bugGNU Octave - Bugs: bug #53612, need accurate gammainc algorithm...

 
 

bug #53612: need accurate gammainc algorithm for "large" negative x and small a

Submitter:  Rik <rik5>
Submitted:  Wed 11 Apr 2018 06:22:53 PM UTC
   
 
Category:  Test Suite Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Inaccurate Result
Status:  Confirmed Assigned to:  None
Originator Name:  Open/Closed:  * Open
Release:  * dev Operating System:  * Any
Fixed Release:  None Planned Release:  None
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Thu 12 Apr 2018 03:37:07 PM UTC, comment #2: 

Thanks for the insight.  I've re-titled the bug report to reflect that we really just need a better algorithm.

Rik <rik5>
Group administrator
Thu 12 Apr 2018 07:47:52 AM UTC, comment #1: 

Dear Rik,

gammainc is quite inaccurate when x is a "large" negative and a is small. Unfortunately I was not able to find an efficient numerical expansion for such values. I put that test in the hope that someone will be able to fix it.

Michele Ginesi <m_ginesi>
Wed 11 Apr 2018 06:22:53 PM UTC, original submission:  

The following test is failing


%! assert (gammainc (-20, 1.1, "upper"), ...
%!         6.50986687074979e8 + 2.11518396291149e8*i, -1e-13);


On my system, the result is


***** xtest
 assert (gammainc (-20, 1.1, "upper"), ...
         6.50986687074979e8 + 2.11518396291149e8*i, -1e-13);
!!!!! known failure
ASSERT errors for:  assert (gammainc (-20, 1.1, "upper"),6.50986687074979e8 + 2.11518396291149e8 * i,-1e-13)

  Location  |  Observed  |  Expected  |  Reason
     ()      650986687.1916+211518396.654i 650986687.075+211518396.2911i   Rel err 5.5676e-10 exceeds tol 1e-13 by 6e-10


I don't know if the tolerance should be relaxed, or the algorithm within gammainc should be improved.


Rik <rik5>
Group administrator

 

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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2018-04-12 rik5 Release4.3.90 dev
        Summarygammainc fails one test with &quot;upper&quot; flag and complex input need accurate gammainc algorithm for "large" negative x and small a
    2018-04-11 rik5 Carbon-Copy- Added m_ginesi

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