[bug #57978] Incorrect result from cosine function with inf input

[Jackson Vanover]

URL:
  <https://savannah.gnu.org/bugs/?57978>

                 Summary: Incorrect result from cosine function with inf input
                 Project: GNU Scientific Library
            Submitted by: jacksonvanover
            Submitted on: Mon 09 Mar 2020 08:48:52 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

Description: when passing positive infinity to gsl_sf_cos_pi, the
result is 1 which doesn't make sense as the cosine function oscillates
between -1 and 1

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

int main(void){
double out;
out = gsl_sf_cos_pi(GSL_POSINF);

printf("%f\n", out);
return 0;
}




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