#10072: Bug in log gamma evaluation
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Reporter: kcrisman | Owner: AlexGhitza
Type: defect | Status: new
Priority: major | Milestone: sage-4.6.1
Component: basic arithmetic | Keywords:
Author: | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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Comment(by kcrisman):
Replying to [comment:8 zimmerma]:
> I don't see how the wrong result is related (if any) to the correct one.
The difference in the
> imaginary part is {{{1.70425232898877}}}, which is neither 2pi nor pi
nor even pi/2.
Correct. My comment about the `2*pi` was in reference to the Ginac
output, not the Sage output. But I can't for the life of me figure out
how Sage is actually doing `log_gamma(i).n()`, because the conversion to
the complex field pretty clearly just converts it to a Ginac object and
then numerically evaluates.
As to the precision, I think there must be something missing in our code,
because the Ginac docs state
{{{
The function evalf that was used above converts any number in GiNaC's
expressions into floating point numbers. This can be done to arbitrary
predefined accuracy:
> evalf(1/7);
0.14285714285714285714
> Digits=150;
150
> evalf(1/7);
0.1428571428571428571428571428571428571428571428571428571428571428571428
5714285714285714285714285714285714285
}}}
So maybe we're not taking that into account, though I have no idea how I
would do so.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10072#comment:9>
Sage <http://www.sagemath.org>
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