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[bug #58065] gsl_sf_gegenpoly_n returning unexpected values

From: Jackson Vanover
Subject: [bug #58065] gsl_sf_gegenpoly_n returning unexpected values
Date: Sat, 28 Mar 2020 13:25:35 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:74.0) Gecko/20100101 Firefox/74.0


                 Summary: gsl_sf_gegenpoly_n returning unexpected values
                 Project: GNU Scientific Library
            Submitted by: jacksonvanover
            Submitted on: Sat 28 Mar 2020 05:25:33 PM UTC
                Category: None
                Severity: 3 - Normal
        Operating System: Ubuntu 18.04
                  Status: None
             Assigned to: None
             Open/Closed: Open
                 Release: 2.6
         Discussion Lock: Any



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

When the second argument is 0, mpmath and scipy's implementations of
the gegenbauer polynomial agree that the output should be uniformly 0,
but gsl_sf_gegenpoly_n consistently returns nonzero values. This
happens regardless of the value of the first argument, though in
particular when the first argument is 1 and the second is 0, the
function returns twice the third argument...

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

int main (void){
        double out;
        out = gsl_sf_gegenpoly_n(1, 0, 5);

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


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