#include <math.h>

float bessk0(x)
float x;
{
	double y,ans;
	float bessi0();

	if (x <= 2.0) {
		y=x*x/4.0;
		ans=(-log(x/2.0)*bessi0(x))+(-0.57721566+y*(0.42278420
			+y*(0.23069756+y*(0.3488590e-1+y*(0.262698e-2
			+y*(0.10750e-3+y*0.74e-5))))));
	} else {
		y=2.0/x;
		ans=(exp(-x)/sqrt(x))*(1.25331414+y*(-0.7832358e-1
			+y*(0.2189568e-1+y*(-0.1062446e-1+y*(0.587872e-2
			+y*(-0.251540e-2+y*0.53208e-3))))));
	}
	return ans;
}