#include <iostream>
#include "boost/format.hpp"
#include <gsl/gsl_sf_bessel.h>
int main(void) {
  using boost::format;
  double x = 5.0;
  double y = gsl_sf_bessel_J0(x);
  std::cout<<format("J0(%1$g) = %2$.18e\n") %x %y;
  return 0;
}