9 Fixed-point-free automorphism groups

An automorphism group Phi of a group G acts fixed-point-free (fpf) on G if Phi has more than 1 element and no automorphism in Phi except the identity mapping has a fixed point besides the group identity of G.

Phi is fpf on G, iff the semidirect product of G and Phi, with Phi acting naturally on G, is a Frobenius group.

The function IsFpfAutomorphismGroup returns the according value true or false for a group of automorphisms phi on the group G.

    gap> C9 := CyclicGroup( 9 );
    <pc group of size 9 with 2 generators>
    gap> a := GroupHomomorphismByFunction( C9, C9, x -> x^-1 );;
    gap> phi := Group( a );;
    gap> Size( phi );
    2
    gap> IsFpfAutomorphismGroup( phi, C9 );
    true

FpfAutomorphismGroupsMaxSize returns a list with integers kmax and dmax where kmax is an upper bound for the size of an fpf automorphism group on the group G; for example, the order of G is congruent to 1 modulo kmax and kmax is odd for nonabelian groups G. The order of any fpf automorphism group phi on G divides kmax.

Let phi be a metacyclic fpf automorphism group acting on G. Then phi has a cyclic normal subgroup whose index in phi divides dmax. Thus, if dmax is 1, then G admits cyclic fpf automorphism groups only.

    gap> G := ElementaryAbelianGroup( 49 );;
    gap> FpfAutomorphismGroupsMaxSize( G );
    [ 48, 2 ]
    gap> C15 := CyclicGroup( 15 );;
    gap> FpfAutomorphismGroupsMaxSize( C15 );
    [ 2, 1 ]

FrobeniusGroup constructs the semidirect product of G with the fpf automorphism group phi of G with the multiplication (a,g)*(b,h)=(ab,g^ah) by using the function SemidirectProduct. Thus a Frobenius group with Frobenius kernel G and Frobenius complement phi where the action of phi on G is the natural action of automorphisms on the group is returned.

The unique Frobenius group with kernel G = (Z₃)²times(Z₅)² and quaternion complement is constructed as follows:

    gap> aux := FpfAutomorphismGroupsMetacyclic( [3,3,5,5], 4, -1 ); 
    [ [ [ [ f1, f2, f3, f4 ] -> [ f1^2, f2^3, f3*f4, f3*f4^2 ], 
              [ f1, f2, f3, f4 ] -> [ f2^4, f1, f4^2, f3 ] ] ], 
      <pc group of size 225 with 4 generators> ]
    gap> phi := Group( aux[1][1] );
    <group with 2 generators>
    gap> G := aux[2];
    <pc group of size 225 with 4 generators>
    gap> FrobeniusGroup( phi, G );
    <pc group of size 1800 with 7 generators>

9.2 Fixed-point-free representations

Let pi be a representation of the group Phi over the finite field F. If for all varphiinPhi except for the identity the matrix pi(varphi) does not have 1 as an eigenvalue, then pi is said to be fpf.

Let pi be an fpf representation of Phi over F with degree d. Then pi is faithful, the order of Phi and the characteristic of F are coprime and pi is a sum of irreducible faithful fpf F-representations. The matrix group pi(Phi) acts fpf on the vectorspace F^d.

For a list of dtimesd matrices, matrices, over the field F, the function IsFpfRepresentation returns true if the group generated by matrices acts fpf on the d-dimensional vectorspace over F, and false otherwise.

    gap> F := GF(5);;
    gap> A := [[2,0],[0,3]]*One(F);
    [ [ Z(5), 0*Z(5) ], [ 0*Z(5), Z(5)^3 ] ]
    gap> IsFpfRepresentation( [A], F );
    true

returns the degree of the irreducible fpf representations of the cyclic group of order m over GF(p), where m and p are coprime.

Note, that all irreducible fpf representations of the cyclic group of order m over GF(p) have the same degree, the multiplicative order of p modulo m, OrderMod( p, m ).

    gap> DegreeOfIrredFpfRepCyclic( 5, 9 );
    6

returns the degree of the irreducible fpf representations of the metacyclic group Phi determined by parameters m and r over GF(p). If the parameters are not feasible, then an error is returned. See FpfRepresentationsMetacyclic for a presentation of this group.

All irreducible fpf representations of the metacyclic group over GF(p) have the same degree, namely the size of multiplicative group generated by p and r modulo m.

We determine the degree of the irreducible fpf representation of the quaternion group over GF(5):

    gap> DegreeOfIrredFpfRepMetacyclic( 5, 4, -1 );
    2

returns the degree of the irreducible fpf representations of the type-II-group Phi determined by parameters m, r, and k over GF(p). If the parameters are not feasible or if the parameters describe the presentation of a metacyclic group, then an error is returned. See FpfRepresentations2 for a presentation of this group.

All irreducible fpf representations of Phi over GF(p) have the same degree, namely the size of the multiplicative group generated by p, r, and k modulo m.

We determine the degree of the irreducible fpf representation of the smallest, not metacyclic type-2-group (order 120) over the field GF(7):

    gap> DegreeOfIrredFpfRep2( 7, 30, 11, -1 );    
    8

returns the degree of the irreducible fpf representations of the type-III-group Phi determined by parameters m and r over GF(p). If the parameters are not feasible, then an error is returned. See FpfRepresentations3 for a presentation of this group.

All irreducible fpf representations of this group over GF(p) have the same degree.

We determine the degree of the irreducible fpf representation of SL(2,3) over GF(5):

    gap> DegreeOfIrredFpfRep3( 5, 3, 1 );                                  
    2

returns the degree of the irreducible fpf representations of the type-IV-group Phi determined by parameters m, r, and k over GF(p). If the parameters are not feasible, then an error is returned. See FpfRepresentations4 for a presentation of this group.

All irreducible fpf representations of Phi over GF(p) have the same degree.

We determine the degree of the irreducible fpf representation of the smallest type-4-group, the binary octahedral group of order 48, over GF(5):

    gap> DegreeOfIrredFpfRep4( 5, 3, 1, -1 );   
    4

Let a generate a cyclic group of order m. For p and m coprime FpfRepresentationsCyclic returns a list of matrices { Aⁱ | i in indexlist } over GF(p) as well as the list indexlist. For all i in indexlist the representation a mapstoAⁱ is irreducible and fpf. The Aⁱ with i in indexlist describe all irreducible fpf representations up to equivalence; each irreducible fpf representation is equivalent to one a mapstoAⁱ and no two representations a mapstoAⁱ, a mapstoA^j with ineqj and i,j in indexlist are equivalent.

Note, that every faithful irreducible representation of a cyclic group is fpf. The number of nonequivalent faithful irreducible representations over GF(p) is given as phi(m)/d, where the degree d is given as the multiplicative order of p modulo m and phi(m) denotes the number of residues coprime to m.

We determine the irreducible matrix representations of the cyclic group of size 8 over GF(5):

    gap> aux := FpfRepresentationsCyclic( 5, 8 );
    [ [ [ [ Z(5)^3, Z(5)^2 ], [ Z(5), Z(5) ] ], 
          [ [ Z(5)^2, Z(5) ], [ Z(5)^0, Z(5)^0 ] ] ], [ 1, 7 ] ]
    gap> mats := aux[1];
    [ [ [ Z(5)^3, Z(5)^2 ], [ Z(5), Z(5) ] ], 
      [ [ Z(5)^2, Z(5) ], [ Z(5)^0, Z(5)^0 ] ] ]
    gap> indexlist := aux[2];
    [ 1, 7 ]

Let Phi be a metacyclic group (i.e., Phi has a cyclic normal subgroup with cyclic factor) admitting an fpf representation. Then Phi fulfills one of the following two presentations, I or II. Both presentations are determined by integers m and r satisfying certain conditions:

Type I. Presentation of an fpf metacyclic group Phi with all Sylow subgroups cyclic. Let m and r satisfy the following conditions:

(a)

m and r are coprime.

(b)

Let n be the multiplicative order of r modulo m. Then each prime divisor of n divides m.

(c)

Let m' be maximal such that m' divides m and m' is coprime to n. Then r = 1 mod (m/m').

Type II. Presentation of an fpf metacyclic group Phi with generalized quaternion 2-Sylow subgroup. Let m and r satisfy the following conditions:

(a)

m and r are coprime.

(b)

Let n be the multiplicative order of r modulo m. Then n is 2 times an odd number and each prime divisor of n divides m.

(c)

Let s be maximal such that 2^s divides m. Then 2^sgeq4 and r = -1 mod 2^s.

(d)

Let m' be maximal such that m' divides m/2 and m' is coprime to n/2. Then r = 1 mod (m/m').

Then the group Phi with 2 generators a,b satisfying the relations a^m = 1, bⁿ = a^m', b^-1ab = a^r is metacyclic and fpf and has size mn.

A group satisfying presentation I is of type I in the notation of Zassenhaus, Wolf; presentation II gives a type-II-group.

Let m, r be as above, and let p coprime to m. Additionally, we require that m does not divide r-1. (Otherwise, Phi= langlea,b | a^m = 1, bⁿ = a^m', b^-1ab = a^r rangle would be cyclic.) Then FpfRepresentationsMetacyclic returns a list of matrices { (Aⁱ,B_i) | i in indexlist } over GF(p) as well as the list indexlist. The GF(p)-representations determined by a mapstoAⁱ and b mapstoB_i are all irreducible and fpf representations of Phi= langlea,b | a^m = 1, bⁿ = a^m', b^-1ab = a^r rangle up to equivalence.

We determine the irreducible matrix representation of the quaternion group (parameters m = 4, r = -1) over GF(7):

    gap> aux := FpfRepresentationsMetacyclic( 7, 4, -1 );
    [ [ [ [ [ Z(7)^2, Z(7) ], [ Z(7), Z(7)^5 ] ], 
              [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, 0*Z(7) ] ] ] ], [ 1 ] ]
    gap> mats := aux[1];
    [ [ [ [ Z(7)^2, Z(7) ], [ Z(7), Z(7)^5 ] ], 
          [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, 0*Z(7) ] ] ] ]

The presentation of a type-II-group which is not metacyclic is determined by integers m,r,k satisfying the following conditions:

(a)

m and r are coprime, m and k are coprime.

(b)

Let n be the multiplicative order of r modulo m. Then n is 2 times an odd number and each prime divisor of n divides m.

(c)

Let m' be maximal such that m' divides m and m' is coprime to n. Then r = 1 mod (m/m').

(d)

Let 2^s-1 be maximal such that 2^s-1 divides m. Define l = -1 mod 2^s-1 and l = 1 mod (nm/(2^s-1m')). Then k = l mod (m/m').

(e)

The multiplicative order of k modulo m equals 2 and kneqr^(n/2) mod m.

Then the group Phi with generators a,b,q satisfying the relations a^m = 1, bⁿ = a^m', b^-1ab = a^r and furthermore q^-1a q = a^k, q^-1b q = b^l is fpf of type II and has size 2mn.

a,b generate a metacyclic group with all Sylow subgroups cyclic (see conditions (a), (b), (c)) of index 2 in Phi.

For m, r, k as above and p coprime to m FpfRepresentations2 returns a list of matrices { (A_i,B_i,Q_i) | i in indexlist } over GF(p) as well as the list indexlist. The GF(p)-representations determined by a mapstoA_i, b mapstoB_i and q mapstoQ_i are all irreducible, fpf representations of Phi upto equivalence.

We determine the irreducible matrix representations of the smallest type-II-group which is not metacyclic (parameters m = 30, r = 11, k = -1, size 120) over the field GF(11) and obtain 2 nonequivalent fpf representations, each of degree 4:

    gap> DegreeOfIrredFpfRep2( 11, 30, 11, -1 );
    4
    gap> aux := FpfRepresentations2( 11, 30, 11, -1 );
    [ [ [ <block matrix of dimensions (2*2)x(2*2)>, 
              <block matrix of dimensions (2*2)x(2*2)>, 
              <block matrix of dimensions (2*2)x(2*2)> ], 
          [ <block matrix of dimensions (2*2)x(2*2)>, 
              <block matrix of dimensions (2*2)x(2*2)>, 
              <block matrix of dimensions (2*2)x(2*2)> ] ], [ 1, 13 ] ]

A group Phi admitting an fpf representation is said to be of type III if Phi is the semidirect product of the quaternion group and a metacyclic fpf group H of odd size, with the quaternion group normal and H permuting the 3 subgroups of order 4.

The presentation of a type-III-group is determined by integers m and r, describing the metacyclic group H and its action on the normal quaternion subgroup. The following conditions have to be satisfied for m,r:

(a)

3 divides m; m is odd; m and r are coprime.

(b)

Let n be the multiplicative order of r modulo m. Then each prime divisor of n divides m.

(c)

Let m' be maximal such that m' divides m and m' is coprime to n. Then r = 1 mod (m/m').

Let p,q with relations p⁴ = 1, q² = p², q^-1pq = p^-1 generate the quaternion group. Let a,b generate a metacyclic group determined by m and r (See FpfRepresentationsMetacyclic).

If 3 divides n, then let a commute with p,q and let b^-1pb = q, b^-1qb = pq.

If 3 does not divide n, then let b commute with p,q and let a^-1pa = q, a^-1qa = pq

Then the group Phi with generators p,q,a,b is of type III and has size 8mn.

For r neq1 mod m, FpfRepresentations3 returns a list of matrices { (P, Q, A_i,B_i) | i in indexlist } over GF(p) as well as the list indexlist.

For r = 1 mod m, the group H is cyclic and FpfRepresentations3 returns { (P, Q, A_i) | i in indexlist } over GF(p) and indexlist.

The GF(p)-representations determined by p mapstoP, q mapstoQ and a mapstoA_i, b mapstoB_i are all irreducible, fpf representations of Phi upto equivalence.

We determine the irreducible matrix representation of the smallest type-III-group, namely SL(2,3), (parameters m = 3, r = 1, size 24) over the field GF(5):

    gap> aux := FpfRepresentations3( 5, 3, 1 );
    [ [ [ [ [ Z(5), 0*Z(5) ], [ 0*Z(5), Z(5)^3 ] ], 
              [ [ 0*Z(5), Z(5)^2 ], [ Z(5)^0, 0*Z(5) ] ], 
              [ [ Z(5)^3, Z(5)^0 ], [ Z(5), Z(5)^0 ] ] ] ], [ 1 ] ]

A group Phi= langlep,q,a,b,zrangle admitting an fpf representation is said to be of type IV, if it has a normal subgroup H = langlep,q,a,brangle of type III and index 2.

The presentation of a type-IV-group is determined by integers m,r,k satisfying the following conditions:

(a)

Let s be maximal such that 3^s divides m. Then sgeq1; m is odd; m and r are coprime.

(b)

Let n be the multiplicative order of r modulo m. Then 3 does not divide n; each prime divisor of n divides m.

(c)

Let m' be maximal such that m' divides m and m' is coprime to n. Then r = 1 mod (m/m').

(d)

k = -1 mod 3^s, k = 1 mod (m/m') and k² = 1 modulo m.

Let p,q,a,b generate a type-III-group determined by m,r with relations as given in Section FpfRepresentations3. Additionally, let z² = p², z^-1pz = qp, z^-1qz = q^-1 and z^-1a z = a^k,z^-1b z = b.

Then the group Phi with generators p,q,a,b and z is of type IV and has size 16mn.

For r neq1 mod m, FpfRepresentations4 returns a list of matrices { (P, Q, A_i,B_i, Z_i) | i in indexlist } over GF(p) as well as the list indexlist.

For r = 1 mod m, the function FpfRepresentations4 returns { (P, Q, A_i, Z_i) | i in indexlist } over GF(p) and indexlist.

The GF(p)-representations determined by p mapstoP, q mapstoQ and a mapstoA_i, b mapstoB_i, z mapstoZ_i are all irreducible, fpf representations of Phi upto equivalence.

We determine the 2 nonequivalent irreducible matrix representations of the smallest type-IV-group (binary octahedral group, m = 3, r = 1, k = -1, size 48) over the field GF(7):

    gap> aux := FpfRepresentations4( 7, 3, 1, -1 );
    [ [ [ [ [ Z(7)^2, Z(7) ], [ Z(7), Z(7)^5 ] ], 
              [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, 0*Z(7) ] ], 
              [ [ Z(7)^2, 0*Z(7) ], [ Z(7)^0, Z(7)^4 ] ], 
              [ [ Z(7)^5, Z(7) ], [ Z(7), Z(7)^2 ] ] ], 
          [ [ [ Z(7)^2, Z(7) ], [ Z(7), Z(7)^5 ] ], 
              [ [ 0*Z(7), Z(7)^3 ], [ Z(7)^0, 0*Z(7) ] ], 
              [ [ Z(7)^2, 0*Z(7) ], [ Z(7)^0, Z(7)^4 ] ], 
              [ [ Z(7)^2, Z(7)^4 ], [ Z(7)^4, Z(7)^5 ] ] ] ], 
      [ [ 1, 1 ], [ -1, 1 ] ] ]

9.3 Fixed-point-free automorphism groups

If AbelianGroup(ints) admits a cyclic fpf automorphism group of size m, then FpfAutomorphismGroupsCyclic determines one representative for each conjugacy class of such fpf automorphism groups. Conjugacy is determined within the whole automorphism group of AbelianGroup(ints).

ints has to be a list of prime power integers and is sorted in the function, according to the order pⁱleqq^j Leftrightarrowp < q or (p=q and j < i).

AbelianGroup(ints) admits a cyclic fpf automorphism group of size m iff the multiplicity of each prime power pⁱ in ints is divisible by DegreeOfIrredFpfRepCyclic( p, m ).

A list of generators of the nonconjugate fpf automorphism groups is returned together with the group AbelianGroup(ints), on which the automorphisms act. Here ints is sorted with the order above.

The generators, as, of the 2 nonconjugate cyclic fpf automorphism groups of order 4 on Z₂₅timesZ₅ are computed as follows:

    gap> aux := FpfAutomorphismGroupsCyclic( [25,5], 4 ); 
    [ [ [ f1, f3 ] -> [ f1^2*f2, f3^2 ], 
          [ f1, f3 ] -> [ f1^2*f2, f3^3 ] ], 
      <pc group of size 125 with 2 generators> ]
    gap> as := aux[1];
    [ [ f1, f3 ] -> [ f1^2*f2, f3^2 ], [ f1, f3 ] -> [ f1^2*f2, f3^3 ] ]
    gap> G := aux[2];
    <pc group of size 125 with 2 generators>

If AbelianGroup(ints) admits a metacyclic fpf automorphism group determined by parameters m and r that is not cyclic (see FpfRepresentationsMetacyclic for a presentation), then FpfAutomorphismGroupsMetacyclic determines one representative for each conjugacy class of such fpf automorphism groups. Conjugacy is determined within the whole automorphism group of AbelianGroup(ints).

ints has to be a list of prime power integers and is sorted in the function, according to the order pⁱleqq^j Leftrightarrowp < q or (p = q and igeqj).

Moreover, the multiplicity of each prime power pⁱ in ints has to be divisible by DegreeOfIrredFpfRepMetacyclic( p, m, r ), which is a multiple of the multiplicative order of r modulo m.

A list of pairs of generators (a,b satisfying b^-1ab = a^r, a^m = 1 and bⁿ = a^m') of the nonconjugate fpf automorphism groups is returned together with the group AbelianGroup(ints), on which the automorphisms act. Here ints is sorted with the order above.

For G = (Z₃)²times(Z₅)² the quaternion fpf automorphism group of size 8 (parameters m = 4, r = -1) is computed as follows:

    gap> aux := FpfAutomorphismGroupsMetacyclic( [3,3,5,5], 4, -1 );
    [ [ [ [ f1, f2, f3, f4 ] -> [ f1^2, f2^3, f3*f4, f3*f4^2 ], 
              [ f1, f2, f3, f4 ] -> [ f2^4, f1, f4^2, f3 ] ] ], 
      <pc group of size 225 with 4 generators> ]
    gap> fs := aux[1];
    [ [ [ f1, f2, f3, f4 ] -> [ f1^2, f2^3, f3*f4, f3*f4^2 ], 
          [ f1, f2, f3, f4 ] -> [ f2^4, f1, f4^2, f3 ] ] ]
    gap> phi := Group( fs[1] );
    <group with 2 generators>
    gap> G := aux[2];
    <pc group of size 225 with 4 generators>

On G = (Z₇)²times(Z₁₇)² there are 2 nonconjugate fpf automorphism groups isomorphic to the generalized quaternion group of size 16 (parameters m = 8, r = -1):

    gap> aux := FpfAutomorphismGroupsMetacyclic( [7,7,17,17], 8, -1 );;
    gap> fs := aux[1];
    [ [ [ f1, f2, f3, f4 ] -> [ f1^9, f2^2, f3^4*f4^2, f3*f4^6 ], 
          [ f1, f2, f3, f4 ] -> [ f2^16, f1, f3^4*f4^5, f3^5*f4^3 ] ], 
      [ [ f1, f2, f3, f4 ] -> [ f1^9, f2^2, f3^3*f4^5, f3^6*f4 ], 
          [ f1, f2, f3, f4 ] -> [ f2^16, f1, f3^3*f4^4, f3*f4^4 ] ] ]
    gap> phis := List( fs, Group );
    [ <group with 2 generators>, <group with 2 generators> ]
    gap> G := aux[2];
    <pc group of size 14161 with 4 generators>

If AbelianGroup(ints) admits an fpf automorphism group of type II, determined by parameters m, r, k that is not metacyclic (see FpfRepresentations2 for a presentation), then FpfAutomorphismGroups2 determines one representative for each conjugacy class of such fpf automorphism groups. Conjugacy is determined within the whole automorphism group of AbelianGroup(ints).

ints has to be a list of prime power integers and is sorted in the function, according to the order pⁱleqq^j Leftrightarrowp < q or (p = q and igeqj).

Note, that the degree of an irreducible fpf representation of a type-II-group which is not metacyclic is divisible by 4 and that the multiplicity of each prime power pⁱ in ints has to be divisible by DegreeOfIrredFpfRep2( p, m, r, k ).

A list of triples of generators (a,b,z satisfying b^-1ab = a^r, a^m = 1 and z^-1az = a^k) of the nonconjugate fpf automorphism groups is returned together with the group AbelianGroup(ints), on which the automorphisms act. Here ints is sorted with the order above.

Upto conjugacy there is only one fpf automorphism group of type II with parameters m = 30, r = 11, k = -1, size 120 on the elementary abelian group of size 11⁴:

    gap> aux := FpfAutomorphismGroups2( [11,11,11,11], 30, 11, -1 );
    [ [ [ [ f1, f2, f3, f4 ] -> [ f1^5*f2^4, f1^3*f2^10, f3^2*f4^8, 
                  f3^6*f4 ], 
              [ f1, f2, f3, f4 ] -> [ f1^3*f2^10, f1^10*f2^8, f3^8*f4, 
                  f3*f4^3 ], 
              [ f1, f2, f3, f4 ] -> [ f3^10, f4^10, f1, f2 ] ] ], 
      <pc group of size 14641 with 4 generators> ]
    gap> phi := Group( aux[1][1] );
    <group with 3 generators>
    gap> G := aux[2];
    <pc group of size 14641 with 4 generators>

If AbelianGroup(ints) admits an fpf automorphism group of type III determined by parameters m and r (see FpfRepresentations3 for a presentation), then FpfAutomorphismGroups3 determines one representative for each conjugacy class of such fpf automorphism groups. Conjugacy is determined within the whole automorphism group of AbelianGroup(ints).

ints has to be a list of prime power integers and is sorted in the function, according to the order pⁱleqq^j Leftrightarrowp < q or (p = q and igeqj).

Moreover, the multiplicity of each prime power pⁱ in ints has to be divisible by DegreeOfIrredFpfRep3( p, m, r ), which is a multiple of 2n where n is the multiplicative order of r modulo m.

A list of tuples of generators, [p,q,a,b], (p,q generating the quaternion group, a,b satisfying b^-1ab = a^r, a^m = 1 and bⁿ = a^m') of the nonconjugate fpf automorphism groups is returned together with the group AbelianGroup(ints), on which the automorphisms act. Here ints is sorted with the order above.

For G = (Z₅)² the fpf automorphism type-III-group isomorphic to SL(2,3) is computed as follows (parameters m = 3, r = 1):

    gap> aux := FpfAutomorphismGroups3( [5,5], 3, 1 ); 
    [ [ [ [ f1, f2 ] -> [ f1^2, f2^3 ], [ f1, f2 ] -> [ f2^4, f1 ], 
              [ f1, f2 ] -> [ f1^3*f2, f1^2*f2 ] ] ], 
      <pc group of size 25 with 2 generators> ]
    gap> phi := Group( aux[1][1] );
    <group with 3 generators>
    gap> G := aux[2];
    <pc group of size 25 with 2 generators>

If AbelianGroup(ints) admits an fpf automorphism group of type IV determined by parameters m, r, k (see FpfRepresentations4 for a presentation), then FpfAutomorphismGroups4 determines one representative for each conjugacy class of such fpf automorphism groups. Conjugacy is determined within the whole automorphism group of AbelianGroup(ints).

ints has to be a list of prime power integers and is sorted in the function, according to the order pⁱleqq^j Leftrightarrowp < q or (p = q and igeqj).

Moreover, the multiplicity of each prime power pⁱ in ints has to be divisible by DegreeOfIrredFpfRep4( p, m, r ), which is a multiple of 2n where n is the multiplicative order of r modulo m.

A list of tuples of generators, [p,q,a,b,z], of the nonconjugate fpf automorphism groups is returned together with the group AbelianGroup(ints), on which the automorphisms act. Here ints is sorted with the order above. If r = 1 mod m, then a list of tuples, [p,q,a,z], is returned instead.

For G = (Z₇)² the fpf automorphism type-IV-group isomorphic the binary octahedral group of size 48 (parameters m = 3, r = 1, k = -1) is computed as follows:

    gap> aux := FpfAutomorphismGroups4( [7,7], 3, 1, -1 );
    [ [ [ [ f1, f2 ] -> [ f1^2*f2^3, f1^3*f2^5 ], 
              [ f1, f2 ] -> [ f2^6, f1 ], [ f1, f2 ] -> [ f1^2, f1*f2^4 ], 
              [ f1, f2 ] -> [ f1^5*f2^3, f1^3*f2^2 ] ] ], 
      <pc group of size 49 with 2 generators> ]
    gap> phi := Group( aux[1][1] );
    <group with 4 generators>
    gap> G := aux[2];
    <pc group of size 49 with 2 generators>

[Up] [Previous] [Next] [Index]