Dear GAP-Forum,

This is to announce a bugfix for the GAP library. The priority
of this fix is high.

This fix corrects:
1) An error in the Tietze transformation routines when handling presentations
including a redundant generator g and a relator of form g^2 or g^-2.

2) An error in 'RewriteSubgroupRelators' when encountering the empty word
as group relator.

Error 1 may give wrong results, error 2 may cause GAP to crash.

To apply the fix, load the zoo archive 'fix4.zoo' from the
bugfixes web page
http://www-groups.dcs.st-and.ac.uk/~gap/Info/bugfixes.html
Alternatively you can find this file in the 'bugfixes' directory of the
GAP ftp distribution.

Unpack the file in the home directory of your GAP distribution (the
directory containing the 'lib' and 'etc' directories) using 'unzoo -x
fix4.zoo' (On a PC or a Mac you will have to enter the argument line after
the call to 'unzoo'.) This will replace the erraneous file by fixed versions.

You can remove the file 'description4' afterwards.

You won't need to recompile anything.

#############################################################################
# these commands should run without error if fix4 has been applied.
#
F := FreeGroup( 1 );
G := F / [ F.1^4, F.1*F.1^-1 ];
Size( G );
H := Subgroup( G, [ G.1^2 ] );
Size( H );
P := PresentationSubgroupMtc( G, H );
F := FreeGroup( 2 );;
G := F / [ F.1*F.2, F.2^2 ];;
Size( G );
S := SimplifiedFpGroup( G );
Size( S );

Best regards,

Alexander Hulpke