅ (D12 `Y` D12) : C2. The corresponding Q-classes, R1, …, R6, say, are pairwise different except that R3 coincides with R4. The groups G1, G2, and G3 are i.m.f. subgroups of GL4(Q), but G5 and G6 are not because they are conjugate under GL4(Q) to proper subgroups of G1 and G2, respectively. So we have Q1(4) = { R1, R2, R3 }, Q2(4) = { R5, R6 }, q1(4) = 3, and q2(4) = 2.

  The q1(n) Q-classes of i.m.f. subgroups of GLn(Q) have been determined for each dimension n ≤ 31. The current GAP library provides integral representative groups for all these classes. Moreover, all Z-classes of i.m.f. subgroups of GLn(Z) are known for n ≤ 11 and for n ∈ {13,17,19,23}. For these dimensions, the library offers integral representative groups for all Q-classes in Q1(n) and Q2(n) as well as for all Z-classes of i.m.f. subgroups of GLn(Z).

  Any group G of dimension n given in the library is represented as the automorphism group G = Aut(F,L) = { g ∈ GLn(Z) | Lg = L and g F gtr = F } of a positive definite symmetric n ×n matrix F ∈ Zn ×n on an n-dimensional lattice L ≅ Z1 ×n (for details see e.g. PN95). GAP provides for G a list of matrix generators and the Gram matrix F.

  The positive definite quadratic form defined by F defines a norm v F vtr for each vector v ∈ L, and there is only a finite set of vectors of minimal norm. These vectors are often simply called the short vectors. Their set splits into orbits under G, and G being irreducible acts faithfully on each of these orbits by multiplication from the right. GAP provides for each of these orbits the orbit size and a representative vector.

  Like most of the other GAP libraries, the library of i.m.f. integral matrix groups supplies an extraction function, `ImfMatrixGroup`. However, as the library involves only 525 different groups, there is no need for a selection or an example function. Instead, there are two functions, `ImfInvariants` and `DisplayImfInvariants`, which provide some Z-class invariants that can be extracted from the library without actually constructing the representative groups themselves. The difference between these two functions is that the latter one displays the resulting data in some easily readable format, whereas the first one returns them as record components so that you can properly access them.

  We shall give an individual description of each of the library functions, but first we would like to insert a short remark concerning their names: Any self-explaining name of a function handling irreducible maximal finite integral matrix groups would have to include this term in full length and hence would grow extremely long. Therefore we have decided to use the abbreviation `Imf` instead in order to restrict the names to some reasonable length.

  The first three functions can be used to formulate loops over the classes.

- `ImfNumberQQClasses(` dim`) F`
- `ImfNumberQClasses(` dim`) F`
- `ImfNumberZClasses(` dim`,` q`) F`

  `ImfNumberQQClasses` returns the number q1(dim) of Q-classes of i.m.f. rational matrix groups of dimension dim. Valid values of dim are all positive integers up to 31.

  Note: In order to enable you to loop just over the classes belonging to Q1(dim), we have arranged the list of Q-classes of dimension dim for any dimension dim in the library such that, whenever the classes of Q2(dim) are known, too, i.e., in the cases dim ≤ 11 or dim ∈ {13,17,19,23}, the classes of Q1(dim) precede those of Q2(dim) and hence are numbered from 1 to q1(dim).

  `ImfNumberQClasses` returns the number of Q-classes of groups of dimension dim which are available in the library. If dim ≤ 11 or dim ∈ {13,17,19,23}, this is the number q1(dim) + q2(dim) of Q-classes of i.m.f. subgroups of GLdim(Z). Otherwise, it is just the number q1(dim) of Q-classes of i.m.f. subgroups of GLdim(Q). Valid values of dim are all positive integers up to 31.

  `ImfNumberZClasses` returns the number of Z-classes in the qth Q-class of i.m.f. integral matrix groups of dimension dim. Valid values of dim are all positive integers up to 11 and all primes up to 23.

- `DisplayImfInvariants(` dim`,` q`) F`
- `DisplayImfInvariants(` dim`,` q`,` z`) F`

  `DisplayImfInvariants` displays the following Z-class invariants of the groups in the zth Z-class in the qth Q-class of i.m.f. integral matrix groups of dimension dim:

  - its Z-class number in the form dim.q.z, if dim is at most 11 or a prime at most 23, or its Q-class number in the form dim.q, else,
  - a message if the group is solvable,
  - the size of the group,
  - the isomorphism type of the group,
  - the elementary divisors of the associated quadratic form,
  - the sizes of the orbits of short vectors (these sizes are the degrees of the faithful permutation representations which you may construct using the functions `IsomorphismPermGroup` or `IsomorphismPermGroupImfGroup` below),
  - the norm of the associated short vectors,
  - only in case that the group is not an i.m.f. group in GLn(Q): an appropriate message, including the Q-class number of the corresponding rational i.m.f. class.

  If you specify the value 0 for any of the parameters dim, q, or z, the command will loop over all available dimensions, Q-classes of given dimension, or Z-classes within the given Q-class, respectively. Otherwise, the values of the arguments must be in range. A value z ≠ 1 must not be specified if the Z-classes are not known for the given dimension, i.e., if dim > 11 and dim ∉ {13,17,19,23}. The default value of z is 1. This value of z will be accepted even if the Z-classes are not known. Then it specifies the only representative group which is available for the qth Q-class. The greatest legal value of dim is 31.

      gap> DisplayImfInvariants( 3, 1, 0 );
      #I Z-class 3.1.1:  Solvable, size = 2^4*3
      #I   isomorphism type = C2 wr S3 = C2 x S4 = W(B3)
      #I   elementary divisors = 1^3
      #I   orbit size = 6, minimal norm = 1
      #I Z-class 3.1.2:  Solvable, size = 2^4*3
      #I   isomorphism type = C2 wr S3 = C2 x S4 = C2 x W(A3)
      #I   elementary divisors = 1*4^2
      #I   orbit size = 8, minimal norm = 3
      #I Z-class 3.1.3:  Solvable, size = 2^4*3
      #I   isomorphism type = C2 wr S3 = C2 x S4 = C2 x W(A3)
      #I   elementary divisors = 1^2*4
      #I   orbit size = 12, minimal norm = 2
      gap> DisplayImfInvariants( 8, 15, 1 );
      #I Z-class 8.15.1:  Solvable, size = 2^5*3^4
      #I   isomorphism type = C2 x (S3 wr S3)
      #I   elementary divisors = 1*3^3*9^3*27
      #I   orbit size = 54, minimal norm = 8
      #I   not maximal finite in GL(8,Q), rational imf class is 8.5
      gap> DisplayImfInvariants( 20, 23 );
      #I Q-class 20.23:  Size = 2^5*3^2*5*11
      #I   isomorphism type = (PSL(2,11) x D12).C2
      #I   elementary divisors = 1^18*11^2
      #I   orbit size = 3*660 + 2*1980 + 2640 + 3960, minimal norm = 4

  Note that the function `DisplayImfInvariants` uses a kind of shorthand to display the elementary divisors. E. g., the expression `1*3^3*9^3*27` in the preceding example stands for the elementary divisors 1,3,3,3,9,9,9,27. (See also the next example which shows that the function `ImfInvariants` provides the elementary divisors in form of an ordinary GAP list.)

  In the description of the isomorphism types the following notations are used:

  A `x` B
    denotes a direct product of a group A by a group B,

  A `subd` B
    denotes a subdirect product of A by B,

  A `Y` B
    denotes a central product of A by B,

  A `wr` B
    denotes a wreath product of A by B,

  A`:`B
    denotes a split extension of A by B,

  A·B
    denotes just an extension of A by B (split or nonsplit).

  The groups involved are

  - the cyclic groups Cn, dihedral groups Dn, and generalized quaternion groups Qn of order n, denoted by `C` n, `D` n, and `Q` n, respectively,
  - the alternating groups An and symmetric groups Sn of degree n, denoted by `A` n and `S` n, respectively,
  - the linear groups GLn(q), PGLn(q), SLn(q), and PSLn(q), denoted by `GL`(n,q), `PGL`(n,q), `SL`(n,q), and `PSL`(n,q), respectively,
  - the unitary groups SUn(q) and PSUn(q), denoted by `SU`(n,q) and `PSU`(n,q), respectively,
  - the symplectic groups Sp(n,q) and PSp(n,q), denoted by `Sp`(n,q) and `PSp`(n,q), respectively,
  - the orthogonal groups O8 +(2) and PO8 +(2), denoted by `O+`(8,2) and `PO+`(8,2), respectively,
  - the extraspecial groups 2+ 1+8, 3+ 1+2, 3+ 1+4, and 5+ 1+2, denoted by `2+^(1+8)`, `3+^(1+2)`, `3+^(1+4)`, and `5+^(1+2)`, respectively,
  - the Chevalley group G2(3), denoted by `G2(3)`,
  - the twisted Chevalley group 3D4(2), denoted by `3D4(2)`,
  - the Suzuki group Sz(8), denoted by `Sz(8)`,
  - the Weyl groups W(An), W(Bn), W(Dn), W(En), and W(F4), denoted by `W(A` n`)`, `W(B` n`)`, `W(D` n`)`, `W(E` n`)`, and `W(F4)`, respectively,
  - the sporadic simple groups Co1, Co2, Co3, HS, J2, M12, M22, M23, M24, and Mc, denoted by `Co1`, `Co2`, `Co3`, `HS`, `J2`, `M12`, `M22`, `M23`, `M24`, and `Mc`, respectively,
  - a point stabilizer of index 11 in M11, denoted by `M10`.

  As mentioned above, the data assembled by the function `DisplayImfInvariants` are ``cheap data'' in the sense that they can be provided by the library without loading any of its large matrix files or performing any matrix calculations. The following function allows you to get proper access to these cheap data instead of just displaying them.

- `ImfInvariants(` dim`,` q`) F`
- `ImfInvariants(` dim`,` q`,` z`) F`

  `ImfInvariants` returns a record which provides some Z-class invariants of the groups in the zth Z-class in the qth Q-class of i.m.f. integral matrix groups of dimension dim. A value z ≠ 1 must not be specified if the Z-classes are not known for the given dimension, i.e., if dim > 11 and dim ∉ {13,17,19,23}. The default value of z is 1. This value of z will be accepted even if the Z-classes are not known. Then it specifies the only representative group which is available for the qth Q-class. The greatest legal value of dim is 31.

  The resulting record contains six or seven components:

  `size`
    the size of any representative group G,

  `isSolvable`
    is `true` if G is solvable,

  `isomorphismType`
    a text string describing the isomorphism type of G (in the same notation as used by the function `DisplayImfInvariants` above),

  `elementaryDivisors`
    the elementary divisors of the associated Gram matrix F (in the same format as the result of the function `ElementaryDivisorsMat`, see ElementaryDivisorsMat),

  `minimalNorm`
    the norm of the associated short vectors,

  `sizesOrbitsShortVectors`
    the sizes of the orbits of short vectors under F,

  `maximalQClass`
    the Q-class number of an i.m.f. group in GLn(Q) that contains G as a subgroup (only in case that not G itself is an i.m.f. subgroup of GLn(Q)).

  Note that four of these data, namely the group size, the solvability, the isomorphism type, and the corresponding rational i.m.f. class, are not only Z-class invariants, but also Q-class invariants.

  Note further that, though the isomorphism type is a Q-class invariant, you will sometimes get different descriptions for different Z-classes of the same Q-class (as, e.g., for the classes 3.1.1 and 3.1.2 in the last example above). The purpose of this behaviour is to provide some more information about the underlying lattices.

      gap> ImfInvariants( 8, 15, 1 );
      rec( size := 2592, isSolvable := true, isomorphismType := "C2 x (S3 wr S3)",
        elementaryDivisors := [ 1, 3, 3, 3, 9, 9, 9, 27 ], minimalNorm := 8,
        sizesOrbitsShortVectors := [ 54 ], maximalQClass := 5 )
      gap> ImfInvariants( 24, 1 ).size;
      10409396852733332453861621760000
      gap> ImfInvariants( 23, 5, 2 ).sizesOrbitsShortVectors;
      [ 552, 53130 ]
      gap> for i in [ 1 .. ImfNumberQClasses( 22 ) ] do
      >    Print( ImfInvariants( 22, i ).isomorphismType, "\n" ); od;
      C2 wr S22 = W(B22)
      (C2 x PSU(6,2)).S3
      (C2 x S3) wr S11 = (C2 x W(A2)) wr S11
      (C2 x S12) wr C2 = (C2 x W(A11)) wr C2
      C2 x S3 x S12 = C2 x W(A2) x W(A11)
      (C2 x HS).C2
      (C2 x Mc).C2
      C2 x S23 = C2 x W(A22)
      C2 x PSL(2,23)
      C2 x PSL(2,23)
      C2 x PGL(2,23)
      C2 x PGL(2,23)

- `ImfMatrixGroup(` dim`,` q`) F`
- `ImfMatrixGroup(` dim`,` q`,` z`) F`

  `ImfMatrixGroup` is the essential extraction function of this library (note that its name has been changed from `ImfMatGroup` in GAP 3 to `ImfMatrixGroup` in GAP 4). It returns a representative group, G say, of the zth Z-class in the qth Q-class of i.m.f. integral matrix groups of dimension dim. A value z ≠ 1 must not be specified if the Z-classes are not known for the given dimension, i.e., if dim > 11 and dim ∉ {13,17,19,23}. The default value of z is 1. This value of z will be accepted even if the Z-classes are not known. Then it specifies the only representative group which is available for the qth Q-class. The greatest legal value of dim is 31.

      gap> G := ImfMatrixGroup( 5, 1, 3 );
      ImfMatrixGroup(5,1,3)
      gap> for m in GeneratorsOfGroup( G ) do PrintArray( m ); od;
      [ [  -1,   0,   0,   0,   0 ],
        [   0,   1,   0,   0,   0 ],
        [   0,   0,   0,   1,   0 ],
        [  -1,  -1,  -1,  -1,   2 ],
        [  -1,   0,   0,   0,   1 ] ]
      [ [  0,  1,  0,  0,  0 ],
        [  0,  0,  1,  0,  0 ],
        [  0,  0,  0,  1,  0 ],
        [  1,  0,  0,  0,  0 ],
        [  0,  0,  0,  0,  1 ] ]

  The attributes `Size` and `IsSolvable` will be properly set in the resulting matrix group G. In addition, it has two attributes `IsImfMatrixGroup` and `ImfRecord` where the first one is just a logical flag set to true and the latter one is a record. Except for the group size and the solvability flag, this record contains the same components as the resulting record of the function `ImfInvariants` described above (see ImfInvariants), namely the components `isomorphismType`, `elementaryDivisors`, `minimalNorm`, and `sizesOrbitsShortVectors` and, if G is not a rational i.m.f. group, `maximalQClass`. Moreover, it has the two components

  `form`
    the associated Gram matrix F, and

  `repsOrbitsShortVectors`
    representatives of the orbits of short vectors under F.

  The last one of these components will be required by the function `IsomorphismPermGroup` below.

  Example:

      gap> Size( G );
      3840
      gap> imf := ImfRecord( G );;
      gap> imf.isomorphismType;
      "C2 wr S5 = C2 x W(D5)"
      gap> PrintArray( imf.form );
      [ [  4,  0,  0,  0,  2 ],
        [  0,  4,  0,  0,  2 ],
        [  0,  0,  4,  0,  2 ],
        [  0,  0,  0,  4,  2 ],
        [  2,  2,  2,  2,  5 ] ]
      gap> imf.elementaryDivisors;
      [ 1, 4, 4, 4, 4 ]
      gap> imf.minimalNorm;
      4

  If you want to perform calculations in such a matrix group G you should be aware of the fact that the permutation group routines of GAP are much more efficient than the matrix group routines. Hence we recommend that you do your computations, whenever possible, in the isomorphic permutation group which is induced by the action of G on one of the orbits of the associated short vectors. You may call one of the following functions `IsomorphismPermGroup` or `IsomorphismPermGroupImfGroup` to get an isomorphism to such a permutation group (note that these GAP 4 functions have replaced the GAP 3 functions `PermGroup` and `PermGroupImfGroup`).

- `IsomorphismPermGroup(` G`) M`

  returns an isomorphism, ϕ say, from the given i.m.f. integral matrix group G to a permutation group P : = ϕ(G ) acting on a minimal orbit, S say, of short vectors of G such that each matrix m ∈ G is mapped to the permutation induced by its action on S.

  Note that in case of a large orbit the construction of ϕ may be space and time consuming. Fortunately, there are only six Q-classes in the library for which the smallest orbit of short vectors is of size greater than 20000, the worst case being the orbit of size 196560 for the Leech lattice (dim = 24, q = 3).

  The inverse isomorphism ϕ−1 from P to G is constructed by determining a Q-base B ⊂ S of Q1 ×dim in S and, in addition, the associated base change matrix M which transforms B into the standard base of Z1 ×dim. This allows a simple computation of the preimage ϕ−1(p) of any permutation p ∈ P as follows. If, for 1 ≤ i ≤ dim, bi is the position number in S of the ith base vector in B, it suffices to look up the vector whose position number in S is the image of bi under p and to multiply this vector by M to get the ith row of ϕ−1(p).

  You may use the functions `Image` and `PreImage` (see Image and PreImage) to switch from G to P and back from P to G.

  As an example, let us continue the preceding example and compute the solvable residuum of the group G.

      gap> # Perform the computations in an isomorphic permutation group.
      gap> phi := IsomorphismPermGroup( G );;
      gap> P := Image( phi );
      Group([ (1,7,6)(2,9)(4,5,10), (2,3,4,5)(6,9,8,7) ])
      gap> D := DerivedSubgroup( P );
      Group([ (1,2,10,9)(3,8)(4,5)(6,7), (1,3,10,8)(2,5)(4,7)(6,9),
        (1,2,8,7,5)(3,4,6,10,9) ])
      gap> Size( D );
      960
      gap> IsPerfectGroup( D );
      true
      gap> # We have found the solvable residuum of P,
      gap> # now move the results back to the matrix group G.
      gap> R := PreImage( phi, D );
      <matrix group of size 960 with 3 generators>
      gap> for m in GeneratorsOfGroup( R ) do PrintArray( m ); od;
      [ [  -1,  -1,  -1,  -1,   2 ],
        [   0,  -1,   0,   0,   0 ],
        [   0,   0,   0,   1,   0 ],
        [   0,   0,   1,   0,   0 ],
        [  -1,  -1,   0,   0,   1 ] ]
      [ [   0,   0,   0,   1,   0 ],
        [  -1,  -1,  -1,  -1,   2 ],
        [   0,   0,  -1,   0,   0 ],
        [   1,   0,   0,   0,   0 ],
        [   0,  -1,  -1,   0,   1 ] ]
      [ [   0,  -1,   0,   0,   0 ],
        [   0,   0,   1,   0,   0 ],
        [   0,   0,   0,  -1,   0 ],
        [   1,   1,   1,   1,  -2 ],
        [   0,   0,   1,   0,  -1 ] ]

- `IsomorphismPermGroupImfGroup(` G`,` n`) F`

  `IsomorphismPermGroupImfGroup` returns an isomorphism, ϕ say, from the given i.m.f. integral matrix group G to a permutation group P acting on the n th orbit, S say, of short vectors of G such that each matrix m ∈ G is mapped to the permutation induced by its action on S.

  The only difference to the above function `IsomorphismPermGroup` is that you can specify the orbit to be used. In fact, as the orbits of short vectors are sorted by increasing sizes, the function `IsomorphismPermGroup(` G`)` has been implemented such that it is equivalent to `IsomorphismPermGroupImfGroup(` G`, 1 )`.

      gap> ImfInvariants( 12, 9 ).sizesOrbitsShortVectors;
      [ 120, 300 ]
      gap> G := ImfMatrixGroup( 12, 9 );
      ImfMatrixGroup(12,9)
      gap> phi1 := IsomorphismPermGroupImfGroup( G, 1 );;
      gap> P1 := Image( phi1 );
      <permutation group of size 2400 with 2 generators>
      gap> LargestMovedPoint( P1 );
      120
      gap> phi2 := IsomorphismPermGroupImfGroup( G, 2 );;
      gap> P2 := Image( phi2 );
      <permutation group of size 2400 with 2 generators>
      gap> LargestMovedPoint( P2 );
      300

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  GAP 4 manual
  September 2006