This mail contains a fix for the complement routines for AgGroups.

The priority of this bug fix is *high*, because one of the bugs might
lead to wrong results without a warning.

This bug fix changes only the library. Thus you need not recompile the
GAP kernel.

Go to the GAP directory (the directory with the 'lib/' subdirectory),
name this bug fix file 'bugfix02.dif', and issue the command:

patch -p0 < bugfix02.dif

COMMENT

This fix simply disables the faulty treatment of the coprime case when
complements in AgGroups can be obtained as Hall subgroups.

The current treatment of this case produces elements at an interim stage
which should be in the normal subgroup but are not. In the examples
considered so far this did not lead to wrong results. In fact the bug
was only detected when including an extra check function intended for
the test of extended routines.

We cannot show that this misbehaviour will produce errors at all.
However we also cannot show the contrary. Thus we recommend to
apply this fix to your local installation.

VERSION

GAP 3.4.3.0
@(#)$Id: 1.html,v 1.2 2004/04/21 15:04:32 felsch Exp $

DESCRIPTION

'Complementclasses' and 'Complement' might produce wrong results in
coprime situations.

FIX

Prereq: 3.11.1.1
--- lib/agcomple.g      1995/10/09 09:51:50
+++ lib/agcomple.g      1996/02/22 10:59:28
@@ -2,13 +2,16 @@
 ##
 #A  agcomple.g                  GAP library                      Frank Celler
 ##
-#A  @(#)$Id: 1.html,v 1.2 2004/04/21 15:04:32 felsch Exp $
+#A  @(#)$Id: 1.html,v 1.2 2004/04/21 15:04:32 felsch Exp $
 ##
 #Y  Copyright 1990-1992,  Lehrstuhl D fuer Mathematik,  RWTH Aachen,  Germany
 ##
 ##  This file contains all polymorph functions for groups.
 ##
 #H  $Log: 1.html,v $
 #H  Revision 1.2  2004/04/21 15:04:32  felsch
 #H  Corrected links in the Forum Archive pages.   VF
 #H
 #H  Revision 1.1.1.1  2004/04/20 13:39:35  felsch
 #H  The final GAP-Forum archive until 2003.
 #H
 #H  Revision 1.6  2003/06/12 19:20:33  gap
 #H  Further update. AH
 #H
 #H  Revision 1.5  2003/06/12 17:28:25  gap
 #H  Address updates by JN. AH
 #H
 #H  Revision 1.4  1999/03/18 10:17:42  gap
 #H  Fixed a mail that was corrupted in the files stemming from Martin.
 #H
 #H  Revision 1.3  1997/08/15 11:19:30  gap
 #H  New forum setup. AH
 #H
 #H  Revision 1.2  1997/04/24 15:32:10  gap
 #H  These files were replaced by the versions in WWW. The content is basically the
 #H  same but the formatting has been much more friendly towards the HTML-Converter.
 #H  AH
 #H
 #H  Revision 1.1  1996/10/30 13:07:02  gap
 #H  added forum archive and translation files.
 #H
+#H  Revision 3.11.1.2  1996/02/22  10:59:28  ahulpke
+#H  disabled erraneous coprime case (only temporary fix)
+#H
 #H  Revision 3.11.1.1  1995/10/09  09:51:50  fceller
 #H  fixed the parameter sequence of 'SumAgGroup' (normal subgroup first)
 #H
@@ -901,13 +904,18 @@
        cor := rec();

     # otherwise we compute a hall system for <G>/<N>
-    else
-       InfoAgCo2( "#I  Complements: computing p prime sets\n" );
+    elif false  then
+        InfoAgCo2( "#I  Complements: computing p prime sets\n" );
         a   := NaturalHomomorphism( G, G / N );
         cor := PPrimeSetsOC( Image( a ) );
-        cor.generators := List( cor.generators, x ->
-                               PreImagesRepresentative( a, x ) );
-       cor.useCentralSK := true;
+        cor.generators := List( cor.generators, x ->
+                                PreImagesRepresentative( a, x ) );
+        cor.useCentralSK := true;
+
+    # use standard algorithm until CoprimeComplement is fixed
+    else
+        cor := rec();
+        cor.useCentralSK := true;
     fi;

# we want our nice elementary abelian series

END OF mail