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Jacob Hirbawi asked about the possibility to calculate with the

elements of an infinite matrix group over the rationals. That is

indeed possible as well as the calculation with matrices over

cyclotomic fields, it is just that most of the general group functions

cannot be used. Frank Celler is answering that in more detail.

However,in a PS you say that this has to do with a number of problems

in crystallography and since in the past I have been working on

crystallographic groups making heavy use of computer methods (e.g. for

the classification of the four-dimensional point and space groups), I

am curious to know what you are after. possibly we or others might

even have some suggestions how to use GAP for the problems.

Joachim Neubueser

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