#15364: Introduce factor_or_zero into global namespace
-------------------------------------+-------------------------------------
Reporter: zabrocki | Owner:
Type: enhancement | Status: needs_work
Priority: minor | Milestone: sage-6.2
Component: factorization | Resolution:
Keywords: | Merged in:
Authors: Mike Zabrocki | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
public/ticket/15364/zabrocki/factor_or_zero|
1e0e3da44e609c65c2b6b269f80991d01a33b844
Dependencies: | Stopgaps:
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Comment (by zabrocki):
I noticed another problem with `factor` and the factorization objects that
it returns. I took a symmetric function expression and used
`.map_coefficients(factor)` and my coefficients which were 1 and -1
disappeared (I was applying it to an expression with polynomial
coefficients, but you get the idea from the following expression).
{{{
sage: s = SymmetricFunctions(QQ).s()
sage: (s[3,2]+2*s[2,2,1]).map_coefficients(factor)
2*s[2, 2, 1]
}}}
This happens because `map_coefficients` is calling `s.sum_of_terms` which
calls `s.term` which calls `s._from_dict` and we have the strange behavior
that `s._from_dict({Partition([3,2]) : factor(1)})` is 0.
If you trace it further, you see that `_from_dict` removes the zeros and
computes
{{{dict( (key, coeff) for key, coeff in d.iteritems() if coeff)}}}
Now `bool(factor(2))` is `True` while `bool(factor(1))` and
`bool(factor(-1))` is `False`. I don't know if there is a good reason
that `factor(1)` and `factor(-1)` should be `False` or if this bug should
be fixed in `combinat/free_module.py` in the code for `_from_dict`.
--
Ticket URL: <http://trac.sagemath.org/ticket/15364#comment:16>
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