Dear Gap-forum,

Leonard Soicher wrote:

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?

Not at present, but in the next patch or release.

The above question is asking for a change of the current concept of
the functions which create or handle presentation records, i. e., the
functions

PresentationFpGroup,
PresentationSubgroupMtc,
PresentationSubgroupRrs,
PresentationNormalClosureRrs,

and all the Tietze transformations functions.

In fact, this change will come with the next patch or release of Gap,
for it is part of an extension of the Tietze transformations functions
which is presently under work. It will provide a possibility to trace
the images of the given generators as words in the final generators as
well as the preimages of the final generators as words in the given
generators under the isomorphism which is defined by all the Tietze
transformations that are applied to a presentation.

Unfortunately, these extensions are not yet available. Therefore, if
there are Gap users who urgently need an intermediate ad hoc solution
which guarantees the above mentioned bijection, I would suggest that
they contact me directly.

Volkmar Felsch volkmar.felsch@math.rwth-aachen.de