> < ^ Date: Wed, 19 Jun 1996 14:53:38 +0100
> < ^ From: Leonard Soicher <l.h.soicher@qmul.ac.uk >
> < ^ Subject: Re: Computing a presentation for a group

Dear Gap-forum,

In a letter to the Forum on 22 April 1996, Volkmar Felsch wrote:

As described in the manual section 'Tietze Transformations', each
presentation record, 'P' say, carries with it certain parameters,
the so-called 'Tietze options'. One of these is the parameter
'P.generatorsLimit' which by default is initialized to be zero.
Its purpose is to prevent the Tietze transformations functions
from eliminating too many generators.

More precisely: The functions 'SimplifyPresentation', 'TzGo', and
'TzGoGo' are not allowed to eliminate a generator if the resulting
number of generators would be smaller than this bound.

However, it does appear that SimplifyPresentation(P) will eliminate a
generator found to be trivial, even if P.generatorsLimit has first
been set to Length(P.generators). This certainly can cause me
problems when I need there to be a bijection between two sets of
generators, even if one of the generators turns out to be trivial. Is
there any way to prevent SimplifyPresentation from removing trivial
generators? Am I right in assuming that the function TzSearch would
not do this?

Thanks for your help.

Regards, Leonard.

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