#14793: Unique representation for homsets
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Reporter: nthiery | Owner: nthiery
Type: enhancement | Status: new
Priority: major | Milestone: sage-5.12
Component: categories | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Simon King | Merged in:
Dependencies: | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:13 nbruin]:
> > While unpickling of the modular symbols M, we need to construct a
homset with domain and codomain M. At this point, calling M.category()
results in an error, since M.base() returns None and M.category() wants to
return Modules(M.base()).
>
> Is `None` ever a valid value for `M.base()`?
I don't know if it is valid, but at least it is easily possible to get
None:
{{{
sage: Parent().base() is None
True
}}}
> At this point, is there enough information available on `M` to derive
what `base()` should return?
I don't think so---unless modular symbols are *always* defined over the
rationals. That's not my field of mathematical expertise.
> In that case, I'd think the cleanest way would be to make `base()` a
caching routine: return a stored value if available and otherwise derive
the correct value, store it, and return that. Whenever someone asks for
`base` they're probably not interested in an invalid value. Or is
computing `base` possibly expensive and not really necessary for the
unpickling?
`base` is usually set when you call `Parent.__init__`.
Anyway. If it is really the case that the base of modular symbols will
always be the rational field, then it is fine.
Best regards,
Simon
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14793#comment:14>
Sage <http://www.sagemath.org>
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