possible bug in gsl_sf_hyperg_U

[Stefan Gerlach]

Hi,

I have a problem using gsl_sf_hyperg_U with certain arguments.
running this example

       for(z=3.0;z>0.0;z-=0.1)
                printf("%g %g\n",z,gsl_sf_hyperg_U(-2.0,4.0,z));

i get 

        ...
        2.2 2.84
        2.1 3.41
        2 4
        gsl: psi.c:381: ERROR: domain error
        Default GSL error handler invoked.
        Aborted

AFAIK the hypergeometricU is well defined for z> 0.0.
I get the same error with gsl-1.11. Any ideas?

-- 
Stefan Gerlach

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