2.1 An Example Program

The following short program demonstrates the use of the library by computing the value of the Bessel function J_0(x) for x=5,

     #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;
     }

The output is shown below, and should be correct to double-precision accuracy,¹

     J0(5) = -1.775967713143382920e-01

The steps needed to compile this program are described in the following sections.