bessj1.cpp

这是C++数值算法(第二版)的源代码,其中包含了目前一些比较常用的数值计算的算法.

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

DP NR::bessj1(const DP x)
{
	DP ax,z,xx,y,ans,ans1,ans2;

	if ((ax=fabs(x)) < 8.0) {
		y=x*x;
		ans1=x*(72362614232.0+y*(-7895059235.0+y*(242396853.1
			+y*(-2972611.439+y*(15704.48260+y*(-30.16036606))))));
		ans2=144725228442.0+y*(2300535178.0+y*(18583304.74
			+y*(99447.43394+y*(376.9991397+y*1.0))));
		ans=ans1/ans2;
	} else {
		z=8.0/ax;
		y=z*z;
		xx=ax-2.356194491;
		ans1=1.0+y*(0.183105e-2+y*(-0.3516396496e-4
			+y*(0.2457520174e-5+y*(-0.240337019e-6))));
		ans2=0.04687499995+y*(-0.2002690873e-3
			+y*(0.8449199096e-5+y*(-0.88228987e-6
			+y*0.105787412e-6)));
		ans=sqrt(0.636619772/ax)*(cos(xx)*ans1-z*sin(xx)*ans2);
		if (x < 0.0) ans = -ans;
	}
	return ans;
}