[bug #58066] Digamma function returning incorrect values for (most) negative integers

[Jackson Vanover]

URL:
  <https://savannah.gnu.org/bugs/?58066>

                 Summary: Digamma function returning incorrect values for
(most) negative integers
                 Project: GNU Scientific Library
            Submitted by: jacksonvanover
            Submitted on: Sat 28 Mar 2020 05:27:05 PM UTC
                Category: Accuracy problem
                Severity: 3 - Normal
        Operating System: Ubuntu 18.04
                  Status: None
             Assigned to: None
             Open/Closed: Open
                 Release: 2.6
         Discussion Lock: Any

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Details:

Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

Though the digamma function has poles at all negative integer values,
once the magnitude of the input is larger than 2,
GSL does not complain and returns incorrect values. For the inputs -1
and -2, the error handler is invoked with a domain error as might be
expected.

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>
#include <float.h>

int main (void){
        double out;
        out = gsl_sf_psi(-3);

        printf("%.*f\n", DBL_DIG-1, out);
        printf("%.21e\n", out);
        return 0;
}




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