[Octave-bug-tracker] [bug #47800] gammainc(x, a, "upper") rounds down to zero if output is below eps

[Marco Caliari]

Follow-up Comment #14, bug #47800 (project octave):

Hi,

I include my final proposal. It has corner cases first, then a series and
finally a continued fraction expansion. All tests gammainc(200,200,tail) are
passed with at most relative error 78*eps. I used all the ideas discussed here
(thanks Nir). Two points are still missing

1) accurate implementation of D(a,x) (seems hard to implement)
2) Matlab is *exact* for a integer. I found a close form
(http://mathworld.wolfram.com/IncompleteGammaFunction.html, formula (2)), but
can't implement without over/underflow.

(file #37113)
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Additional Item Attachment:

File name: ngammainc.m                    Size:4 KB


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