FIX: Scaling of gcd of polynomials to be standard

This fixes #3212
gap-system · Jan 24, 2019 · 5963695 · 5963695
1 parent 0d25d1f
commit 5963695
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Showing 3 changed files with 17 additions and 5 deletions.
diff --git a/hpcgap/lib/polyrat.gi b/hpcgap/lib/polyrat.gi
@@ -775,13 +775,17 @@ local brci,gcd,fam,fc,gc;
   if gcd=fail then
     # fall back to the original version:
     gcd:=RPIGcd(f,g);
-    if Length(gcd)>0 and not IsOne(gcd[1]) then gcd:=gcd/gcd[1];fi;
+    if Length(gcd)>0 and not IsOne(gcd[Length(gcd)]) then
+      gcd:=gcd/gcd[Length(gcd)];
+    fi;
     return gcd;
 
   fi;
   fc:=Minimum(fc[2],gc[2]);
   fc:=fc+RemoveOuterCoeffs(gcd,fam!.zeroCoefficient);
-  if Length(gcd)>0 and not IsOne(gcd[1]) then gcd:=gcd/gcd[1];fi;
+  if Length(gcd)>0 and not IsOne(gcd[Length(gcd)]) then
+    gcd:=gcd/gcd[Length(gcd)];
+  fi;
   return LaurentPolynomialByExtRepNC(fam,gcd,fc,brci);
 end);
 
@@ -1859,7 +1863,7 @@ local   fc,ind, v, g, q, s, r, x,shift;
   Append(s,List([1..v],f->x));
   Sort(s);
 
-  if not IsBound(opt.stopdegs) and Sum(s,DegreeOfLaurentPolynomial)<>DegreeOfLaurentPolynomial(f)+v then
+  if not (IsBound(opt.stopdegs) or IsBound(opt.onlydegs)) and Sum(s,DegreeOfLaurentPolynomial)<>DegreeOfLaurentPolynomial(f)+v then
     Error("degree discrepancy!");
   fi;
 

diff --git a/lib/polyrat.gi b/lib/polyrat.gi
@@ -764,13 +764,17 @@ local brci,gcd,fam,fc,gc;
   if gcd=fail then
     # fall back to the original version:
     gcd:=RPIGcd(f,g);
-    if Length(gcd)>0 and not IsOne(gcd[1]) then gcd:=gcd/gcd[1];fi;
+    if Length(gcd)>0 and not IsOne(gcd[Length(gcd)]) then
+      gcd:=gcd/gcd[Length(gcd)];
+    fi;
     return gcd;
 
   fi;
   fc:=Minimum(fc[2],gc[2]);
   fc:=fc+RemoveOuterCoeffs(gcd,fam!.zeroCoefficient);
-  if Length(gcd)>0 and not IsOne(gcd[1]) then gcd:=gcd/gcd[1];fi;
+  if Length(gcd)>0 and not IsOne(gcd[Length(gcd)]) then
+    gcd:=gcd/gcd[Length(gcd)];
+  fi;
   return LaurentPolynomialByExtRepNC(fam,gcd,fc,brci);
 end);
 

diff --git a/tst/testinstall/ratfun.tst b/tst/testinstall/ratfun.tst
@@ -104,6 +104,10 @@ gap> (t^24-1)/(t^16-1);
 gap> (t^24-1)/(t^-16-1);
 (t^32+t^24+t^16)/(-t^8-1)
 
+#
+gap> Gcd(t-2,t^2-2*t);
+t-2
+
 #
 # multivariate
 #