[GAP Forum] bag with IsomorphismPermGroup

[Rene Hartung]

Dear Evgeny,

sorry for the delay. 

The method NqEpimorphismNilpotentQuotient computes an epimorphism onto a 
PcpGroup. Since a PcpGroup might (in general) be infinite, the (current) 
method implemented for IsomorphismPermGroup asks for a group lying in the 
filter 'IsPcpGroup and IsFinite'. 

When computing the size of the group, the filter 'IsFinite' is set and the 
current method applies to your (finite) PcpGroup. If, on the other hand, the 
PcpGroup does not know whether it is finite or not, then there's no method 
implemented. This will be changed in an future update of polycyclic (it is 
already contained in the cvs-branch of gap). 

Cheers, ren\'e. 

On Thursday 22 January 2009 11:54, Evgeny Vdovin wrote:
> Dear all,
> This program runs ok on my GAP
>
> F:=FreeGroup(1);
> G:=F/[F.1^5];    # Z_5
> h:=NqEpimorphismNilpotentQuotient(G,1);;
> H:=Image(h);
> Size(H);    # critical line
> IsomorphismPermGroup(H);
>
> but if I remove the "critical line"
>
> F:=FreeGroup(1);
> G:=F/[F.1^5];    # Z_5
> h:=NqEpimorphismNilpotentQuotient(G,1);;
> H:=Image(h);
> IsomorphismPermGroup(H);
>
> it runs into error as follows:
>
>
> gap> F:=FreeGroup(1);
> <free group on the generators [ f1 ]>
> gap> G:=F/[F.1^5];    # Z_5
> <fp group on the generators [ f1 ]>
> gap> h:=NqEpimorphismNilpotentQuotient(G,1);
> [ f1 ] -> [ g1 ]
> gap> H:=Image(h);
> Pcp-group with orders [ 5 ]
> gap> IsomorphismPermGroup(H);
> Error, no method found! For debugging hints type ?Recovery from
> NoMethodFound
> Error, no 2nd choice method found for `IsomorphismPermGroup' on 1
> arguments 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>
>
> I have no idea why choosing not to calculate Size(H) makes the program
> break.
> Can anyone help me out?