#18386: fix polylog evalf
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Reporter: rws | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-7.3
Component: symbolics | Resolution:
Keywords: pynac special | Merged in:
Authors: | Reviewers:
Report Upstream: Reported upstream. Developers | Work issues:
acknowledge bug. | Commit:
Branch: | Stopgaps:
Dependencies: |
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Changes (by rws):
* milestone: sage-6.8 => sage-7.3
Old description:
> The `polylog` function (from Pynac) treats `1.0` like `1` and does not
> immediately `evalf` with some numeric arguments.
> {{{
> sage: polylog(2,1)
> 1/6*pi^2
> sage: polylog(2.,1)
> 1.64493406684823
> sage: polylog(2,1.0)
> 1/6*pi^2
> sage: polylog(2,0.9)
> polylog(2, 0.900000000000000)
> sage: _.n()
> TypeError: cannot evaluate symbolic expression numerically
> }}}
> What makes `polylog` different is that the Sage `polylog` has no special
> value logic and calls Pynac's `Li_eval` for everything. This handles
> special values (incorrectly if an arg is FP) and sends everything else
> back with `.hold()`. So you need `N()` to get FP results that are not
> special. With FP Pynac is called which then calls Sage/mpmath. But this
> then bombs with FP args.
>
> It seems better to simply implement everything in Sage because only the
> special value logic is still in Pynac, and buggy too.
>
> Consequently, `polylog` should be removed from the Pynac function
> registry.
New description:
The `polylog` function (from Pynac) treats `1.0` like `1` and does not
immediately `evalf` with some numeric arguments.
{{{
sage: polylog(2,1)
1/6*pi^2
sage: polylog(2.,1)
1.64493406684823
sage: polylog(2,1.0)
1/6*pi^2
sage: polylog(2,0.9)
polylog(2, 0.900000000000000)
sage: _.n()
TypeError: cannot evaluate symbolic expression numerically
}}}
What makes `polylog` different is that the Sage `polylog` has no special
value logic and calls Pynac's `Li_eval` for everything. This handles
special values (incorrectly if an arg is FP) and sends everything else
back with `.hold()`. So you need `N()` to get FP results that are not
special. With FP Pynac is called which then calls Sage/mpmath. But this
then bombs with FP args.
--
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Ticket URL: <http://trac.sagemath.org/ticket/18386#comment:3>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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