Dear Forum,

This is to announce bugfix number 13 for GAP3. You should not apply this
bugfix to GAP4.
The priority of this fix is high.

This fix corrects:
1) A bug in the function `PresentationViaCosetTable'.
2) A bug in the functions `ImfInvariants' and `DisplayImfInvariants'.

Bug 1) may cause wrong results (without evidence), or unexpected error
messages, or a segmentation fault.

Bug 2) produces a wrong list of elementary divisors for the representative
group of the 7th Q-class of the irreducible maximal finite integral matrix
groups of dimension 20.

At the end of this message there are some examples illustrating the fix.

Many thanks to Isabel Araujo and Peter Campbell for reporting bug 1).

To apply the fix, load the zoo archive 'fix13.zoo' from the
bugfixes web page
http://www-gap.dcs.st-and.ac.uk/~gap/Info/bugfixes.html
Alternatively you can find this file in the 'gap-3.4.4/bugfixes' directory
of the GAP ftp distribution.

Unpack the file in the home directory of your GAP distribution (the
directory containing the 'lib' and 'grp' directories) using 'unzoo -x
fix13.zoo'. `unzoo' may ask you for permissions to overwrite files,
answer this with Y)es or A)ll. This will replace the erroneous file by
fixed versions.

This fix does not change the source of the kernel, so you will not have to
recompile, nor do you need any new binaries.

You can remove the file 'description13' afterwards.

Willem de Graaf 19-May-00


#############################################################################
#
# This test job should run without error and produce the same results as
# given below if fix13 has been applied.
#

ImfInvariants(20,7).elementaryDivisors;
DisplayImfInvariants(20,7);
f := FreeGroup(2);;
g := Group( [[-1,0],[0,1]], [[1,0],[0,-1]] );;
PresentationViaCosetTable( g, f, [ f.1 ] );
g := Group( [[0,1],[-1,0]], [[0,1],[1,0]] );;
PresentationViaCosetTable( g, f, [ f.1 ] );
g := Group( [[0,1],[1,0]], [[0,1],[-1,0]] );;
PresentationViaCosetTable( g, f, [ f.1 ] );
quit;

#############################################################################

gap> ImfInvariants(20,7).elementaryDivisors;
[ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 3, 6, 6, 6, 6 ]
gap> DisplayImfInvariants(20,7);
#I Q-class 20.7:  Size = 2^17*3^10*5^2
#I   isomorphism type = ((SU(4,2) x C6):C2) wr C2
#I   elementary divisors = 1^10*3^6*6^4
#I   orbit size = 540, minimal norm = 4
gap> f := FreeGroup(2);;
gap> g := Group( [[-1,0],[0,1]], [[1,0],[0,-1]] );;
gap> PresentationViaCosetTable( g, f, [ f.1 ] );
<< presentation with 2 gens and 3 rels of total length 8 >>
gap> g := Group( [[0,1],[-1,0]], [[0,1],[1,0]] );;
gap> PresentationViaCosetTable( g, f, [ f.1 ] );
<< presentation with 2 gens and 3 rels of total length 10 >>
gap> g := Group( [[0,1],[1,0]], [[0,1],[-1,0]] );;
gap> PresentationViaCosetTable( g, f, [ f.1 ] );
<< presentation with 2 gens and 3 rels of total length 10 >>
gap> quit;