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Chris Wensley writes:

Martin Sch"onert, in his reply to Tim Hsu, writes:

" ..use Tietze transformations.. If one keeps track of the transformations.."

Since this is precisely what I want to do, I am asking how!

Specifically, if a messy presentation with generators _x1 .. _x20 (say)

is reduced, using TzGo , to a presentation with generating set

[ _x3, _x12, _x24 ] ,I want to remember expressions for the

21 eliminated generators as words in the three remaining ones.I have not been able to discover a solution in the manual but 3 possible

methods suggest themselves. Guidance as to which way to go would be much

appreciated.1. Avoid using the high-level TzGo and use the low-level commands

so that at each elimination the required formula can be saved. Surely

this would amount to rewriting TzGo ?2. Edit appropriate parts of the file "fptietze.g" .

This file is 40 pages long, with many interdependant functions,

so I should probably create a host of bugs?

In any case, the functions use the <Tietze record> type, which

appears to be defined elsewhere, possibly in the kernel?3. Start a LOG file before using TzGo and so produce a file

containing messages of the form:

#I eliminating _x7 = _x18^5*_x5*_x4

As far as I can tell, GAP cannot Read this LOG file, but perhaps it

would be possible to write some utility to convert the LOG file into

a GAP file, Exec this utility, and Read the resulting GAP file?

As the discussion on keeping track of Tietze transformations is too

technical to be continued in the GAP forum, I have sent my answer to

the above letter directly to Chris Wensley. However, I would like to

encourage all those forum members who are interested in that question

to ask me for a copy of that correspondence.

Volkmar Felsch, Aachen

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