bessi1.c

Numerical Recipes 是国际公认的高水平的、关于数值计算的书

#include <math.h>

float bessi1(x)
float x;
{
	float ax,ans;
	double y;

	if ((ax=fabs(x)) < 3.75) {
		y=x/3.75;
		y*=y;
		ans=ax*(0.5+y*(0.87890594+y*(0.51498869+y*(0.15084934
			+y*(0.2658733e-1+y*(0.301532e-2+y*0.32411e-3))))));
	} else {
		y=3.75/ax;
		ans=0.2282967e-1+y*(-0.2895312e-1+y*(0.1787654e-1
			-y*0.420059e-2));
		ans=0.39894228+y*(-0.3988024e-1+y*(-0.362018e-2
			+y*(0.163801e-2+y*(-0.1031555e-1+y*ans))));
		ans *= (exp(ax)/sqrt(ax));
	}
	return x < 0.0 ? -ans : ans;
}