#19040: rewrite Expression.__nonzero__()
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Reporter: rws | Owner:
Type: enhancement | Status: needs_info
Priority: major | Milestone: sage-6.9
Component: symbolics | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by rws):
Replying to [comment:41 vdelecroix]:
> What would be the difference between the output `NotImplemented` and
`Undecidable`!?
>
> As I already mentioned in [comment:24 comment 24], it makes few sense
that `satisfiable(expr)` returns `Undecidable`. Each formula is either
`True` or `False`. I am here only assuming that mathematics are
consistent. Of course `satisfiable` can not work for all input and when it
can not it should return `NotImplemented`. That being said, some formula
`expr` have no proof of their truthness (from Godel), but then I doubt any
computer would actually be able to prove it.
Agreed if we look at equalities. Inequalities can be undecidable if we
know one side has no order relation. I don't know if there could be other
reasons. I haven't decided yet if `__nonzero__` should throw an exception
for this.
>`for all x, exists y, for all z expr(x,y,z)`
Such will not be in the first versions.
--
Ticket URL: <http://trac.sagemath.org/ticket/19040#comment:43>
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