From 5b8dd6c49d8db411de6e6ef25b133e67659967ac Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?G=C3=A1bor=20Horv=C3=A1th?= <ghorvath@science.unideb.hu>
Date: Sat, 9 Apr 2016 12:52:08 +0200
Subject: [PATCH] Add efficiency note and infinite example to
 MaximalNormalSubgroups

---
 lib/grp.gd | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/grp.gd b/lib/grp.gd
index 85c409981f..4e6784a279 100644
--- a/lib/grp.gd
+++ b/lib/grp.gd
@@ -1713,9 +1713,19 @@ DeclareAttribute( "NilpotencyClassOfGroup", IsGroup );
 ##  is a list containing those proper normal subgroups of the group <A>G</A>
 ##  that are maximal among the proper normal subgroups. Returns fail if there
 ##  are infinitely many maximal normal subgroups.
+##
+##  Note, that the maximal normal subgroups of a group <A>G</A> can be
+##  computed more efficiently if the character table of <A>G</A> is known or
+##  if <A>G</A> is known to be abelian or solvable (even if infinite). So if
+##  the character table is needed, anyhow, or <A>G</A> is suspected to be
+##  abelian or solvable, then these should be computed before computing the
+##  maximal normal subgroups.
 ##  <Example><![CDATA[
 ##  gap> MaximalNormalSubgroups( g );
 ##  [ Group([ (1,2,3), (2,3,4) ]) ]
+##  gap> f := FreeGroup("x", "y");; x := f.1;; y := f.2;;
+##  gap> List(MaximalNormalSubgroups(f/[x^2, y^2]), GeneratorsOfGroup);
+##  [ [ x, y*x*y^-1 ], [ y, x*y*x^-1 ], [ y*x^-1 ] ]
 ##  gap> MaximalNormalSubgroups( AbelianGroup( [ 0 ] ) );
 ##  fail
 ##  ]]></Example>