From ce68bc347106e19ded19d87b5425e07f19640127 Mon Sep 17 00:00:00 2001
From: ThomasBreuer <sam@math.rwth-aachen.de>
Date: Wed, 13 Sep 2017 10:49:47 +0200
Subject: [PATCH] fixed a bug in `RadicalOfAlgebra`

For associative algebras that are not matrix algebras,
the radical is computed in an isomorphic matrix algebra.
Up to now, one ran into an error if `Coefficients`
returned an immutable list.

(One might argue that `Coefficients` should always return
a new mutable list, but this would cause the creation of
many superfluous lists.
Thus we solve the problem in the place where the list
has to be changed.)
---
 lib/algebra.gi            | 3 +++
 tst/testinstall/algsc.tst | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/lib/algebra.gi b/lib/algebra.gi
index 69f08fef37..c17c6557ff 100644
--- a/lib/algebra.gi
+++ b/lib/algebra.gi
@@ -2757,6 +2757,9 @@ InstallMethod( RadicalOfAlgebra,
         M:=[];
         for j in [1..n] do
           col:= Coefficients( BA, bv[i] * bv[j] );
+          if not IsMutable( col ) then
+            col:= ShallowCopy( col );
+          fi;
           col[n+1]:= Zero( F );
           Add( M, col );
         od;
diff --git a/tst/testinstall/algsc.tst b/tst/testinstall/algsc.tst
index 12c6292268..a3dc908a8b 100644
--- a/tst/testinstall/algsc.tst
+++ b/tst/testinstall/algsc.tst
@@ -675,6 +675,15 @@ gap> c:= Centre( t );
 gap> c = t;
 true
 
+#############################################################################
+##
+##  Some bugfixes
+##
+gap> a:= Algebra( Rationals, [ [ [ 0, 0 ], [ 1, 0 ] ] ] );;
+gap> sc:= Image( IsomorphismSCAlgebra( a ) );;
+gap> Dimension( RadicalOfAlgebra( sc ) );
+1
+
 #############################################################################
 gap> STOP_TEST( "algsc.tst", 1);