Merge pull request #618 from hulpke/additions

Collection of minor improvements

Married, as no problems arose.

lib/factgrp.gi

@@ -715,28 +715,32 @@ end;
 InstallGlobalFunction(SmallerDegreePermutationRepresentation,function(G)
 local o, s, k, gut, erg, H, hom, b, ihom, improve, map, loop,
   i,cheap,first;
+#if not HasSize(G) then Error("SZ");fi;
+  Info(InfoFactor,1,"Smaller degree for order ",Size(G),", deg: ",NrMovedPoints(G));
   cheap:=ValueOption("cheap");
   if cheap="skip" then
     return IdentityMapping(G);
   fi;
 
   cheap:=cheap=true;
 
-  # deal with large abelian cases first (which could be direct)
-  hom:=MaximalAbelianQuotient(G);
-  i:=IndependentGeneratorsOfAbelianGroup(Image(hom));
-  o:=List(i,Order);
-  if ValueOption("norecurse")<>true and 
-    Product(o)>20 and Sum(o)*4<NrMovedPoints(G) then
-    Info(InfoFactor,2,"append abelian rep");
-    s:=AbelianGroup(IsPermGroup,o);
-    ihom:=GroupHomomorphismByImagesNC(Image(hom),s,i,GeneratorsOfGroup(s));
-    erg:=SubdirectDiagonalPerms(
-	  List(GeneratorsOfGroup(G),x->Image(ihom,Image(hom,x))),
-	  GeneratorsOfGroup(G));
-    k:=Group(erg);SetSize(k,Size(G));
-    hom:=GroupHomomorphismByImagesNC(G,k,GeneratorsOfGroup(G),erg);
-    return hom*SmallerDegreePermutationRepresentation(k:norecurse);
+  # deal with large abelian components first (which could be direct)
+  if cheap<>true then
+    hom:=MaximalAbelianQuotient(G);
+    i:=IndependentGeneratorsOfAbelianGroup(Image(hom));
+    o:=List(i,Order);
+    if ValueOption("norecurse")<>true and 
+      Product(o)>20 and Sum(o)*4<NrMovedPoints(G) then
+      Info(InfoFactor,2,"append abelian rep");
+      s:=AbelianGroup(IsPermGroup,o);
+      ihom:=GroupHomomorphismByImagesNC(Image(hom),s,i,GeneratorsOfGroup(s));
+      erg:=SubdirectDiagonalPerms(
+	    List(GeneratorsOfGroup(G),x->Image(ihom,Image(hom,x))),
+	    GeneratorsOfGroup(G));
+      k:=Group(erg);SetSize(k,Size(G));
+      hom:=GroupHomomorphismByImagesNC(G,k,GeneratorsOfGroup(G),erg);
+      return hom*SmallerDegreePermutationRepresentation(k:norecurse);
+    fi;
   fi;
 
 
@@ -779,7 +783,7 @@ local o, s, k, gut, erg, H, hom, b, ihom, improve, map, loop,
     fi;
     return IdentityMapping(G);
   # simple test is comparatively cheap for permgrp
-  elif IsSimpleGroup(G) and not IsAbelian(G) then 
+  elif HasIsSimpleGroup(G) and IsSimpleGroup(G) and not IsAbelian(G) then 
     H:=SimpleGroup(ClassicalIsomorphismTypeFiniteSimpleGroup(G));
     if IsPermGroup(H) and NrMovedPoints(H)>=NrMovedPoints(G) then
       return IdentityMapping(G);

lib/ghomfp.gi

@@ -1032,12 +1032,17 @@ local m,s,g,i,j,rel,gen,img,fin,hom,gens;
     SetAbelianInvariants(f,AbelianInvariantsOfList(DiagonalOfMat(s.normal)));
 
     if Length(m[1])>s.rank then
+      img:=[];
       for i in [1..s.rank] do  
+	Add(img,s.normal[i][i]);
         Add(rel,g.(i)^s.normal[i][i]);
       od;
+      Append(img,ListWithIdenticalEntries(Length(gen)-s.rank,0));
+      SetAbelianInvariants(f,img);
       g:=g/rel;
       fin:=false;
     else  
+      SetAbelianInvariants(f,DiagonalOfMat(s.normal));
       g:=AbelianGroup(DiagonalOfMat(s.normal));
       fin:=true;
     fi;
@@ -1056,6 +1061,7 @@ local m,s,g,i,j,rel,gen,img,fin,hom,gens;
     g:=g/rel;
     fin:=Length(gen)=0;
     img:=GeneratorsOfGroup(g);
+    SetAbelianInvariants(f,ListWithIdenticalEntries(Length(gen),0));
   fi;
 
   SetIsFinite(g,fin);

lib/gprdmat.gi

@@ -413,3 +413,38 @@ local  info,proj,H;
   return proj;
 end);
 
+# tensor wreath -- dimension d^e This is not a faithful representation of
+# the tensor product if the matrix group has a center.
+DeclareGlobalFunction("TensorWreathProduct");
+
+InstallGlobalFunction(TensorWreathProduct,function(M,P)
+local d,e,f,s,dim,gens,mimgs,pimgs,tup,act,a,g,i;
+  d:=Length(One(M));
+  f:=DefaultFieldOfMatrixGroup(M);
+  e:=LargestMovedPoint(P);
+  s:=SymmetricGroup(e);
+  dim:=d^e;
+  gens:=[];
+  mimgs:=[];
+  for g in GeneratorsOfGroup(M) do
+    a:=NullMat(dim,dim,f);
+    for i in [1..dim/d] do
+      tup:=[1..d]+(i-1)*d;
+      a{tup}{tup}:=g;
+    od;
+    a:=ImmutableMatrix(f,a);
+    Add(gens,a);
+    Add(mimgs,a);
+  od;
+  tup:=List(Tuples([1..d],e),Reversed); # reverse to be consistent with Magma
+  act:=ActionHomomorphism(s,tup,Permuted,"surjective");
+  pimgs:=[];
+  for g in GeneratorsOfGroup(P) do
+    a:=PermutationMat(Image(act,g),dim,f);
+    a:=ImmutableMatrix(f,a);
+    Add(gens,a);
+    Add(pimgs,a);
+  od;
+  return Group(gens);
+end);
+

lib/groebner.gd

@@ -485,6 +485,8 @@ DeclareOperation("GroebnerBasis",
 DeclareOperation("GroebnerBasis",[IsPolynomialRingIdeal,IsMonomialOrdering]);
 DeclareGlobalFunction("GroebnerBasisNC");
 
+DeclareSynonym("GrobnerBasis",GroebnerBasis);
+
 #############################################################################
 ##
 #O  ReducedGroebnerBasis( <L>, <O> )
@@ -533,6 +535,7 @@ DeclareOperation("ReducedGroebnerBasis",
   [IsHomogeneousList and IsRationalFunctionCollection,IsMonomialOrdering]);
 DeclareOperation("ReducedGroebnerBasis",
   [IsPolynomialRingIdeal,IsMonomialOrdering]);
+DeclareSynonym("ReducedGrobnerBasis",ReducedGroebnerBasis);
 
 #############################################################################
 ##

lib/grp.gi

@@ -781,7 +781,13 @@ InstallMethod( DerivedSeriesOfGroup,
     S := [ G ];
     Info( InfoGroup, 2, "DerivedSeriesOfGroup: step ", Length(S) );
     D := DerivedSubgroup( G );
-    while D <> S[ Length(S) ]  do
+
+    while
+      # we don't know that the group has no generators
+      (not HasGeneratorsOfGroup(S[Length(S)]) or
+	    Length(GeneratorsOfGroup(S[Length(S)]))>0) and
+      ( (not HasAbelianInvariants(S[Length(S)]) and D <> S[ Length(S) ]) or
+	    Length(AbelianInvariants(S[Length(S)]))>0) do
         Add( S, D );
         Info( InfoGroup, 2, "DerivedSeriesOfGroup: step ", Length(S) );
         D := DerivedSubgroup( D );

lib/grpfp.gi

@@ -937,7 +937,14 @@ function(G)
 local iso,hom,u;
   iso:=IsomorphismFpGroup(G);
   hom:=MaximalAbelianQuotient(Range(iso));
-  if Size(Range(hom))=1 then
+  if HasAbelianInvariants(Range(iso)) then
+    SetAbelianInvariants(G,AbelianInvariants(Range(iso)));
+  fi;
+  if HasIsAbelian(G) and IsAbelian(G) then
+    return TrivialSubgroup(G);
+  elif Size(Image(hom))=infinity then
+    Error("Derived subgroup has infinite index, cannot represent");
+  elif Size(Range(hom))=1 then
     return G; # this is needed because the trivial quotient is represented
               # as fp group on no generators
   fi;

lib/helpbase.gi

@@ -104,7 +104,8 @@ BindGlobal( "TRANSATL",
               [ "ise", "ize" ],
               [ "abeling", "abelling" ],
               [ "olvable", "oluble" ],
-              [ "yse", "yze" ] ] );
+              [ "yse", "yze" ],
+	      [ "roebner", "robner"]] );
 
 
 #############################################################################

lib/matobjplist.gi

@@ -1172,6 +1172,20 @@ InstallMethod( CopySubMatrix, "for two plist matrices and four lists",
     od;
   end );
 
+InstallOtherMethod( CopySubMatrix,
+  "for two plists -- fallback in case of bad rep.",
+  [ IsPlistRep, IsPlistRep and IsMutable,
+    IsList, IsList, IsList, IsList ],
+  function( m, n, srcrows, dstrows, srccols, dstcols )
+    local i;
+    # in this representation all access probably has to go through the
+    # generic method selection, so it is not clear whether there is an
+    # improvement in moving this into the kernel.
+    for i in [1..Length(srcrows)] do
+        n[dstrows[i]]{dstcols}:=m[srcrows[i]]{srccols};
+    od;
+  end );
+
 InstallMethod( CopySubMatrix,
   "for a plist matrix and a checking plist matrix and four lists",
   [ IsPlistMatrixRep,

lib/oprt.gd

@@ -898,7 +898,9 @@ end );
 ##  <Description>
 ##  computes a homomorphism from <A>G</A> into the symmetric group on
 ##  <M>|<A>Omega</A>|</M> points that gives the permutation action of
-##  <A>G</A> on <A>Omega</A>.
+##  <A>G</A> on <A>Omega</A>. (In particular, this homomorphism is a
+##  permutation equivalence, that is the permutation image of a group element
+##  is given by the positions of points in <A>Omega</A>.)
 ##  <P/>
 ##  By default the homomorphism returned by
 ##  <Ref Func="ActionHomomorphism" Label="for a group, an action domain, etc."/>

lib/pcgs.gi

@@ -1623,6 +1623,7 @@ local G,e,ind,s,m,i,p;
   if not IsSolvableGroup(G) then
     Error("<G> must be solvable");
   fi;
+  IsFinite(G); # trigger finiteness test
   e:=ElementaryAbelianSeriesLargeSteps(param);
   ind:=[];
   s:=[];

lib/wordrep.gi

@@ -110,10 +110,32 @@ end );
 # code for printing words in factored form. This pattern searching clearly
 # is improvable
 BindGlobal("FindSubstringPowers",function(l,n)
-local new,t,i,step,lstep,z,zz,j,a,k,good,bad,lim;
+local new,t,i,step,lstep,z,zz,j,a,k,good,bad,lim,plim;
   new:=0;
   t:=[];
   z:=Length(l);
+  # first deal with large powers to avoid x^1000 being an obstacle.
+  plim:=9; # length for treating large powers-1
+  j:=1;
+  while j+plim<=Length(l) do
+    if ForAll([1..plim],x->l[j]=l[j+x]) then
+      k:=j+plim;
+      while k<Length(l) and l[j]=l[k+1] do
+	k:=k+1;
+      od;
+      zz:=[0,l[j],k-j+1];
+      a:=Position(t,zz);
+      if a=fail then
+	new:=new+1;
+	t[new]:=zz;
+	a:=new;
+      fi;
+      l:=Concatenation(l{[1..j-1]},[a+n],l{[k+1..Length(l)]});
+    fi;
+    j:=j+1;
+  od;
+  z:=Length(l);
+
   # long matches first, we then treat the subpatterns themselves again
   j:=QuoInt(z,2);
   lstep:=j;
@@ -186,8 +208,8 @@ end);
 # threshold up to which to try.
 PRINTWORDPOWERS:=true;
 
-DoNSAW:=function(l,names)
-local a,n,
+DoNSAW:=function(l,names,tseed)
+local a,n,t,
       word,
       exp,
       i,j,
@@ -204,6 +226,7 @@ local a,n,
     a:=[l,[]];
   fi;
   word:=a[1];
+  a[2]:=Concatenation(tseed,a[2]);
 
   i:= 1;
   str:= "";
@@ -213,10 +236,24 @@ local a,n,
     fi;
     exp:=1;
     if word[i]>n then
-      # decode longer word -- it will occur as power, so use ()
-      Add(str,'(');
-      Append(str,DoNSAW(a[2][word[i]-n],names));
-      Add(str,')');
+      t:=a[2][word[i]-n];
+      # is it a power stored specially?
+      if t[1]=0 then
+	if t[2]<0 then
+	  Append( str, names[ -t[2] ] );
+	  Append( str, "^-" );
+	  Append( str, String(t[3]));
+	else
+	  Append( str, names[ t[2] ] );
+	  Append( str, "^" );
+	  Append( str, String(t[3]));
+	fi;
+      else
+	# decode longer word -- it will occur as power, so use ()
+	Add(str,'(');
+	Append(str,DoNSAW(t,names,Filtered(a[2],x->x[1]=0)));
+	Add(str,')');
+      fi;
     elif word[i]<0 then
       Append( str, names[ -word[i] ] );
       exp:=-1;
@@ -251,7 +288,7 @@ local names,word;
   if Length(word)=0 then
     return "<identity ...>";
   fi;
-  word:=DoNSAW(word,names);
+  word:=DoNSAW(word,names,[]);
   return word;
 end);