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Re: [Bug-gsl] gsl_sf_hyperg_2F1

From: Brian Gough
Subject: Re: [Bug-gsl] gsl_sf_hyperg_2F1
Date: Wed, 06 Sep 2006 22:46:10 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20060815 SeaMonkey/1.0.4

Olaf Wucknitz wrote:

I am having a problem with gsl_sf_hyperg_2F1.
With the parameters (-0.5, 0.5, 1, x) the returned values show a jump at x=0.5. For x<0.5 the results seem to be correct, while for x>0.5 they aren't.
The function gsl_sf_hyperg_2F1_e calls hyperg_2F1_series for x<0.5, but
hyperg_2F1_reflect for x>0.5. The latter function checks for c-a-b being an integer (which it is in my case). If I change one of the parameters a,b,c by a small amount, the other branch of the function is taken and the results are correct again. Unfortunately I know too little about the numerics of 2F1 to suggest a patch at the moment.

Thanks for the bug report -- I think this may be related to bug #1
in the BUGS file. I have added your description to it -- as yet we don't have a resolution for the problem.

best regards,

Brian Gough

Network Theory Ltd,
Publishing the GSL Manual - http://www.network-theory.co.uk/gsl/manual/

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