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# 43 Polycyclic Groups

### Sections

1. Polycyclic Generating Systems
2. Computing a Pcgs
3. Defining a Pcgs Yourself
4. Elementary Operations for a Pcgs
5. Elementary Operations for a Pcgs and an Element
6. Exponents of Special Products
7. Subgroups of Polycyclic Groups - Induced Pcgs
8. Subgroups of Polycyclic Groups - Canonical Pcgs
9. Factor Groups of Polycyclic Groups - Modulo Pcgs
10. Factor Groups of Polycyclic Groups in their Own Representation
11. Pcgs and Normal Series
12. Sum and Intersection of Pcgs
13. Special Pcgs
14. Action on Subfactors Defined by a Pcgs
15. Orbit Stabilizer Methods for Polycyclic Groups
16. Operations which have Special Methods for Groups with Pcgs
17. Conjugacy Classes in Solvable Groups

A group G is polycyclic if there exists a subnormal series G = C1 > C2 > ¼ > Cn > Cn+1 = {1} with cyclic factors. Such a series is called pc series of G.

Every polycyclic group is solvable and every finite solvable group is polycyclic. However, there are infinite solvable groups which are not polycyclic.

In GAP there exists a large number of methods for polycyclic groups which are based upon the polycyclic structure of these groups. These methods are usually very efficient and hence GAP tries to use them whenever possible.

In GAP 3 these methods have been available for AgGroups only; that is, for groups defined via a power-commutator presentation, see Chapter Pc Groups for the GAP 4 analogon. This has changed in GAP 4 where these methods can be applied to many types of groups. For example, the methods can be applied to permutation groups or matrix groups which are known to be polycyclic. The only exception is the representation as finitely presented group for which the polycyclic methods cannot be used in general.

At the current state of implementations the methods for polycyclic groups can only be applied to finite groups. However, a more general implementation is planned.

## 43.1 Polycyclic Generating Systems

Let G be a polycyclic group with a pc series as above. A polycyclic generating sequence (pcgs for short) of G is a sequence P : = (g1, ¼, gn) of elements of G such that Ci = á Ci+1, gi ñ for 1 £ i £ n. Note that each polycyclic group has a pcgs, but except for very small groups, a pcgs is not unique.

For each index i the subsequence of elements (gi, ¼, gn) forms a pcgs of the subgroup Ci. In particular, these tails generate the subgroups of the pc series and hence we say that the pc series is determined by P.

Let ri be the index of Ci+1 in Ci which is either a finite positive number or infinity. Then ri is the order of gi Ci+1 and we call the resulting list of indices the relative orders of the pcgs P.

Moreover, with respect to a given pcgs (g1, ¼, gn) each element g of G can be represented in a unique way as a product g = g1e1 ·g2e2 ¼ gnen with exponents ei Î {0, ¼, ri-1}, if ri is finite, and ei Î Z otherwise. Words of this form are called normal words or words in normal form. Then the integer vector [e1, ¼, en] is called the exponent vector of the element g. Furthermore, the smallest index k such that ek ¹ 0 is called the depth of g and ek is the leading exponent of g.

For many applications we have to assume that each of the relative orders ri is either a prime or infinity. This is equivalent to saying that there are no trivial factors in the pc series and the finite factors of the pc series are maximal refined. Then we obtain that ri is the order of g Ci+1 for all elements g in Ci \Ci+1 and we call ri the relative order of the element g.

## 43.2 Computing a Pcgs

Suppose a group G is given; for example, let G be a permutation or matrix group. Then we can ask GAP to compute a pcgs of this group. If G is not polycyclic, the result will be `fail`.

Note that these methods can only be applied if G is not given as finitely presented group. For finitely presented groups one can try to compute a pcgs via the polycyclic quotient methods, see Quotient Methods.

Note also that a pcgs behaves like a list.

- `Pcgs(` G`) A`

  returns a pcgs for the group G. If grp is not polycyclic it returns `fail` and this result is not stored as attribute value, in particular in this case the filter `HasPcgs` is not set for G!

- `IsPcgs(` obj`) C`

  The category of pcgs.

      gap> G := Group((1,2,3,4),(1,2));;
      gap> p := Pcgs(G);
      Pcgs([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
      gap> IsPcgs( p );
      true
      gap> p[1];
      (3,4)

      gap> G := Group((1,2,3,4,5),(1,2));;
      gap> Pcgs(G);
      fail

- `CanEasilyComputePcgs(` grp`) F`

  This filter indicates whether it is possible to compute a pcgs for grp cheaply. Clearly, grp must be polycyclic in this case. However, not for every polycyclic group there is a method to compute a pcgs at low costs. This filter is used in the method selection mainly. Note that this filter may change its value from false to true.

      gap> G := Group( (1,2,3,4),(1,2) );
      Group([ (1,2,3,4), (1,2) ])
      gap> CanEasilyComputePcgs(G);
      false
      gap> Pcgs(G);
      Pcgs([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
      gap> CanEasilyComputePcgs(G);
      true

  ## 43.3 Defining a Pcgs Yourself

  In a number of situations it might be useful to supply a pgcs to a group.

- `PcgsByPcSequence(` fam`,` pcs`) O`
- `PcgsByPcSequenceNC(` fam`,` pcs`) O`

  constructs a pcgs for the elements family fam from the elements in the list pcs. The elements must lie in the family fam. `PcgsByPcSequence`(`NC`) will always create a new pcgs which is not induced by any other pcgs.

      gap> fam := FamilyObj( (1,2) );; # the family of permutations
      gap> p := PcgsByPcSequence( fam, [(1,2),(1,2,3)] );
      Pcgs([ (1,2), (1,2,3) ])
      gap> RelativeOrders(p);
      [ 2, 3 ]
      gap> ExponentsOfPcElement( p, (1,3,2) );
      [ 0, 2 ]

  Note that the elementary operations for such a pcgs might be rather inefficient, since GAP has to use generic methods in this case. It might be helpful to supply the relative orders of the self-defined pcgs as well by `SetRelativeOrders( pcgs, orders )`. See also IsPrimeOrdersPcgs.

  ## 43.4 Elementary Operations for a Pcgs

- `RelativeOrders(` pcgs`) A`

  returns the list of relative orders of the pcgs pcgs.

  the list of relative orders of the pcgs pcgs.

- `IsFiniteOrdersPcgs(` pcgs`) P`

  tests whether the relative orders of pcgs are all finite.

- `IsPrimeOrdersPcgs(` pcgs`) P`

  tests whether the relative orders of pcgs are prime numbers. Many algorithms require a pcgs to have this property. The operation `IsomorphismRefinedPcGroup` (see IsomorphismRefinedPcGroup) can be of help here.

- `PcSeries(` pcgs`) A`

  returns the subnormal series determined by pcgs.

- `GroupOfPcgs(` pcgs`) A`

  The group generated by pcgs.

- `OneOfPcgs(` pcgs`) A`

  The identity of the group generated by pcgs.

      gap> G := Group( (1,2,3,4),(1,2) );; p := Pcgs(G);;
      gap> RelativeOrders(p);
      [ 2, 3, 2, 2 ]
      gap> IsFiniteOrdersPcgs(p);
      true
      gap> IsPrimeOrdersPcgs(p);
      true
      gap> PcSeries(p);
      [ Group([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ]),
        Group([ (2,4,3), (1,4)(2,3), (1,3)(2,4) ]),
        Group([ (1,4)(2,3), (1,3)(2,4) ]), Group([ (1,3)(2,4) ]), Group(()) ]

  ## 43.5 Elementary Operations for a Pcgs and an Element

- `RelativeOrderOfPcElement(` pcgs`,` elm`) O`

  The relative order of elm with respect to the prime order pcgs pcgs.

- `ExponentOfPcElement(` pcgs`,` elm`,` pos`) O`

  returns the pos-th exponent of elm with respect to pcgs.

- `ExponentsOfPcElement(` pcgs`,` elm`) O`
- `ExponentsOfPcElement(` pcgs`,` elm`,` posran`) O`

  returns the exponents of elm with respect to pcgs. The second form returns the exponents in the positions given in posran.

- `DepthOfPcElement(` pcgs`,` elm`) O`

  returns the depth of the element elm with respect to pcgs.

- `LeadingExponentOfPcElement(` pcgs`,` elm`) O`

  returns the leading exponent of elm with respect to pcgs.

- `PcElementByExponents(` pcgs`,` list`) O`
- `PcElementByExponentsNC(` pcgs`,` list`) O`
- `PcElementByExponentsNC(` pcgs`,` basisind`,` list`) O`

  returns the element corresponding to the exponent vector list with respect to pcgs. The exponents in list must be in the range of permissible exponents for pcgs. It is not guaranteed that `PcElementByExponents` will reduce the exponents modulo the relative orders. (You should use the operation `LinearCombinationPcgs` for this purpose.) The NC version does not check that the lengths of the lists fit together and does not check the exponent range.

  The third version gives exponents only wrt. the generators in pcgs indexed by basisind.

- `LinearCombinationPcgs(` pcgs`,` list`[,` one`] ) O`

  returns the product Õipcgs [i]list [i]. In contrast to `PcElementByExponents` this permits negative exponents. pcgs might be an list of group elements, in this case, an appropriate one must be given. if list can be empty.

      gap> G := Group( (1,2,3,4),(1,2) );; P := Pcgs(G);;
      gap> g := PcElementByExponents(P, [0,1,1,1]);
      (1,2,3)
      gap> ExponentsOfPcElement(P, g);
      [ 0, 1, 1, 1 ]

- `SiftedPcElement(` pcgs`,` elm`) O`

  sifts elm through pcgs, reducing it if the depth is the same as the depth of one of the generators in pcgs. Thus the identity is returned if elm lies in the group generated by pcgs. pcgs must be an induced pcgs and elm must lie in the span of the parent of pcgs.

- `CanonicalPcElement(` ipcgs`,` elm`) O`

  reduces elm at the induces pcgs ipcgs such that the exponents of the reduced result r are zero at the depths for which there are generators in ipcgs. Elements, whose quotient lies in the group generated by ipcgs yield the same canonical element.

- `ReducedPcElement(` pcgs`,` x`,` y`) O`

  reduces the element x by dividing off (from the left) a power of y such that the leading coefficient of the result with respect to pcgs becomes zero. The elements x and y therefore have to have the same depth.

- `CleanedTailPcElement(` pcgs`,` elm`,` dep`) O`

  returns an element in the span of pcgs whose exponents for indices 1 to dep -1 with respect to pcgs are the same as those of elm, the remaining exponents are undefined. This can be used to obtain more ``simple'' elements if only representatives in a factor are required, see Factor Groups of Polycyclic Groups - Modulo Pcgs.

  The difference to `HeadPcElementByNumber` (see HeadPcElementByNumber) is that `HeadPcElementByNumber` is guaranteed to zero out trailing coefficients while `CleantedTailPcElement` will only do this if it can be done cheaply.

- `HeadPcElementByNumber(` pcgs`,` elm`,` dep`) O`

  returns an element in the span of pcgs whose exponents for indices 1 to dep -1 with respect to pcgs are the same as those of elm, the remaining exponents are zero. This can be used to obtain more ``simple'' elements if only representatives in a factor are required.

  ## 43.6 Exponents of Special Products

  There are certain products of elements whose exponents are used often within algorithms, and which might be obtained more easily than by computing the product first and to obtain its exponents afterwards. The operations in this section provide a way to obtain such exponent vectors directly.

  (The circumstances under which these operations give a speedup depend very much on the pcgs and the representation of elements that is used. So the following operations are not guaranteed to give a speedup in every case, however the default methods are not slower than to compute the exponents of a product and thus these operations should always be used if applicable.)

- `ExponentsConjugateLayer(` mpcgs`,` elm`,` e`) O`

  Computes the exponents of elm e with respect to mpcgs; elm must be in the span of mpcgs, e a pc element in the span of the parent pcgs of mpcgs and mpcgs must be the modulo pcgs for an abelian layer. (This is the usual case when acting on a chief factor). In this case if mpcgs is induced by the family pcgs, the exponents can be computed directly by looking up exponents without having to compute in the group and having to collect a potential tail.

  The second class are exponents of products of the generators which make up the pcgs. If the pcgs used is a `FamilyPcgs` these exponents can be looked up and do not need to be computed.

- `ExponentsOfRelativePower(` pcgs`,` i`) O`

  For p =pcgs [i ] this function returns the exponent vector with respect to pcgs of the element pe where e is the relative order of p in pcgs. For the family pcgs or pcgs induced by it, this might be faster than computing the element and computing its exponent vector.

- `ExponentsOfConjugate(` pcgs`,` i`,` j`) O`

  returns the exponents of pcgs [i ]pcgs [j ] with respect to pcgs. For the family pcgs or pcgs induced by it, this might be faster than computing the element and computing its exponent vector.

- `ExponentsOfCommutator(` pcgs`,` i`,` j`) O`

  returns the exponents of the commutatior Comm(pcgs [i ],pcgs [j ]) with respect to pcgs. For the family pcgs or pcgs induced by it, this might be faster than computing the element and computing its exponent vector.

  ## 43.7 Subgroups of Polycyclic Groups - Induced Pcgs

  Let U be a subgroup of G and let P be a pcgs of G as above such that P determines the subnormal series G = C1 > ¼ > Cn+1 = {1}. Then the series of subgroups U Ç Ci is a subnormal series of U with cyclic or trivial factors. Hence, if we choose an element uij Î (U Ç Cij) \(U Ç Cij+1) whenever this factor is non-trivial, then we obtain a pcgs Q = (ui1, ¼, uim) of U. We say that Q is an induced pcgs with respect to P. The pcgs P is the parent pcgs to the induced pcgs Q.

  Note that the pcgs Q is induced with respect to P if and only if the matrix of exponent vectors of the elements uij with respect to P is in upper triangular form. Thus Q is not unique in general.

  In particular, the elements of an induced pcgs do not necessarily have leading coefficient 1 relative to the inducing pcgs. The attribute `LeadCoeffsIGS` (see LeadCoeffsIGS) holds the leading coefficients in case they have to be renormed in an algorithm.

  Each induced pcgs is a pcgs and hence allows all elementary operations for pcgs. On the other hand each pcgs could be transformed into an induced pcgs for the group defined by the pcgs, but note that an arbitrary pcgs is in general not an induced pcgs for technical reasons.

  An induced pcgs is ``compatible'' with its parent.

- `IsInducedPcgs(` pcgs`) C`

  The category of induced pcgs. This a subcategory of pcgs.

- `InducedPcgsByPcSequence(` pcgs`,` pcs`) O`
- `InducedPcgsByPcSequenceNC(` pcgs`,` pcs`) O`
- `InducedPcgsByPcSequenceNC(` pcgs`,` pcs`,` depths`) O`

  If pcs is a list of elements that form an induced pcgs with respect to pcgs this operation returns an induced pcgs with these elements.

  In the third version, the depths of pcs with respect to pcgs can be given (they are computed anew otherwise).

- `ParentPcgs(` pcgs`) A`

  returns the pcgs by which pcgs was induced. If pcgs was not induced, it simply returns pcgs.

      gap> G := Group( (1,2,3,4),(1,2) );;
      gap> P := Pcgs(G);;
      gap> K := InducedPcgsByPcSequence( P, [(1,2,3,4),(1,3)(2,4)] );
      Pcgs([ (1,2,3,4), (1,3)(2,4) ])
      gap> ParentPcgs( K );
      Pcgs([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
      gap> IsInducedPcgs( K );
      true

  In SOGOS a ``non-commutative gauss'' algorithm is described to compute an induced pcgs of a subgroup U from a generating set of U. This can be called in GAP via one of the following commands.

- `InducedPcgs(` pcgs`,` grp`) O`

  computes a pcgs for grp which is induced by pcgs. If pcgs has a parent pcgs, then the result is induced with respect to this parent pcgs.

- `InducedPcgsByGenerators(` pcgs`,` gens`) O`
- `InducedPcgsByGeneratorsNC(` pcgs`,` gens`) O`

  returns an induced pcgs with respect to pcgs for the subgroup generated by gens.

- `InducedPcgsByPcSequenceAndGenerators(` pcgs`,` ind`,` gens`) O`

  returns an induced pcgs with respect to pcgs of the subgroup generated by ind and gens. Here ind must be an induced pcgs with respect to pcgs (or a list of group elements that form such an igs) and it will be used as initial sequence for the computation.

      gap> G := Group( (1,2,3,4),(1,2) );;  P := Pcgs(G);;
      gap> I := InducedPcgsByGenerators( P, [(1,2,3,4)] );
      Pcgs([ (1,2,3,4), (1,3)(2,4) ])
      gap> J := InducedPcgsByPcSequenceAndGenerators( P, I, [(1,2)] );
      Pcgs([ (1,2,3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])

- `LeadCoeffsIGS(` igs`) A`

  This attribute is used to store leading coefficients with respect to the parent pcgs. the i-th entry - if bound - is the leading exponent of the element of igs that has depth i in the parent. (It cannot be assigned to a component in `InducedPcgsByPcSequenceNC` as the permutation group methods call it from within the postprocessing, before this postprocessing however no coefficients may be computed.)

- `ExtendedPcgs(` N`,` gens`) O`

  extends the pcgs N (induced wrt. home) to a new induced pcgs by prepending gens. No checks are performed that this really yields an induced pcgs.

  To create a subgroup generated by an induced pcgs such that the induced pcgs gets stored automatically there is the following operation.

- `SubgroupByPcgs(` G`,` pcgs`) O`

  ## 43.8 Subgroups of Polycyclic Groups - Canonical Pcgs

  The induced pcgs Q of U is called canonical if the matrix of exponent vectors contains normed vectors only and above each leading entry in the matrix there are 0's only. The canonical pcgs of U with respect to P is unique and hence such pcgs can be used to compare subgroups.

- `IsCanonicalPcgs(` pcgs`) P`

  An induced pcgs is canonical if the matrix of the exponent vectors of the elements of pcgs with respect to `ParentPcgs(`pcgs`)` is in Hermite normal form (see SOGOS). While a subgroup can have various induced pcgs with respect to a parent pcgs a canonical pcgs is unique.

- `CanonicalPcgs(` pcgs`) A`

  returns the canonical pcgs corresponding to the induced pcgs pcgs.

      gap> G := Group((1,2,3,4),(5,6,7));
      Group([ (1,2,3,4), (5,6,7) ])
      gap> P := Pcgs(G);
      Pcgs([ (5,6,7), (1,2,3,4), (1,3)(2,4) ])
      gap> I := InducedPcgsByPcSequence(P, [(5,6,7)*(1,3)(2,4),(1,3)(2,4)] );
      Pcgs([ (1,3)(2,4)(5,6,7), (1,3)(2,4) ])
      gap> CanonicalPcgs(I);
      Pcgs([ (5,6,7), (1,3)(2,4) ])

  ## 43.9 Factor Groups of Polycyclic Groups - Modulo Pcgs

  Let N be a normal subgroup of G such that G/N is polycyclic with pcgs (h1 N, ¼, hr N). Then we call the sequence of preimages (h1, ¼ hr) a modulo pcgs of G/N. G is called the numerator of the modulo pcgs and N is the denominator of the modulo pcgs.

  Modulo pcgs are often used to facilitate efficient computations with factor groups, since they allow computations with factor groups without formally defining the factor group at all.

  All elementary operations of pcgs, see Sections Elementary Operations for a Pcgs and Elementary Operations for a Pcgs and an Element, apply to modulo pcgs as well. However, it is in general not possible to compute induced pcgs with respect to a modulo pcgs.

- `ModuloPcgs(` G`,` N`) O`

  returns a modulo pcgs for the factor G /N which must be solvable, which N may be insolvable.

  `ModuloPcgs` will return a pcgs for the factor, there is no guarantee that it will be ``compatible'' with any other pcgs. If this is required, the `mod` operator must be used on induced pcgs, see below.

- `IsModuloPcgs(` obj`) C`

  The category of modulo pcgs. Note that each pcgs is a modulo pcgs for the trivial subgroup.

  Additionally there are two more elementary operations for modulo pcgs.

- `NumeratorOfModuloPcgs(` pcgs`) A`

  returns a generating set for the numerator of the modulo pcgs pcgs.

- `DenominatorOfModuloPcgs(` pcgs`) A`

  returns a generating set for the denominator of the modulo pcgs pcgs.

      gap> G := Group( (1,2,3,4,5),(1,2) );
      Group([ (1,2,3,4,5), (1,2) ])
      gap> P := ModuloPcgs(G, DerivedSubgroup(G) );
      Pcgs([ (4,5) ])
      gap> NumeratorOfModuloPcgs(P);
      [ (1,2,3,4,5), (1,2) ]
      gap> DenominatorOfModuloPcgs(P);
      [ (1,3,2), (2,4,3), (2,3)(4,5) ]
      gap> RelativeOrders(P);
      [ 2 ]
      gap> ExponentsOfPcElement( P, (1,2,3,4,5) );
      [ 0 ]
      gap> ExponentsOfPcElement( P, (4,5) );
      [ 1 ]

  Modulo Pcgs can also be built from compatible induced pcgs. Let G be a group with pcgs P and let I be an induced pcgs of a normal subgroup N of G. (Respectively: P and I are both induced with respect to the same Pcgs.) Then we can compute a modulo pcgs of G mod N by

- ``P `mod` I``

  Note that in this case we obtain the advantage that the `NumeratorOfModuloPcgs` and the `DenominatorOfModuloPcgs` are just P and I, respectively, and hence are unique.

  The resulting modulo pcgs will consist of a subset of P and will be ``compatible'' with P (or its parent).

      gap> G := Group((1,2,3,4));;
      gap> P := Pcgs(G);
      Pcgs([ (1,2,3,4), (1,3)(2,4) ])
      gap> I := InducedPcgsByGenerators(P, [(1,3)(2,4)]);
      Pcgs([ (1,3)(2,4) ])
      gap> M := P mod I;
      [ (1,2,3,4) ]
      gap> NumeratorOfModuloPcgs(M);
      Pcgs([ (1,2,3,4), (1,3)(2,4) ])
      gap> DenominatorOfModuloPcgs(M);
      Pcgs([ (1,3)(2,4) ])

- `CorrespondingGeneratorsByModuloPcgs(` mpcgs`,` imgs`) O`

  let mpcgs be a modulo pcgs for a factor of a group G and let U be a subgroup of G generated by imgs such that U covers the factor for the modulo pcgs. Then this function computes elements in U corresponding to the generators of the modulo pcgs.

  Note that the computation of induced generating sets is not possible for some modulo pcgs.

- `CanonicalPcgsByGeneratorsWithImages(` pcgs`,` gens`,` imgs`) O`

  computes a canonical, pcgs-induced pcgs for the span of gens and simultaneously does the same transformations on imgs, preserving thus a correspondence between gens and imgs. This operation is used to represent homomorphisms from a pc group.

  ## 43.10 Factor Groups of Polycyclic Groups in their Own Representation

  If substantial calculations are done in a factor it might be worth still to construct the factor group in its own representation (for example by calling `PcGroupWithPcgs` on a modulo pcgs, see PcGroupWithPcgs).

  The following functions are intended for working with factor groups obtained by factoring out the tail of a pcgs. They provide a way to map elements or induced pcgs quickly in the factor (respectively to take preimages) without the need to construct a homomorphism.

  The setup is always a pcgs pcgs of G and a pcgs fpcgs of a factor group H =G /N which corresponds to a head of pcgs.

  No tests for validity of the input are performed.

- `ProjectedPcElement(` pcgs`,` fpcgs`,` elm`) F`

  returns the image in H of an element elm of G.

- `ProjectedInducedPcgs(` pcgs`,` fpcgs`,` ipcgs`) F`

  ipcgs must be an induced pcgs with respect to pcgs. This operation returns an induced pcgs with respect to fpcgs consisting of the nontrivial images of ipcgs.

- `LiftedPcElement(` pcgs`,` fpcgs`,` elm`) F`

  returns a preimage in G of an element elm of H.

- `LiftedInducedPcgs(` pcgs`,` fpcgs`,` ipcgs`,` ker`) F`

  ipcgs must be an induced pcgs with respect to fpcgs. This operation returns an induced pcgs with respect to pcgs consisting of the preimages of ipcgs, appended by the elements in ker (assuming there is a bijection of pcgs mod ker to fpcgs). ker might be a simple element list.

  ## 43.11 Pcgs and Normal Series

  By definition, a pcgs determines a pc series of its underlying group. However, in many applications it will be necessary that this pc series refines a normal series with certain properties; for example, a normal series with abelian factors.

  There are functions in GAP to compute a pcgs through a normal series with elementary abelian factors, a central series or the lower p-central series. See also Section Special Pcgs for a more explicit possibility.

- `IsPcgsElementaryAbelianSeries(` pcgs`) P`

  returns `true` if the pcgs pcgs refines an elementary abelian series. `IndicesNormalSteps` gives the indices in the Pcgs, at which the normal subgroups of this series start.

- `PcgsElementaryAbelianSeries(` G`) A`
- `PcgsElementaryAbelianSeries( [`G`,` N1`,` N2`, ....] ) A`

  computes a pcgs for G that refines an elementary abelian series. `IndicesNormalSteps` gives the indices in the Pcgs, at which the normal subgroups of this series start. The second variant returns a pcgs that runs through the normal subgroups N1, N2, etc.

- `IsPcgsCentralSeries(` pcgs`) P`

  returns `true` if the pcgs pcgs refines a central series. `IndicesNormalSteps` gives the indices in the Pcgs, at which the normal subgroups of this series start.

- `PcgsCentralSeries(` G`) A`

  computes a pcgs for G that refines a central series. `IndicesNormalSteps` gives the indices in the Pcgs, at which normal subgroups start. If G is not nilpotent, the result is undefined.

- `IsPcgsPCentralSeriesPGroup(` pcgs`) P`

- `PcgsPCentralSeriesPGroup(` G`) A`

  computes a pcgs for G that refines the lower p-central series. `IndicesNormalSteps` gives the indices in the Pcgs, at which normal subgroups start. If G is not a p-group, the result is undefined.

- `PcgsChiefSeries(` G`) A`

  computes a pcgs for G that refines a chief series. `IndicesNormalSteps` gives the indices in the Pcgs, at which normal subgroups start.

  Let P = (g1, ¼, gn) be a pcgs such that the pc series determined by P refines a normal series of one of the above types. To obtain the subgroups of this series, the following attributes can be used:

- `NormalSeriesByPcgs(` pcgs`) A`

  returns the series of all normal subgroups contained in the pc series determined by pcgs.

- `IndicesNormalSteps(` pcgs`) A`

  Let pcgs be a pcgs obtained as corresponding to a series of normal subgroups with elementary abelian factors (for example `PcgsElementaryAbelianSeries`, `PcgsChiefSeries` or `PcgsCentralSeries`). Then `IndicesNormalSteps` returns a sorted list of integers, indicating the tails of pcgs which generate these normal subgroup of G. If i is an element of this list, (gi, ¼, gn) is a normal subgroup of G. The list always starts with 1 and ends with n+1. (These indices form one series with elementary abelian subfactors, not necessarily the most refined one.)

  For arbitrary pcgs not obtained as belonging to a special series such a set of indices not necessarily exists, and `IndicesNormalSteps` is not guaranteed to work in this situation.

  Typically, `IndicesNormalSteps` is set by calls to operations that compute a particular pcgs such as `PcgsElementaryAbelianSeries`. (Note, however, that in an existing pcgs this attribute might have been set already differently by a previous calculation. In this case the pcgs is never `IsPcgsElementaryAbelianSeries`, even though it could be.)

      gap> g:=Group((1,2,3,4),(1,2));;
      gap> p:=PcgsElementaryAbelianSeries(g);
      Pcgs([ (3,4), (2,4,3), (1,4)(2,3), (1,3)(2,4) ])
      gap> IndicesNormalSteps(p);
      [ 1, 2, 3, 5 ]
      gap> g:=Group((1,2,3,4),(1,5)(2,6)(3,7)(4,8));;
      gap> p:=PcgsCentralSeries(g);
      Pcgs([ (1,5)(2,6)(3,7)(4,8), (5,6,7,8), (5,7)(6,8), (1,4,3,2)(5,6,7,8),
        (1,3)(2,4)(5,7)(6,8) ])
      gap> IndicesNormalSteps(p);
      [ 1, 2, 4, 5, 6 ]
      gap> q:=PcgsPCentralSeriesPGroup(g);
      Pcgs([ (1,5)(2,6)(3,7)(4,8), (5,6,7,8), (5,7)(6,8), (1,4,3,2)(5,6,7,8),
        (1,3)(2,4)(5,7)(6,8) ])
      gap> IndicesNormalSteps(q);
      [ 1, 3, 5, 6 ]

  ## 43.12 Sum and Intersection of Pcgs

- `SumFactorizationFunctionPcgs(` parentpcgs`,` n`,` u`,` kerpcgs`) O`

  computes the sum and intersection of the lists n and u whose elements form modulo pcgs induced by parentpcgs for two subgroups modulo a kernel given by kerpcgs. If kerpcgs is a tail if the parent-pcgs it is sufficient to give the starting depth, this can be more efficient than to construct an explicit pcgs. The factor group modulo kerpcgs generated by n must be elementary abelian and normal under u.

  The function returns a record with components

  `sum`
    Elements that form a modulo pcgs for the span of both subgroups.

  `intersection`
    Elements that form a modulo pcgs for the intersection of both subgroups.

  `factorization`
    A function that returns for an element x in the span of `sum` a record with components `u` and `n` that give its decomposition.
  The record components `sum` and `intersection` are not pcgs but only lists of pc elements (to avoid unnecessary creation of InducedPcgs).

  ## 43.13 Special Pcgs

  In short, a special pcgs is a pcgs which has particularly nice properties, for example it always refines an elementary abelian series, for p-groups it even refines a central series. These nice properties permit particularly efficient algorithms.

  Let G be a finite polycyclic group. A special pcgs of G is a pcgs which is closely related to a Hall system and the maximal subgroups of G. These pcgs have been introduced by C. R. Leedham-Green who also gave an algorithm to compute them. Improvements to this algorithm are due to Bettina Eick. For a more detailed account of their definition the reader is referred to Eick97

  To introduce the definition of special pcgs we first need to define the LG-series and head complements of a finite polycyclic group G. Let G = G1 > G2 > ¼ Gm > Gm+1 = {1} be the lower nilpotent series of G; that is, Gi is the smallest normal subgroup of Gi-1 with nilpotent factor. To obtain the LG-series of G we need to refine this series. Thus consider a factor Fi : = Gi / Gi+1. Since Fi is finite nilpotent, it is a direct product of its Sylow subgroups, say Fi = Pi,1 ¼ Pi,ri. For each Sylow pj-subgroup Pi, j we can consider its lower pj-central series. To obtain a characteristic central series with elementary abelian factors of Fi we loop over its Sylow subgroups. Each time we consider Pi,j in this process we take the next step of its lower pj-central series into the series of Fi. If there is no next step, then we just skip the consideration of Pi,j. Note that the second term of the lower p-central series of a p-group is in fact its Frattini subgroup. Thus the Frattini subgroup of Fi is contained in the computed series of this group. We denote the Frattini subgroup of Fi = Gi / Gi+1 by Gi* / Gi+1.

  The factors Gi / Gi* are called the heads of G, while the (possibly trivial) factors Gi* / Gi+1 are the tails of G. A head complement of G is a subgroup U of G such that U / Gi* is a complement to the head Gi / Gi* in G / Gi* for some i.

  Now we are able to define a special pcgs of G. It is a pcgs of G with three additional properties. First, the pc series determined by the pcgs refines the LG-series of G. Second, a special pcgs exhibits a Hall system of the group G; that is, for each set of primes p the elements of the pcgs with relative order in p form a pcgs of a Hall p-subgroup in a Hall system of G. Third, a special pcgs exhibits a head complement for each head of G.

  To record information about the LG-series with the special pcgs we define the LGWeights of the special pcgs. These weights are a list which contains a weight w for each elements g of the special pcgs. Such a weight w represents the smallest subgroup of the LG-series containing g.

  Since the LG-series is defined in terms of the lower nilpotent series, Sylow subgroups of the factors and lower p-central series of the Sylow subgroup, the weight w is a triple. More precisely, g is contained in the w[1]th term U of the lower nilpotent series of G, but not in the next smaller one V. Then w[3] is a prime such that g V is contained in the Sylow w[3]-subgroup P/V of U/V. Moreover, gV is contained in the w[2]th term of the lower p-central series of P/V.

  There are two more attributes of a special pcgs containing information about the LG-series: the list LGLayers and the list LGFirst. The list of layers corresponds to the elements of the special pcgs and denotes the layer of the LG-series in which an element lies. The list LGFirst corresponds to the LG-series and gives the number of the first element in the special pcgs of the corresponding subgroup.

- `IsSpecialPcgs(` obj`) P`

  tests whether obj is a special pcgs.

- `SpecialPcgs(` pcgs`) A`
- `SpecialPcgs(` G`) A`

  computes a special pcgs for the group defined by pcgs or for G.

- `LGWeights(` pcgs`) A`

  returns the LGWeights of the special pcgs pcgs.

- `LGLayers(` pcgs`) A`

  returns the layers of the special pcgs pcgs.

- `LGFirst(` pcgs`) A`

  returns the first indices for each layer of the special pcgs pcgs.

- `LGLength(` G`) A`

  returns the Length of the LG-series of the group G, if G is solvable and fail otherwise.

      gap> G := SmallGroup( 96, 220 );
      <pc group of size 96 with 6 generators>
      gap> spec := SpecialPcgs( G );
      Pcgs([ f1, f2, f3, f4, f5, f6 ])
      gap> LGWeights(spec);
      [ [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 1, 3 ],
        [ 1, 2, 2 ] ]
      gap> LGLayers(spec);
      [ 1, 1, 1, 1, 2, 3 ]
      gap> LGFirst(spec);
      [ 1, 5, 6, 7 ]
      gap> LGLength( G );
      3

      gap> p := SpecialPcgs( Pcgs( SmallGroup( 96, 120 ) ) );
      Pcgs([ f1, f2, f3, f4, f5, f6 ])
      gap> LGWeights(p);
      [ [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 2, 2 ], [ 1, 3, 2 ],
        [ 2, 1, 3 ] ]

  Thus the first group, `SmallGroup(96, 220)`, has a lower nilpotent series of length 1; that is, the group is nilpotent. It is a direct product of its Sylow subgroups. Moreover the Sylow 2-subgroup is generated by f1, f2, f3, f4, f6 and the Sylow 3-subgroup is generated by f5. The lower 2-central series of the Sylow 2-subgroup has length 2 and the second subgroup in this series is generated by f6.

  The second group, `SmallGroup(96, 120)`, has a lower nilpotent series of length 2 and hence is not nilpotent. The second subgroup in this series is just the Sylow 3-subgroup and it is generated by f6. The subgroup generated by f1, ¼, f5 is a Sylow 2-subgroup of the group and also a head complement to the second head of the group. Its lower 2-central series has length 2.

  In this example the `FamilyPcgs` of the groups used was a special pcgs, but this is not necessarily the case. For performance reasons it can be worth to enforce this, see IsomorphismSpecialPcGroup.

- `IsInducedPcgsWrtSpecialPcgs(` pcgs`) P`

  tests whether pcgs is induced with respect to a special pcgs.

- `InducedPcgsWrtSpecialPcgs(` G`) A`

  computes an induced pcgs with respect to the special pcgs of the parent of G.

  `InducedPcgsWrtSpecialPcgs` will return a pcgs induced by a special pcgs (which might differ from the one you had in mind). If you need an induced pcgs compatible with a given special pcgs use `InducedPcgs` for this special pcgs.

  ## 43.14 Action on Subfactors Defined by a Pcgs

  When working with a polycyclic group, one often needs to compute matrix operations of the group on a factor of the group. For this purpose there are the following functions.

- `VectorSpaceByPcgsOfElementaryAbelianGroup(` mpcgs`,` fld`) F`

  returns the vector space over fld corresponding to the modulo pcgs mpcgs. Note that mpcgs has to define an elementary abelian p-group where p is the characteristic of fld.

- `LinearOperation(` gens`,` basisvectors`,` linear`) O`
- `LinearAction(` gens`,` basisvectors`,` linear`) O`

  returns a list of matrices, one for each element of gens, which corresponds to the matrix action of the elements in gens on the basis basisvectors via linear.

- `LinearOperationLayer(` G`,` gens`,` pcgs`) F`
- `LinearActionLayer(` G`,` gens`,` pcgs`) F`

  returns a list of matrices, one for each element of gens, which corresponds to the matrix action of G on the vector space corresponding to the modulo pcgs pcgs.

  In certain situations, for example within the computation of conjugacy classes of finite soluble groups as described in MeckyNeubueser89, affine actions of groups are required. For this purpose we introduce the following functions.

- `AffineOperation(` gens`,` basisvectors`,` linear`,` transl`) O`
- `AffineAction(` gens`,` basisvectors`,` linear`,` transl`) O`

  return a list of matrices, one for each element of gens, which corresponds to the affine action of the elements in gens on the basis basisvectors via linear with translation transl.

- `AffineOperationLayer(` G`,` gens`,` pcgs`,` transl`) F`
- `AffineActionLayer(` G`,` gens`,` pcgs`,` transl`) F`

  returns a list of matrices, one for each element of gens, which corresponds to the affine action of G on the vector space corresponding to the modulo pcgs pcgs with translation transl.

      gap> G := SmallGroup( 96, 51 );
      <pc group of size 96 with 6 generators>
      gap> spec := SpecialPcgs( G );
      Pcgs([ f1, f2, f3, f4, f5, f6 ])
      gap> LGWeights( spec );
      [ [ 1, 1, 2 ], [ 1, 1, 2 ], [ 1, 1, 3 ], [ 1, 2, 2 ], [ 1, 2, 2 ],
        [ 1, 3, 2 ] ]
      gap> mpcgs := InducedPcgsByPcSequence( spec, spec{[4,5,6]} );
      Pcgs([ f4, f5, f6 ])
      gap> npcgs := InducedPcgsByPcSequence( spec, spec{[6]} );
      Pcgs([ f6 ])
      gap> modu := mpcgs mod npcgs;
      [ f4, f5 ]
      gap> mat:=LinearActionLayer( G, spec{[1,2,3]}, modu );
      [ <an immutable 2x2 matrix over GF2>, <an immutable 2x2 matrix over GF2>,
        <an immutable 2x2 matrix over GF2> ]

      gap> Print(mat);
      [ [ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ], [ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2),
          Z(2)^0 ] ], [ [ Z(2)^0, 0*Z(2) ], [ 0*Z(2), Z(2)^0 ] ] ]

  ## 43.15 Orbit Stabilizer Methods for Polycyclic Groups

  If a pcgs pcgs is known for a group G, then orbits and stabilizers can be computed by a special method which is particularly efficient. Note that within this function only the elements in pcgs and the relative orders of pcgs are needed. Hence this function works effectively even if the elementary operations for pcgs are slow.

- `StabilizerPcgs(` pcgs`,` pt`)`

- `Pcgs_OrbitStabilizer(` pcgs`,` domain`,` pnt`,` oprs`,` opr`) F`

  runs a solvable group orbit-stabilizer algorithm on pnt with pcgs acting via the images oprs and the operation function opr. The domain domain can be used to speed up search, if it is not known, `false` can be given instead. The function returns a record with components `orbit`, `stabpcgs` and `lengths`, the latter indicating the lengths of the orbit whenever it got extended. This can be used to recompute transversal elements. This function should be used only inside algorithms when speed is essential.

  ## 43.16 Operations which have Special Methods for Groups with Pcgs

  For the following methods there are special operations for groups with pcgs installed:

  `IsNilpotent`, `IsSupersolvable`, `Size`, `CompositionSeries`, `ConjugacyClasses`, `Centralizer`, `FrattiniSubgroup`, `PrefrattiniSubgroup`, `MaximalSubgroups` and related operations, `HallSystem` and related operations, `MinimalGeneratingSet`, `Centre`, `Intersection`, `AutomorphismGroup`, `IrreducibleModules`.

  ## 43.17 Conjugacy Classes in Solvable Groups

  There are a variety of algorithms to compute conjugacy classes and centralizers in solvable groups via epimorphic images (FelschNeubueser79, MeckyNeubueser89, Theissen93). Usually these are only invoked as methods, but it is possible to access the algorithm directly.

  @The syntax of this function may change in a future rewrite!@

- `ClassesSolvableGroup(` G`,` mode`[,` opt`] ) F`

  computes conjugacy classes and centralizers in solvable groups. G is the acting group. mode indicates the type of the calculation:

  0 Conjugacy classes

  4 Conjugacy test for the two elements in opt`.candidates`

  In mode 0 the function returns a list of records containing components representative and centralizer. In mode 4 it returns a conjugating element.

  The optional record opt may contain the following components that will affect the algorithms behaviour:

  `pcgs`
    is a pcgs that will be used for the calculation. In the case of the calculation of rational classes, it must be a pcgs refining a central series. The attribute `NormalSeriesByPcgs` must return an appropriate series of normal subgroups with elementary abelian factors among them. The algorithm will step down this series. By default an `ElementaryAbelianSeries` is used.

  `candidates`
    is a list of elements for which canonical representatives are to be computed or for which a conjugacy test is performed. They must be given in mode 4. In mode 0 a list of classes corresponding to candidates is returned (which may contain duplicates). The representatives chosen are canonical with respect to pcgs. The records returned also contain components operator such that (candidate `^` operator) =representative.

  `consider`
    is a function consider(fhome,rep,cenp,K,L). Here fhome is a home pcgs for the factor group F in which the calculation currently takes place, rep is an element of the factor and cenp is a pcgs for the centralizer of rep modulo K. In mode 0, when lifting from F/K to F/L (note: for efficiency reasons, F can be different from G or L might be not trivial) this function is called before performing the actual lifting and only those representatives for which it returns `true` are passed to the next level. This permits for example the calculation of only those classes with small centralizers or classes of restricted orders.

- `CentralizerSizeLimitConsiderFunction(` sz`) F`

  returns a function (of the form func(fhome,rep,cen,K,L) )that can be used in `ClassesSolvableGroup` as the consider component of the options record. It will restrict the lifting to those classes, for which the size of the centralizer (in the factor) is at most sz.

  See also `SubgroupsSolvableGroup` (SubgroupsSolvableGroup).

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  GAP 4 manual
  May 2002