FIX: NaturalHomomorphismByIdeal (if variable is reduced away).

gap-system · Feb 14, 2017 · 0e1b8e7 · 0e1b8e7
1 parent be981f7
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diff --git a/lib/groebner.gi b/lib/groebner.gi
@@ -1361,7 +1361,9 @@ local ord,b,ind,num,c,corners,i,j,a,rem,bound,mon,n,monb,dim,sc,k,l,char,hom;
     if j<>fail then
       Add(l,GeneratorsOfLeftOperatorRing(a)[j]);
     else
-      Add(l,Zero(a));
+      rem:=PolynomialReducedRemainder(i,b,ord);
+      rem:=Coefficients(monb,rem);
+      Add(l,GeneratorsOfLeftOperatorRing(a)*rem);
     fi;
   od;
 

diff --git a/tst/testinstall/alghom.tst b/tst/testinstall/alghom.tst
@@ -70,6 +70,15 @@ gap> f:= AlgebraWithOneHomomorphismByImages( A, B, [ C[2] ], [ m2 ] );
 [ v.2, v.1+v.2 ] -> [ [ [ 0, 0 ], [ 0, 1 ] ], [ [ 1, 0 ], [ 0, 1 ] ] ]
 gap> IsBijective( f );
 true
+gap> P := PolynomialRing(Rationals, ["x","y","z"]);;
+gap> x:=P.1;;y:=P.2;;z:=P.3;;
+gap> pols:=[ x^3-3*x-1, x^2+x*y+y^2-3, x+y+z ];;
+gap> I := Ideal(P, pols);;
+gap> pr := NaturalHomomorphismByIdeal(P, I);
+[ x, y, z, 1 ] -> [ (-1)*(z)+(-1)*(y), (y), (z), (1) ]
+gap> Image(pr,x);
+(-1)*(z)+(-1)*(y)
+
 gap> STOP_TEST( "alghom.tst", 660000);
 
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