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Gcd no longer returns standard associates? #3212

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ThomasBreuer opened this issue Jan 22, 2019 · 2 comments · Fixed by #3227
Closed

Gcd no longer returns standard associates? #3212

ThomasBreuer opened this issue Jan 22, 2019 · 2 comments · Fixed by #3227
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kind: bug: wrong result Issues describing bugs that result in mathematically or otherwise wrong results, and PRs fixing them kind: bug Issues describing general bugs, and PRs fixing them regression A bug that only occurs in the branch, not in a release
Milestone
GAP 4.10.1

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@ThomasBreuer
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The following happens in GAP 4.10.0 as well as in the current master branch,
with or without packages loaded.

gap> x:= Indeterminate( Rationals, "x" );;
gap> g:= Gcd( x-2, x^2-2*x );
-1/2*x+1
gap> StandardAssociate( g );
x-2

Up to GAP 4.9, the value returned by Gcd was the standard associate.
Thanks to Meinolf Geck for this observation.

I think this changed behavior is a bug, the documentation of Gcd says:

Gcd returns the standard associate (see StandardAssociate (56.5-5)) of the
greatest common divisors.
@ThomasBreuer ThomasBreuer added kind: bug: wrong result Issues describing bugs that result in mathematically or otherwise wrong results, and PRs fixing them regression A bug that only occurs in the branch, not in a release backport-to-4.10 labels Jan 22, 2019
@fingolfin
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This regression was introduced in commit fb0cdaf by @hulpke

@fingolfin fingolfin mentioned this issue Jan 22, 2019
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hulpke added a commit to hulpke/gap that referenced this issue Jan 23, 2019
@hulpke
FIX: Scaling of gcd of polynomials to be standard
6fb229f 
This fixes gap-system#3212
@hulpke hulpke mentioned this issue Jan 23, 2019
Merged
@fingolfin fingolfin added the kind: bug Issues describing general bugs, and PRs fixing them label Jan 23, 2019
fingolfin pushed a commit to hulpke/gap that referenced this issue Jan 24, 2019
@hulpke @fingolfin
FIX: Scaling of gcd of polynomials to be standard
5963695 
This fixes gap-system#3212
fingolfin pushed a commit that referenced this issue Jan 24, 2019
@hulpke @fingolfin
FIX: Scaling of gcd of polynomials to be standard
211faf8 
This fixes #3212
fingolfin pushed a commit that referenced this issue Jan 24, 2019
@hulpke @fingolfin
FIX: Scaling of gcd of polynomials to be standard
9a6034d 
This fixes #3212
@olexandr-konovalov
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The fix has been backported to stable-4.10 in commit 9a6034d

@olexandr-konovalov olexandr-konovalov added this to the GAP 4.10.1 milestone Jan 26, 2019
hulpke added a commit to hulpke/gap that referenced this issue Feb 12, 2019
@hulpke
FIX: Scaling of gcd of polynomials to be standard
baed10d 
This fixes gap-system#3212
ssiccha pushed a commit to ssiccha/gap that referenced this issue Mar 27, 2019
@hulpke @ssiccha
FIX: Scaling of gcd of polynomials to be standard
11920cf 
This fixes gap-system#3212
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kind: bug: wrong result Issues describing bugs that result in mathematically or otherwise wrong results, and PRs fixing them kind: bug Issues describing general bugs, and PRs fixing them regression A bug that only occurs in the branch, not in a release
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GAP 4.10.1
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FIX: Scaling of gcd of polynomials to be standard hulpke/gap
3 participants
@fingolfin @olexandr-konovalov @ThomasBreuer