bugGNU Scientific Library - Bugs: bug #54998, Bugs in gsl_sf_hyperg_2F1

 
 

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bug #54998: Bugs in gsl_sf_hyperg_2F1

Submitted by:  Ludvig af Klinteberg <ludvigak>
Submitted on:  Sat 10 Nov 2018 12:42:24 AM UTC  
 
Category:  Accuracy problem Severity:  3 - Normal
Operating System:  Status:  None
Assigned to:  None Open/Closed:  Open
Release:  2.5

Sat 10 Nov 2018 12:42:24 AM UTC, original submission:  

For some reason hyperg_2F1_reflect computes the wrong value when c-a-b is integer, and a and b are of opposite signs, e.g.

gsl_sf_hyperg_2F1_e(-0.25, 0.25, 1.0, 0.9, &r)
=> 0.8800433809321181 (should be 0.92058925138209276)

I don't have access to Moshier, so I can't tell if it's due to the "reflection" method or just a bug. However, hyperg_2F1_series appears to compute the correct value for these cases, so a workaround is to use that when a and b are of opposite signs:
https://github.com/ludvigak/gsl/commit/bc27ee844907567bb11b6acc9c6343bfd8ba5438

This appears to fix some of the issues mentioned in bugs #39056, #32306, #21835.

In addition, the fix introduced in commit 9163448f5e0bfa3591a2535c16a2e37386b91171 could use some amendment. First, it uses a stopping criterion of two terms, not the three that Pearson recommends. Second, the new stopping criterion doesn't associate an error estimate with the new termination criteria. This gives an error estimate of O(1) when it's used:

gsl_sf_hyperg_2F1_e(-0.25, 0.25, 1, 0.25, &r)
=> val=0.9833426507751653, err=1.0000000000000049

This can be fixed as:
https://github.com/ludvigak/gsl/commit/da629add7f0a5228dc7ab965c7c6ef88e60486d0

Ludvig af Klinteberg <ludvigak>

 

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