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\Chapter{Libraries and examples of pcp-groups}

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\Section{Libraries of various types of polycyclic groups}

There are the following generic pcp-groups available.

\>AbelianPcpGroup( <n>, <rels> )

      constructs the   abelian  group  on  <n>  generators  such  that
      generator $i$ has  order $rels[i]$. If  this  order is infinite,
      then $rels[i]$ should be either unbound or 0.

\>DihedralPcpGroup( <n> )

      constructs the dihedral  group of order <n>. If <n>  is an odd
      integer, then 'fail' is returned.  If  <n> is zero or not an 
      integer, then the infinite dihedral group is returned.

\>UnitriangularPcpGroup( <n>, <c> )

      returns a pcp-group isomorphic  to the group of upper triangular
      in $GL(n, R)$ where $R = \Z$ if $c = 0$ and $R = \F_p$ if $c = p$.
      The natural unitriangular matrix representation of the returned 
      pcp-group $G$ can be obtained as $G!.isomorphism$.

\>SubgroupUnitriangularPcpGroup( <mats> )

      <mats> should be a list of upper unitriangular $n \times n$ 
      matrices over $\Z$ or over $\F_p$. This function returns the 
      subgroup of the corresponding 'UnitriangularPcpGroup' generated 
      by the matrices in <mats>.

\>InfiniteMetacyclicPcpGroup( <n>, <m>, <r> )

      Infinite metacyclic groups are classified in \cite{BK00}. Every 
      infinite metacyclic group $G$ is isomorphic to a finitely presented 
      group $G(m,n,r)$ with two generators $a$ and $b$ and relations of the 
      form $a^n = b^n = 1$ and $[a,b] = a^{1-r}$, where $m,n,r$ are three
      non-negative integers with $mn=0$ and $r$ relatively prime to $m$. 
      If $r\equiv -1$ mod $m$ then $n$ is even, and if $r\equiv 1$ mod 
      $m$ then $m=0$. Also $m$ and $n$ must not be $1$.

      Moreover, $G(m,n,r)\cong G(m',n',s)$ if and only if $m=m'$, $n=n'$, 
      and either $r\equiv s$ or $r\equiv s^{-1}$ mod $m$. 

      This function returns the metacyclic group with parameters <n>,
      <m> and <r> as a pcp-group with the pc-presentation $\langle
      x,y | x^n, y^m, y^x = y^r\rangle$.  This presentation is easily
      transformed into the one above via the mapping $x\mapsto b^{-1},
      y \mapsto a$. 

\>HeisenbergPcpGroup( <n> )

      returns the Heisenberg group on <n> generators as pcp-group.

\>MaximalOrderByUnitsPcpGroup( <f> )

      takes as input a normed, irreducible polynomial over the integers.
      Thus <f> defines a field extension <F> over the rationals. This 
      function returns the split extension of the maximal order <O> of <F> 
      by the unit group <U> of <O>, where <U> acts by right multiplication
      on <O>.

\>BurdeGrunewaldPcpGroup( <s>, <t> )
   
      returns a nilpotent group of Hirsch length 11 which has been 
      constructed by Burde und Grunewald. If <s> is not 0, then this 
      group has no faithful 12-dimensional linear representation.

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\Section{Some asorted example groups}

The functions in this section provide some more example groups to play
with. They come with no further description and their investigation is
left to the interested user.

\>ExampleOfMetabelianPcpGroup( <a>, <k> )

      returns an example of a metabelian group. The input parameters must
      be two positive integers greater than 1.

\>ExamplesOfSomePcpGroups( <n> )

      this function takes values <n> in 1 up to 16 and returns for each 
      input an example of a pcp-group. The groups in this example list 
      have been used as test groups for the functions in this package.

