#18210: numerical bug in incomplete gamma function
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Reporter: VivianePons | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.7
Component: symbolics | Resolution:
Keywords: sd67 | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Description changed by rws:
Old description:
> The following code:
>
> {{{
> sage: var('R k')
> (R, k)
> sage: integrated = -(gamma(1/2*k, 1/2*R*k) - gamma(1/2*k))/gamma(1/2*k)
> sage: plot(integrated.subs(k=41), (R,0,6))
> }}}
>
> gives a Seg fault error and crashes Sage entirely:
>
> line 134: 4216 Segmentation fault (core dumped)
>
> I believe it comes from the symbolic computation because the following
> code is working:
>
> {{{
> sage: var('R k')
> (R, k)
> sage: integrated = -(gamma(1/2*k, 1/2*R*k) - gamma(1/2*k))/gamma(1/2*k)
> sage: plot(integrated.subs(k=41.), (R,0,6))
> }}}
>
> (Note the 41. instead of 41)
>
> I have no idea where this comes from but it's really bad, because
> whatever is going wrong, crashing Sage is bad!
New description:
Wrong values from incomplete gamma:
{{{
$ ./sage -v
SageMath Version 6.7.beta1, Release Date: 2015-04-15
$ ./sage
sage: gamma(60, 30).numerical_approx()
-1.28306738270893e74
}}}
Reason is a bug in Pari:
{{{
? \p 50
realprecision = 57 significant digits (50 digits displayed)
? incgam(60,30)
%2 = 1.3868299023788799504133735635863795825413935892945 E80
? \p 30
realprecision = 38 significant digits (30 digits displayed)
? incgam(60,30)
%3 = -1.23084064495468737276106696496 E74
}}}
Previous description:
The following code:
{{{
sage: var('R k')
(R, k)
sage: integrated = -(gamma(1/2*k, 1/2*R*k) - gamma(1/2*k))/gamma(1/2*k)
sage: plot(integrated.subs(k=41), (R,0,6))
}}}
gives a Seg fault error and crashes Sage entirely:
line 134: 4216 Segmentation fault (core dumped)
I believe it comes from the symbolic computation because the following
code is working:
{{{
sage: var('R k')
(R, k)
sage: integrated = -(gamma(1/2*k, 1/2*R*k) - gamma(1/2*k))/gamma(1/2*k)
sage: plot(integrated.subs(k=41.), (R,0,6))
}}}
(Note the 41. instead of 41)
I have no idea where this comes from but it's really bad, because whatever
is going wrong, crashing Sage is bad!
--
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Ticket URL: <http://trac.sagemath.org/ticket/18210#comment:14>
Sage <http://www.sagemath.org>
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