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[Octave-bug-tracker] [bug #54550] inaccurate result of gammainc

From: anonymous
Subject: [Octave-bug-tracker] [bug #54550] inaccurate result of gammainc
Date: Wed, 22 Aug 2018 03:50:41 -0400 (EDT)
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.84 Safari/537.36


                 Summary: inaccurate result of gammainc
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Wed 22 Aug 2018 07:50:39 AM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: Michael Schmidt
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.4.0
        Operating System: GNU/Linux



With octave 4.4.0,  calling 

c = (gammainc(77,2)-1)/eps;

delivers c = 31

wheras octave 4.2.1 (presumably correctly) delivers c = 0. For those arguments
(x=77) and (a=2), a subfunction gammainc_s is called (case 8). If this call is
suppressed (setting flag_s to 0 at line 205), the correct value is computed
and the issue disappears. 

This is not a fix but it indicates that the subfunction gammainc_s may have an
issue or it is applied within a wrong region of arguments.


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