#9457: power series equality fails when trailing coefficients are zero
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Reporter: niles | Owner: malb
Type: defect | Status: needs_work
Priority: major | Milestone: sage-6.2
Component: commutative | Resolution:
algebra | Merged in:
Keywords: | Reviewers:
Authors: niles | Work issues: resolve problems with
Report Upstream: N/A | sha_tate.py
Branch: | Commit:
u/niles/ticket/9457 | e2219602a9d360f465ad79b098e4f923765ab791
Dependencies: | Stopgaps: #15748
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Comment (by wuthrich):
I try to remember what happened 4 years ago. I think I discovered exactly
the same problem as you. First some bug, then fixing that gave an error
for sha_tate in the supersingular case. This was due to the Dp_valued
series return the wrong result. Chasing that I came to #8198, where I got
stuck becasue (at the time) I did not know how I could fix that.
So let's make a list - as there are several overlapping tickets and
issues:
* (see #4656) is_zero is broken for power series. This causes your {{{QpT(
O(5^-1))}}} to print as zero. It remembers that it is not, if you have
.list you see its first coefficient is still {{{O(5^-1)}}}. We should
change {{{__nonzero__}}} in power_series_poly.pyx
* (see #4656, too) power series compare wrongly. Your suggestion to use
padded_list above is better than what I tried to do in the other ticket.
That is cmp in power_series_ring_element.pyx
* (see #4656) power series also print wrongly. You would expect the
inexact coefficients of the form {{{O(p^2)}}} to print, too. I tried to
change that in the other ticket. That is {{{_repr_}}} in
power_series_ring_element.pyx
* (see #8198) Matrix multiplication on p-adics looses precision. I believe
that is the heart of the problem for getting the right answer for this
Dp_series. If I understand correctly you have spotted the same thing
above.
* There is also #5075 which I have never looked at myself.
We could take the out the doc-string in sha-tate out for now, so that this
and #4656 can be closed. Then reintroduce it if we can fix #8198.
--
Ticket URL: <http://trac.sagemath.org/ticket/9457#comment:29>
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