#4513: Action of MatrixGroup on a MPolynomialRing
---------------------------------------------+------------------------------
Reporter: SimonKing | Owner: SimonKing
Type: enhancement | Status: needs_work
Priority: major | Milestone: sage-4.6.2
Component: commutative algebra | Keywords: matrix group,
action, polynomial ring
Author: Simon King | Upstream: N/A
Reviewer: David Loeffler, William Stein | Merged:
Work_issues: |
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Changes (by davidloeffler):
* status: needs_review => needs_work
* reviewer: => David Loeffler, William Stein
Comment:
I've had a look at the patch, and I don't think you've addressed William's
comment #14 from two years back. The following makes me *extremely*
uneasy:
{{{
sage: G = GL(3, 7)
sage: R.<a, b> = GF(7)[]
sage: G.0 * a
[3*a 0 0]
[ 0 a 0]
[ 0 0 a]
sage: R.<a,b,c> = GF(7)[]
sage: G.0 * a
3*a
}}}
It looks like there's some pre-existing coercion mechanism which returns
elements of the matrix space over R, and you're overriding it in one case
with an alternative coercion that returns completely different answers;
this violates a Sage coercion axiom (where there are multiple paths in the
coercion diagram, all must give the same answer up to numerical precision
issues). Moreover, if you look at the patchbot logs it seems to have found
an example where the preexisting coercion gets picked up instead of the
new one.
Sorry, that's a thumbs down from me.
David
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/4513#comment:26>
Sage <http://www.sagemath.org>
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