bessi0.cpp

这是数学计算上常用的计算方法

#include <cmath>
#include "nr.h"
using namespace std;

DP NR::bessi0(const DP x)
{
	DP ax,ans,y;

	if ((ax=fabs(x)) < 3.75) {
		y=x/3.75;
		y*=y;
		ans=1.0+y*(3.5156229+y*(3.0899424+y*(1.2067492
			+y*(0.2659732+y*(0.360768e-1+y*0.45813e-2)))));
	} else {
		y=3.75/ax;
		ans=(exp(ax)/sqrt(ax))*(0.39894228+y*(0.1328592e-1
			+y*(0.225319e-2+y*(-0.157565e-2+y*(0.916281e-2
			+y*(-0.2057706e-1+y*(0.2635537e-1+y*(-0.1647633e-1
			+y*0.392377e-2))))))));
	}
	return ans;
}