[Octave-bug-tracker] [bug #47800] gammainc rounds down to zero if output is below eps

[anonymous]

URL:
  <http://savannah.gnu.org/bugs/?47800>

                 Summary: gammainc rounds down to zero if output is below eps
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Thu 28 Apr 2016 06:02:53 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: A. M. Winkler
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.0.0
        Operating System: Any

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Details:

Consider this:

In Matlab: 

>> gammainc(60,6,'upper')
ans =
   6.1802e-20

In Octave 4.0.0 (same for 3.8.2):

>> gammainc(60,6,'upper')
ans = 0

This causes large problems, as norminv(6.1802e-20) = -9.0659 (the expected
result), whereas norminv(0) = -Inf, and subsequent code crashes.




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