用java计算gamma函数

仿matlab的gamma函数的做法，做的 。在 x >16后 x为整数时 计算误差偏大 。

 

 

 

public class ComputeGamma {

static double x = 0;

static double[] p = {-1.71618513886549492533811e+0,2.47656508055759199108314e+1,

    -3.79804256470945635097577e+2,6.29331155312818442661052e+2,

    8.66966202790413211295064e+2,-3.14512729688483675254357e+4,

    -3.61444134186911729807069e+4,6.64561438202405440627855e+4};

static double[] q = {-3.08402300119738975254353e+1,3.15350626979604161529144e+2,

    -1.01515636749021914166146e+3,-3.10777167157231109440444e+3,

    2.25381184209801510330112e+4,4.75584627752788110767815e+3,

    -1.34659959864969306392456e+5,-1.15132259675553483497211e+5};

static double[] c = {-1.910444077728e-03,8.4171387781295e-04,

    -5.952379913043012e-04,7.93650793500350248e-04,

    -2.777777777777681622553e-03, 8.333333333333333331554247e-02,

    5.7083835261e-03};

public static double getValue(double a){

x = a;

double xp = x;

double res = 0;

double xn = 0;

double z = 0;

double xnum = 0;

double xden = 0;

double ysq = 0;

double sum = 0;

if(x == 0){

return 1;

}

if(x <= 1){

xp = xp + 1;

}

if(x <= 12){

xn = Math.floor(xp) - 1;

xp = xp - xn;

z = xp - 1;

xnum = 0;

xden = xnum + 1;

for(int i = 0;i < p.length;i++){

xnum = (xnum + p[i]) * z;

xden = xden * z + q[i];

}

res = xnum / xden + 1;

}

if(x <= 1){

res = res / x;

}

if(x > 2 && x <= 12){

for(int i = 0;i < xn; i++ ){

res = res * xp;

xp = xp + 1;

}

}

if(x > 12){

ysq = xp * xp;

sum = c[6];

for(int i = 0;i < c.length-1;i++){

sum = sum / ysq + c[i];

}

double spi = 0.9189385332046727417803297;

sum = sum / xp - xp + spi;

sum = sum + (xp-0.5) * Math.log(xp);

res = Math.exp(sum);

}

return res;

}

}