From 82296906a423053b287179ea26ba2cfb4cb3adad Mon Sep 17 00:00:00 2001
From: ThomasBreuer <sam@math.rwth-aachen.de>
Date: Fri, 8 Nov 2019 14:30:09 +0100
Subject: [PATCH] fixed ConstituentsOfCharacter for Brauer character

- The variant of `ConstituentsOfCharacter` with only argument a Brauer character
  was wrong, it regarded the character as an ordinary character.

  (This bug has been reported by Gabriel Navarro.)

- Also the case of a virtual Brauer character was not handled correctly.

The two bugs are now fixed,
some tests for the variants of arguments are available,
and the documentation states which cases are admissible:
ordinary or modular characters,
ordinary of modular virtual characters.

Also, the documentation
---
 lib/ctblfuns.gd               |  9 +++++++--
 lib/ctblfuns.gi               | 24 ++++++++++++-----------
 tst/teststandard/ctblfuns.tst | 37 +++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+), 13 deletions(-)
 create mode 100644 tst/teststandard/ctblfuns.tst

diff --git a/lib/ctblfuns.gd b/lib/ctblfuns.gd
index 935b9d03c4..4172408478 100644
--- a/lib/ctblfuns.gd
+++ b/lib/ctblfuns.gd
@@ -1393,8 +1393,13 @@ DeclareAttribute( "ClassPositionsOfCentre", IsHomogeneousList );
 ##  <Attr Name="ConstituentsOfCharacter" Arg='[tbl, ]chi'/>
 ##
 ##  <Description>
-##  is the set of irreducible characters that occur in the decomposition of
-##  the (virtual) character <A>chi</A> with nonzero coefficient.
+##  Let <A>chi</A> be an ordinary or modular (virtual) character.
+##  If an ordinary or modular character table <A>tbl</A> is given then
+##  <A>chi</A> may also be a list of character values.
+##  <P/>
+##  <Ref Attr="ConstituentsOfCharacter"/> returns
+##  the set of those irreducible characters that occur in the decomposition
+##  of <A>chi</A> with nonzero coefficient.
 ##  <P/>
 ##  <Example><![CDATA[
 ##  gap> nat:= NaturalCharacter( S4 );
diff --git a/lib/ctblfuns.gi b/lib/ctblfuns.gi
index 2bbf451971..59c432aac1 100644
--- a/lib/ctblfuns.gi
+++ b/lib/ctblfuns.gi
@@ -1832,11 +1832,10 @@ InstallMethod( ConstituentsOfCharacter,
     chi -> ConstituentsOfCharacter( UnderlyingCharacterTable( chi ), chi ) );
 
 InstallMethod( ConstituentsOfCharacter,
-    "for a character",
-    [ IsClassFunction and IsCharacter ],
-    function( chi )
-    local tbl,    # underlying table of `chi'
-          irr,    # irreducible characters of `tbl'
+    "for an ordinary table, and a character",
+    [ IsOrdinaryTable, IsClassFunction and IsCharacter ],
+    function( tbl, chi )
+    local irr,    # irreducible characters of `tbl'
           values, # character values
           deg,    # degree of `chi'
           const,  # list of constituents, result
@@ -1881,7 +1880,7 @@ InstallMethod( ConstituentsOfCharacter,
       scpr:= ScalarProduct( tbl, chi, i );
       if scpr <> 0 then
         Add( const, i );
-        proper:= proper and IsInt( scpr ) and ( 0 < scpr );
+        proper:= proper and IsPosInt( scpr );
       fi;
     od;
 
@@ -1897,15 +1896,18 @@ InstallMethod( ConstituentsOfCharacter,
     "for a Brauer table, and a homogeneous list",
     [ IsBrauerTable, IsHomogeneousList ],
     function( tbl, chi )
-    local irr,    # irreducible characters of `tbl'
-          dec;
+    local irr, intA, intB, dec;
 
     irr:= Irr( tbl );
-    dec:= Decomposition( irr, [ chi ], "nonnegative" )[1];
+    intA:= IntegralizedMat( irr );
+    intB:= IntegralizedMat( [ chi ], intA.inforec );
+    dec:= SolutionIntMat( intA.mat, intB.mat[1] );
     if dec = fail then
-      TryNextMethod();
+      Error( "<chi> is not a virtual character of <tbl>" );
     fi;
-    return irr{ Filtered( [ 1 .. Length( dec ) ], i -> dec[i] <> 0 ) };
+
+    return SortedList( irr{ Filtered( [ 1 .. Length( dec ) ],
+                                      i -> dec[i] <> 0 ) } );
     end );
 
 
diff --git a/tst/teststandard/ctblfuns.tst b/tst/teststandard/ctblfuns.tst
new file mode 100644
index 0000000000..c49c45bad4
--- /dev/null
+++ b/tst/teststandard/ctblfuns.tst
@@ -0,0 +1,37 @@
+#@local ordtbl, modtbl, irr, chi, const, ibr, phi
+gap> START_TEST( "ctblfuns.tst" );
+
+#
+gap> ordtbl:= CharacterTable( GL(2,3) );;
+gap> modtbl:= ordtbl mod 2;;
+gap> irr:= Irr( ordtbl );;
+gap> chi:= irr[5] * irr[5];;
+gap> const:= ConstituentsOfCharacter( chi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1, 1, 1, 1, -1, -1, -1 ], [ 3, 0, 3, 0, -1, -1, -1, 1 ] ]
+gap> const:= ConstituentsOfCharacter( -chi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1, 1, 1, 1, -1, -1, -1 ], [ 3, 0, 3, 0, -1, -1, -1, 1 ] ]
+gap> const:= ConstituentsOfCharacter( ordtbl, chi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1, 1, 1, 1, -1, -1, -1 ], [ 3, 0, 3, 0, -1, -1, -1, 1 ] ]
+gap> const:= ConstituentsOfCharacter( ordtbl, ValuesOfClassFunction( chi ) );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1, 1, 1, 1, -1, -1, -1 ], [ 3, 0, 3, 0, -1, -1, -1, 1 ] ]
+gap> ibr:= Irr( modtbl );;
+gap> phi:= ibr[2] * ibr[2];;
+gap> const:= ConstituentsOfCharacter( phi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1 ], [ 2, -1 ] ]
+gap> const:= ConstituentsOfCharacter( -phi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1 ], [ 2, -1 ] ]
+gap> const:= ConstituentsOfCharacter( modtbl, phi );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1 ], [ 2, -1 ] ]
+gap> const:= ConstituentsOfCharacter( modtbl, ValuesOfClassFunction( phi ) );;
+gap> List( const, ValuesOfClassFunction );
+[ [ 1, 1 ], [ 2, -1 ] ]
+
+#
+gap> STOP_TEST( "ctblfuns.tst" );