  
  [1X2 [33X[0;0YMathematical Background and Terminology[133X[101X
  
  [33X[0;0YIn this chapter we will give a brief description of the mathematical notions
  used  in  the  algorithms  implemented  in  the ANU [10Xpq[110X program that are made
  accessible  from  [5XGAP[105X  through  this package. For proofs and details we will
  point  to  relevant  places in the published literature. Also we will try to
  give  some  explanation  of  terminology  that may help to use the [21Xlow-level[121X
  interactive  functions  described  in  Section [14X'[33X[0;0YLow-level  Interactive ANUPQ
  functions  based on menu items of the pq program[133X'[114X. However, users who intend
  to   use  these  functions  are  strongly  advised  to  acquire  a  thorough
  understanding  of the algorithms from the quoted literature. There is little
  or no checking done in these functions and naive use may result in incorrect
  results.[133X
  
  
  [1X2.1 [33X[0;0YBasic notions[133X[101X
  
  
  [1X2.1-1 [33X[0;0Ypc Presentations and Consistency[133X[101X
  
  [33X[0;0YFor details, see e.g. [NNN98].[133X
  
  [33X[0;0YEvery finite [22Xp[122X-group [22XG[122X has a presentation of the form:[133X
  
  
  [24X[33X[0;6Y\{a_1,\dots,a_n  \mid  a_i^p = v_{ii}, 1 \le i \le n, [a_k, a_j] = v_{jk}, 1
  \le j < k \le n \}.[133X
  
  [124X
  
  [33X[0;0Ywhere [22Xv_jk[122X is a word in the elements [22Xa_k+1,dots,a_n[122X for [22X1 ≤ j ≤ k ≤ n[122X.[133X
  
  [33X[0;0YThis  is  called a [13Xpower-commutator[113X presentation (or [13Xpc presentation[113X or [13Xpcp[113X)
  of  [22XG[122X,  generators  from  such  a  presentation  will  be  referred to as [13Xpc
  generators[113X. In terms of such pc generators every element of [22XG[122X can be written
  in  a  [21Xnormal  form[121X [22Xa_1^e_1dots a_n^e_n[122X with [22X0 ≤ e_i < p[122X. Moreover any given
  product  of  the generators can be brought into such a normal form using the
  defining  relations  in  the  above  presentation as rewrite rules. Any such
  process  is  called  [13Xcollection[113X.  For  the  discussion of various collection
  methods see [LGS90] and [VL90b].[133X
  
  [33X[0;0YEvery  [22Xp[122X-group  of  order  [22Xp^n[122X has such a pcp on [22Xn[122X generators and conversely
  every  such  presentation  defines a [22Xp[122X-group. However a [22Xp[122X-group defined by a
  pcp  on  [22Xn[122X  generators  can  be  of  smaller  order [22Xp^m[122X with [22Xm<n[122X. A pcp on [22Xn[122X
  generators  that  does  in  fact  define  a  [22Xp[122X-group  of order [22Xp^n[122X is called
  [13Xconsistent[113X  in  this  manual,  in  line  with  most of the literature on the
  algorithms occurring here. A consistent pcp determines a [13Xconfluent rewriting
  system[113X  (see [2XIsConfluent[102X  ([14XReference:  IsConfluent[114X)  of  the  [5XGAP[105X  Reference
  Manual) for the group it defines and for this reason often (in particular in
  the [5XGAP[105X Reference Manual) such a pcp presentation is also called [13Xconfluent[113X.[133X
  
  [33X[0;0YConsistency  of  a  pcp is tantamount to the fact that for any given word in
  the generators any two collections will yield the same normal form.[133X
  
  [33X[0;0YConsistency  of  a  pcp  can  be  checked  by  a  finite  set of [13Xconsistency
  conditions[113X, demanding that collection of the left hand side and of the right
  hand  side  of  certain  equations,  starting  with subproducts indicated by
  bracketing,  will  result in the same normal form. There are 3 types of such
  equations (that will be referred to in the manual):[133X
  
  
  [24X[33X[0;6Y\begin{array}{rclrl}  (a^n)a  &=&  a(a^n)  &&{\rm  (Type  1)}  \\ (b^n)a &=&
  b^{(n-1)}(ba),  b(a^n)  =  (ba)a^{(n-1)} &&{\rm (Type 2)} \\ c(ba) &=& (cb)a
  &&{\rm (Type 3)} \\ \end{array}[133X
  
  [124X
  
  [33X[0;0YSee  [VL84]  for a description of a sufficient set of consistency conditions
  in the context of the [22Xp[122X-quotient algorithm.[133X
  
  
  [1X2.1-2 [33X[0;0YExponent-[22Xp[122X[101X[1X Central Series and Weighted pc Presentations[133X[101X
  
  [33X[0;0YFor details, see [NNN98].[133X
  
  [33X[0;0YThe  ([13Xdescending[113X or [13Xlower[113X) ([13Xexponent-[113X)[13X[22Xp[122X-central series[113X of an arbitrary group
  [22XG[122X is defined by[133X
  
  
  [24X[33X[0;6YP_0(G) := G, P_i(G) := [G, P_{i-1}(G)] P_{i-1}(G)^p.[133X
  
  [124X
  
  [33X[0;0YFor a [22Xp[122X-group [22XG[122X this series terminates with the trivial group. [22XG[122X has [13X[22Xp[122X-class[113X
  [22Xc[122X  if  [22Xc[122X  is  the smallest integer such that [22XP_c(G)[122X is the trivial group. In
  this  manual, as well as in much of the literature about the [10Xpq[110X- and related
  algorithms, the [22Xp[122X-class is often referred to simply by [13Xclass[113X.[133X
  
  [33X[0;0YLet the [22Xp[122X-group [22XG[122X have a consistent pcp as above. Then the subgroups[133X
  
  
  [24X[33X[0;6Y\langle1\rangle < {\langle}a_n\rangle < {\langle}a_n, a_{n-1}\rangle < \dots
  < {\langle}a_n,\dots,a_i\rangle < \dots < G[133X
  
  [124X
  
  [33X[0;0Yform  a  central  series  of [22XG[122X. If this refines the [22Xp[122X-central series, we can
  define  the [13Xweight function[113X [22Xw[122X for the pc generators by [22Xw(a_i) = k[122X, if [22Xa_i[122X is
  contained in [22XP_k-1(G)[122X but not in [22XP_k(G)[122X.[133X
  
  [33X[0;0YThe  pair  of  such  a  weight  function  and a pcp allowing it, is called a
  [13Xweighted pcp[113X.[133X
  
  
  [1X2.1-3 [33X[0;0Y[22Xp[122X[101X[1X-Cover, [22Xp[122X[101X[1X-Multiplicator[133X[101X
  
  [33X[0;0YFor details, see [NNN98].[133X
  
  [33X[0;0YLet  [22Xd[122X  be  the  minimal number of generators of the [22Xp[122X-group [22XG[122X of [22Xp[122X-class [22Xc[122X.
  Then  [22XG[122X  is isomorphic to a factor group [22XF/R[122X of a free group [22XF[122X of rank [22Xd[122X. We
  denote  [22X[F,  R]  R^p[122X  by  [22XR^*[122X.  It can be proved (see e.g. [O'B90]) that the
  isomorphism  type  of  [22XG^*  :=  F/R^*[122X  depends  only on [22XG[122X. [22XG^*[122X is called the
  [13X[22Xp[122X-covering  group[113X  or  [13X[22Xp[122X-cover[113X of [22XG[122X, and [22XR/R^*[122X the [13X[22Xp[122X-multiplicator[113X of [22XG[122X. The
  [22Xp[122X-multiplicator  is,  of  course, an elementary abelian [22Xp[122X-group; its minimal
  number of generators is called the [13X([22Xp[122X-)multiplicator rank[113X.[133X
  
  
  [1X2.1-4 [33X[0;0YDescendants, Capable, Terminal, Nucleus[133X[101X
  
  [33X[0;0YFor details, see [New77] and [O'B90].[133X
  
  [33X[0;0YLet again [22XG[122X be a [22Xp[122X-group of [22Xp[122X-class [22Xc[122X and [22Xd[122X the minimal number of generators
  of  [22XG[122X.  A [22Xp[122X-group [22XH[122X is a [13Xdescendant[113X of [22XG[122X if the minimal number of generators
  of  [22XH[122X  is  [22Xd[122X  and  [22XH/P_c(H)[122X  is  isomorphic  to [22XG[122X. A descendant [22XH[122X of [22XG[122X is an
  [13Ximmediate  descendant[113X  if  it has [22Xp[122X-class [22Xc+1[122X. [22XG[122X is called [13Xcapable[113X if it has
  immediate descendants; otherwise it is [13Xterminal[113X.[133X
  
  [33X[0;0YLet [22XG^* = F/R^*[122X again be the [22Xp[122X-cover of [22XG[122X. Then the group [22XP_c(G^*)[122X is called
  the  [13Xnucleus[113X  of  [22XG[122X.  Note that [22XP_c(G^*)[122X is contained in the [22Xp[122X-multiplicator
  [22XR/R^*[122X.[133X
  
  [33X[0;0YIt  is  proved  (e.g. in  [O'B90])  that  the immediate descendants of [22XG[122X are
  obtained  as  factor  groups  of  the [22Xp[122X-cover by (proper) supplements of the
  nucleus  in  the (elementary abelian) [22Xp[122X-multiplicator. These are also called
  [13Xallowable[113X.[133X
  
  [33X[0;0YIt  is  further  proved there that every automorphism [22Xα[122X of [22XF/R[122X extends to an
  automorphism [22Xα^*[122X of the [22Xp[122X-cover [22XF/R^*[122X and that the restriction of [22Xα^*[122X to the
  multiplicator  [22XR/R^*[122X is uniquely determined by [22Xα[122X. Each [13Xextended automorphism[113X
  [22Xα^*[122X  induces  a  permutation  of  the allowable subgroups. Thus the extended
  automorphisms  determine a group [22XP[122X of [13Xpermutations[113X on the set [22XA[122X of allowable
  subgroups  (The  group  [22XP[122X  of permutations will appear in the description of
  some  interactive functions). Choosing a representative [22XS[122X from each orbit of
  [22XP[122X  on  [22XA[122X,  the  set of factor groups [22XF/S[122X contains each (isomorphism type of)
  immediate  descendant  of [22XG[122X exactly once. For each immediate descendant, the
  procedure   of  computing  the  [22Xp[122X-cover,  extending  the  automorphisms  and
  computing  the  orbits  on allowable subgroups can be repeated. Iteration of
  this  procedure  can  in principle be used to determine all descendants of a
  [22Xp[122X-group.[133X
  
  
  [1X2.1-5 [33X[0;0YLaws[133X[101X
  
  [33X[0;0YLet  [22Xl(x_1,  dots, x_n)[122X be a word in the free generators [22Xx_1, dots, x_n[122X of a
  free  group  of  rank  [22Xn[122X.  Then  [22Xl(x_1,  dots,  x_n)  = 1[122X is called a [13Xlaw[113X or
  [13Xidentical  relation[113X  in a group [22XG[122X if [22Xl(g_1, dots, g_n) = 1[122X for any choice of
  elements  [22Xg_1,  dots, g_n[122X in [22XG[122X. In particular, [22Xx^e = 1[122X is called an [13Xexponent
  law[113X,  [22X[[x,y],[u,v]]  = 1[122X the [13Xmetabelian law[113X, and [22X[dots [[x_1,x_2],x_2],dots,
  x_2] = 1[122X an [13XEngel identity[113X.[133X
  
  
  [1X2.2 [33X[0;0YThe p-quotient Algorithm[133X[101X
  
  [33X[0;0YFor  details,  see  [HN80],  [NO96]  and  [VL84].  Other descriptions of the
  algorithm are given in [Sim94].[133X
  
  [33X[0;0YThe  [10Xpq[110X algorithm successively determines the factor groups of the groups of
  the  [22Xp[122X-central series of a finitely presented (fp) group [22XG[122X. If a bound [22Xb[122X for
  the [22Xp[122X-class is given, the algorithm will determine those factor groups up to
  at most [22Xp[122X-class [22Xb[122X. If the [22Xp[122X-central series terminates with a subgroup [22XP_k(G)[122X
  with  [22Xk  <  b[122X,  the algorithm will stop with that group. If no such bound is
  given, it will try to find the biggest such factor group.[133X
  
  [33X[0;0Y[22XG/P_1(G)[122X  is the largest elementary abelian [22Xp[122X-factor group of [22XG[122X and this can
  be  found  from the relation matrix of [22XG[122X using matrix diagonalisation modulo
  [22Xp[122X. So it suffices to explain how [22XG/P_i+1(G)[122X is found from [22XG[122X and [22XG/P_i(G)[122X for
  some [22Xi ≥ 1[122X.[133X
  
  [33X[0;0YThis  is  done,  in  principle,  in  two  steps: first the [22Xp[122X-cover of [22XG_i :=
  G/P_i(G)[122X  is  determined  (which  depends  only  on  [22XG_i[122X, not on [22XG[122X) and then
  [22XG/P_i+1(G)[122X as a factor group of this [22Xp[122X-cover.[133X
  
  
  [1X2.2-1 [33X[0;0YFinding the [22Xp[122X[101X[1X-cover[133X[101X
  
  [33X[0;0YA  very  detailed  description  of  the first step is given in [NNN98], from
  which  we  just  extract  some  passages  in  order  to  point to some terms
  occurring in this manual.[133X
  
  [33X[0;0YLet  [22XH[122X be a [22Xp[122X-group and [22Xp^d(b)[122X be the order of [22XH/P_b(H)[122X. So [22Xd := d(1)[122X is the
  minimal  number  of  generators  of  [22XH[122X.  A  weighted pcp of [22XH[122X will be called
  [13Xlabelled[113X  if  for  each  generator  [22Xa_k[122X,  [22Xk  >  d[122X  one relation, having this
  generator as its right hand side, is marked as [13Xdefinition[113X of this generator.[133X
  
  [33X[0;0YAs  described  in  [NNN98],  a  weighted  labelled  pcp  of a [22Xp[122X-group can be
  obtained stepping down its [22Xp[122X-central series.[133X
  
  [33X[0;0YSo  let  us  assume  that  a  weighted  labelled  pcp  of  [22XG_i[122X  is  given. A
  straightforward  way  of  of writing down a (not necessarily consistent) pcp
  for  its  [22Xp[122X-cover is to add generators, one for each relation which is not a
  definition,  and  modify  the  right  hand  side  of  each  such relation by
  multiplying  it  on  the  right  by one of the new generators -- a different
  generator  for  each such relation. Further relations are then added to make
  the  new  generators central and of order [22Xp[122X. This procedure is called [13Xadding
  tails[113X. A more formal description of it is again given in [NNN98].[133X
  
  [33X[0;0YIt  is  important  to  realise  that  the  [21Xnew[121X  generators  will generate an
  elementary abelian group, that is, in additive notation, a vector space over
  the  field  of  [22Xp[122X elements. As said, the pcp of the [22Xp[122X-cover obtained in this
  way  need  not  be consistent. Since the pcp of [22XG_i[122X was consistent, applying
  the  consistency  conditions  to  the  pcp  of  the  [22Xp[122X-cover,  in  case  the
  presentation  obtained  for [22Xp[122X-cover is not consistent, will produce a set of
  equations  between  the new generators, that, written additively, are linear
  equations  over  the  field  of  [22Xp[122X  elements and can hence be used to remove
  redundant generators until a consistent pcp is obtained.[133X
  
  [33X[0;0YIn  reality,  to follow this straightforward procedure would be forbiddingly
  inefficient  except for very small examples. There are many ways of a priori
  reducing  the  number  of  [21Xnew  generators[121X  to be introduced, using e.g. the
  weights  attached to the generators, and the main part of [NNN98] is devoted
  to a detailed discussion with proofs of these possibilities.[133X
  
  
  [1X2.2-2 [33X[0;0YImposing the Relations of the fp Group[133X[101X
  
  [33X[0;0YIn order to obtain [22XG/P_i+1(G)[122X from the pcp of the [22Xp[122X-cover of [22XG_i = G/P_i(G)[122X,
  the  defining relations from the original presentation of [22XG[122X must be imposed.
  Since [22XG_i[122X is a homomorphic image of [22XG[122X, these relations again yield relations
  between the [21Xnew generators[121X in the presentation of the [22Xp[122X-cover of [22XG_i[122X.[133X
  
  
  [1X2.2-3 [33X[0;0YImposing Laws[133X[101X
  
  [33X[0;0YWhile we have so far only considered the computation of the factor groups of
  a  given  fp  group  by  the  groups of its descending [22Xp[122X-central series, the
  [22Xp[122X-quotient  algorithm allows a very important variant of this idea: laws can
  be  prescribed  that  should be fulfilled by the [22Xp[122X-factor groups computed by
  the  algorithm.  The key observation here is the fact that at each step down
  the  descending [22Xp[122X-central series it suffices to impose these laws only for a
  finite  number of words. Again for efficiency of the method it is crucial to
  keep  the  number of such words small, and much of [NO96] and the literature
  quoted in this paper is devoted to this problem.[133X
  
  [33X[0;0YIn  this form, starting with a free group and imposing an exponent law (also
  referred  to  as  an  [13Xexponent check[113X) the [10Xpq[110X program has, in fact, found its
  most  noted application in the determination of (restricted) Burnside groups
  (as reported in e.g. [HN80], [NO96] and [VL90a]).[133X
  
  [33X[0;0YVia  a  [5XGAP[105X  program using the [21Xlocal[121X interactive functions of the [10Xpq[110X program
  made available through this interface also arbitrary laws can be imposed via
  the option [10XIdentities[110X (see [14X6.2[114X).[133X
  
  
  [1X2.3  [33X[0;0YThe  p-group  generation  Algorithm, Standard Presentation, Isomorphism[101X
  [1XTesting[133X[101X
  
  [33X[0;0YFor details, see [New77] and [O'B90].[133X
  
  [33X[0;0YThe  [22Xp[122X-group  generation algorithm determines the immediate descendants of a
  given  [22Xp[122X-group  [22XG[122X  up  to  isomorphism.  From  what  has  been  explained in
  Section [14X'[33X[0;0YBasic  notions[133X'[114X,  it is clear that this amounts to the construction
  of  the  [22Xp[122X-cover, the extension of the automorphisms of [22XG[122X to the [22Xp[122X-cover and
  the  determination  of  representatives of the orbits of the action of these
  automorphisms   on   the   set   of   supplements  of  the  nucleus  in  the
  [22Xp[122X-multiplicator.[133X
  
  [33X[0;0YThe   main   practical   problem   here   is   the  determination  of  these
  representatives.  [O'B90]  describes  methods  for  this  and the [10Xpq[110X program
  allows choices according to whether space or time limitations must be met.[133X
  
  [33X[0;0YAs   well  as  the  descendants  of  [22XG[122X,  the  [10Xpq[110X  program  determines  their
  automorphism  groups from that of [22XG[122X (see [O'B95]), which is important for an
  iteration  of the process; this has been used by Eamonn O'Brien, e.g. in the
  classification  of  the  [22X2[122X-groups that are now also part of the [13XSmall Groups[113X
  library available through [5XGAP[105X.[133X
  
  [33X[0;0YA  variant  of  the  [22Xp[122X-group  generation  algorithm is also used to define a
  [13Xstandard  presentation[113X  of  a given [22Xp[122X-group. This is done by constructing an
  isomorphic  copy  of  the  given group through a chain of descendants and at
  each  step making a choice of a particular representative for the respective
  orbit  of  capable  groups.  In  a fairly delicate process, subgroups of the
  [22Xp[122X-multiplicator  are  represented  by [13Xechelonised matrices[113X and a first among
  the  [13Xlabels  for standard matrices[113X is chosen (this is described in detail in
  [O'B94]).[133X
  
  [33X[0;0YFinally,  the  standard  presentation provides a way of testing if two given
  [22Xp[122X-groups  are  isomorphic:  the  standard  presentations  of  the groups are
  computed,  for  practical  purposes  [13Xcompacted[113X  and the results compared for
  being  identical,  i.e. the  groups  are  isomorphic  if  and  only if their
  standard presentations are identical.[133X
  
