#14403: Symbolic charpoly broken
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Reporter: nbruin | Owner: burcin
Type: defect | Status: new
Priority: major | Milestone: sage-5.11
Component: symbolics | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Dependencies:
Stopgaps: |
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Comment (by gagern):
OK, here is an attempt to address this.
I modified the `charpoly` method to always use a unique variable name. I
found no ready-to-use utility function to achieve this, but I expect there
might be, so if you know one please let me know and I'll update my code.
So far I simply append numbers to the letter `x` until the resulting
string is not among the list of variable names used inside the matrix. The
resulting variable name will ''always'' be used for the cached polynomial,
and will ''by default'' be used for the returned polynomial.
If the caller overrides the variable name, there is no check to ensure
that this name does not conflict with the names used in the matrix. I'm
unsure myself whether I'd consider this behavior a bug or a feature, i.e.
what intended behavior for that case should be. If you want I can add a
piece of code to perform that check before the `change_variable_name`
call, and throw an error on conflicting variable names.
I've kept the `expression.polynomial(None, ring=SR[var])` approach, and
not used the `expression.coefficient(var, i)` version suggested in the
ticket report. Instead, I adjusted the conversion of expressions to
polynomials to only treat those variable powers as possible monomials
where the variable itself corresponds to one of the variables from the
target ring. All other expressions are treated as coefficients, even if
they are powers of a symbolic variable. This avoids the conversion to
integers for these cases, and hence the error mentioned in the ticket.
This fix might be useful to other applications as well.
--
Ticket URL: <http://trac.sagemath.org/ticket/14403#comment:2>
Sage <http://www.sagemath.org>
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