#3979: Power series composition messes up precision
--------------------------------------------------------+-------------------
Reporter: kedlaya | Owner:
somebody
Type: defect | Status:
needs_review
Priority: critical | Milestone:
sage-5.3
Component: algebra | Resolution:
Keywords: power series, composition, precision | Work issues:
Report Upstream: N/A | Reviewers: Kiran
Kedlaya, Luis Felipe Tabera Alonso
Authors: Francis Clarke | Merged in:
Dependencies: | Stopgaps: 12783
--------------------------------------------------------+-------------------
Old description:
> The composition of two power series is sometimes returned with the wrong
> precision. A trivial example:
>
> {{{
> sage: pow.<u> = PowerSeriesRing(Rationals()); print (1 + O(u^4))(u)
> 1
> }}}
> where the return value should have precision 4 rather than infinity. A
> more nontrivial example:
>
> {{{
> sage: pow.<u> = PowerSeriesRing(Rationals()); print (1 + u^2 +
> O(u^4))(u^2)
> 1 + u^4 + O(u^10)
> }}}
> where the return value should have precision 8 instead of 10.
>
> ----
>
> Apply only [attachment:trac_3979_power_series_substitution_rev4.patch]
New description:
The composition of two power series is sometimes returned with the wrong
precision. A trivial example:
{{{
sage: pow.<u> = PowerSeriesRing(Rationals()); print (1 + O(u^4))(u)
1
}}}
where the return value should have precision 4 rather than infinity. A
more nontrivial example:
{{{
sage: pow.<u> = PowerSeriesRing(Rationals()); print (1 + u^2 +
O(u^4))(u^2)
1 + u^4 + O(u^10)
}}}
where the return value should have precision 8 instead of 10.
----
Apply [attachment:trac_3979_power_series_substitution_rev4.patch] followed
by [attachment:trac_3979_power_series_substitution_rev4_extra.patch]
--
Comment (by fwclarke):
Replying to [comment:21 chapoton]:
> I would like to see, when possible, a more specific error instead of
> {{{
> raise ValueError, "Cannot substitute this value"
> }}}
> In particular, when this is because of negative valuation, one should
say it.
A good point. The new patch (to be applied after
trac_3979_power_series_substitution_rev4.patch) gives a more explicit
error message.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/3979#comment:22>
Sage <http://www.sagemath.org>
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