#18259: comparison of symbolic functions
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Reporter: dkrenn | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.7
Component: symbolics | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nbruin):
This is a consequence of how coercion and comparison are implemented:
{{{
sage: var('y')
y
sage: f(x)=x*y
sage: A=x*y
sage: cSR=parent(f)
sage: cSR.coerce_map_from(SR)
Conversion map:
From: Symbolic Ring
To: Callable function ring with arguments (x,)
sage: SR.coerce_map_from(cSR) is None
True
}}}
So, A coerces into the parent of f (and not the other way around): That
means that for equality testing, A is coerced into the parent of f and
then the comparison is done: comparison testing in sage is defined to be
"equal up to coercion" (which probably necessarily breaks in all kinds of
particular cases).
If you do *not* want these things to compare equal then you should break
the coercion. That might be reasonable. Conversion can still exist.
Currently you can do:
{{{
sage: f+y^2
x |--> x*y + y^2
}}}
without coercion but with conversion, this would need to be:
{{{
sage: f+cSR(y^2)
x |--> x*y + y^2
}}}
which does not seem unreasonable to me.
--
Ticket URL: <http://trac.sagemath.org/ticket/18259#comment:2>
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