#16507: Better handling of subspace construction in pushout
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Reporter: pbruin | Owner:
Type: defect | Status: needs_review
Priority: minor | Milestone: sage-6.4
Component: categories | Resolution:
Keywords: pushout subspace, | Merged in:
sd59 | Reviewers:
Authors: Peter Bruin | Work issues:
Report Upstream: N/A | Commit:
Branch: | 1e75320fbe3cbc7d0b639f789aa0c0ef71cad746
u/pbruin/16507-pushout_coercion_reversed| Stopgaps:
Dependencies: |
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Comment (by SimonKing):
Replying to [ticket:16507 pbruin]:
> This ticket addresses this by equipping the class `ConstructionFunctor`
with an attribute `coercion_reversed`, which is `False` by default; if it
is `True` for a construction `F`, then the pushout of two objects omits
`F` in the case where (at a given step of constructing the pushout)
exactly one of the two constructions is `F`. Of course, we set
`coercion_reversed = True` for the subspace construction.
I hope this will not end up with a pullback construction.
What do you mean by "exactly one of the two constructions is `F`"? Isn't
it the case that one can simply drop all "reversed" construction functors,
''before'' constructing the pushout? Say, if you have direct construction
functors F1,...,F5 and reversed construction functors R1,R2, and consider
`(F5*R2*F4*F3*R1*F2*F1)(ZZ)`, then `(R2*F4*F3*R1*F2*F1)(ZZ)` coerces into
`(F4*F3*R1*F2*F1)(ZZ)` and `(F4*F3*R1*F2*F1)(ZZ)` coerces into
`(F5*F4*F3*R1*F2*F1)(ZZ)`. Would this already imply that
`(F5*R2*F4*F3*R1*F2*F1)(ZZ)` coerces into `(F5*F4*F3*R1*F2*F1)(ZZ)`? If
yes, then one could inductively remove all the reversed construction
functors, and will eventually construct the pushout of
`(F5*F4*F3*F2*F1)(ZZ)` with the other given parent.
--
Ticket URL: <http://trac.sagemath.org/ticket/16507#comment:6>
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