GSL Example

#include <stdio.h>
     #include <gsl/gsl_sf_bessel.h>
     
     int
     main (void)
     {
       double x = 5.0;
       double y = gsl_sf_bessel_J0 (x);
       printf ("J0(%g) = %.18e\n", x, y);
       return 0;
     }