People here made for me the installation of GAP under UNIX. When I
tried to run it, as soon as I call to some function I get the
following message:

the library file 'filename' must exist and be readable in
ReadLib("filename") called from main loop

What have we to do?

You need to tell GAP where to find it's library files. If you haven't
installed the library it can be obtained from whereever you got GAP in
the file lib3r2.zoo or lib3r2.tar.Z. If you have installed it already
then you should call GAP with the -l option to indicate where the
library is.

For example

gap -l /usr/local/gap/lib

An easy way to do this is to call gap by way of a shell script.
Typically the shell script is called /usr/local/bin/gap, and is
something like

#!/bin/sh
/usr/local/gap/src/gap -l /usr/local/gap/lib -h /usr/local/gap/doc $*

The -h option specifies the location of the documentation files used
to provide the on-line help.

You will have to change the path-names to suit your system.

Steve Linton