#18940: Polynomials ignore the step argument in __getitem__
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Reporter: pbruin | Owner:
Type: defect | Status: needs_info
Priority: major | Milestone: sage-6.10
Component: algebra | Resolution:
Keywords: | Merged in:
Authors: Peter Bruin | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/pbruin/18940-getitem_step | 1c56779b3140d12fbe5d0973cdcad0ac9dbb8054
Dependencies: #19409 | Stopgaps:
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Comment (by pbruin):
Replying to [comment:17 jdemeyer]:
> Are you sure that the current behaviour is really what we want?
> {{{
> sage: pol = PolynomialRing(QQ, 'x')(range(10))
> sage: pol
> 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2 + x
> sage: pol[2:]
> 9*x^9 + 8*x^8 + 7*x^7 + 6*x^6 + 5*x^5 + 4*x^4 + 3*x^3 + 2*x^2
> }}}
Yes. The fact that polynomials are usually written with decreasing
exponents is just a historical convention. I think it is absolutely
reasonable that `pol[2:]` should return the monomials of degree 2 and
higher, whether as a polynomial or as a list. And in fact
{{{
sage: list(pol)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
}}}
so doing `list(pol)[2:]` also extracts the coefficients of the monomials
of degree at least 2.
Similarly, in PARI, my experience is that `Vecrev` (which does the
equivalent of `list` above) is more useful than `Vec` to extract the list
of coefficients of a polynomial.
--
Ticket URL: <http://trac.sagemath.org/ticket/18940#comment:29>
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