besjan.cpp

代码

double t(double y)
{
	double aaa,bbb,z,p,temp;
    if (y <= 10.0)
	{
        aaa = 0.000057941 * y - 0.00176148 * y + 0.0208645;
        bbb = 1;
        temp = ((aaa * y - 0.129013) * y + 0.85777) * y + 1.0125;
	}
    else
	{
        z = log(y) - 0.775;
        p = (0.775 - log(z)) / (1.0 + z);
        temp = y / (p + 1.0) / z;
    }
	return temp;
}

void jap(double &x, double &a, double &nmax, double f[])
{
	int n,m;
    double fa[10], rr[10],mmax,e,eps,sum,d1,r,s,nu,al,li,am;
    mmax = 10;
    e = 4;
    eps = 0.5 * pow(10 , (-e));    
    for (n = 0;n<=nmax;n++)
	{
        fa[n] = 0.0;
    }
    sum = exp(gammln(1.0 + a));
    sum = exp(a * log(x / 2.0)) / sum;
    d1 = 2.3026 * e + 1.3863;
    if( nmax > 0 )
	{
        r = t(0.5 * d1 / nmax) * nmax;
	}
    else
	{
        r = 0.0;
    }
    s = t(0.73576 * d1 / x) * 1.3591 * x;
    if (r <= s)
	{
        nu = 1 + int(s);
	}
    else
	{
        nu = 1 + int(r);
	}
yi: m = 0;
    al = 1.0;
    li = int(nu / 2);
er: m = m + 1;
    al = al * (m + a) / (m + 1);
    if (m < li) 
		goto er;
    n = 2 * m;
    r = 0.0;
    s = 0.0;
san: r = 1.0 / (2.0 * (a + n) / x - r);
    if (int(n / 2) != n / 2.0)
	{
        am = 0.0;
	}
    else
	{
        al = al * (n + 2) / (n + 2.0 * a);
        am = al * (n + a);
	}
    s = r * (am + s);
    if (n <= nmax)
		rr[n - 1] = r;
    n = n - 1;
    if (n >= 1) 
		goto san;
    f[0] = sum / (1.0 + s);
    for (n = 0;n<=nmax - 1;n++)
        f[n + 1] = rr[n] * f[n];
    for (n = 0 ;n<=nmax;n++)
	{
        if (fabs((f[n] - fa[n]) / f[n]) > eps)
		{
            for( m = 0;m<=nmax;m++)
                fa[m] = f[m];            
            nu = nu + 5;
            goto yi;
        }
    }
}

void besjan(double x,double a,double nm, double ih,double f[])
{
	int i;
	double temp;
	temp=1;
    if (ih == 1)
	{
        jap(x, a, nm, f);
        _c_exit(); 
	}
    else
	{
        jap(x, a, temp, f);
        f[1] = 2.0 * a * f[0] / x - f[1];
        for (i = 1;i<=nm;i++)
            f[i + 1] = 2.0 * (a - i) * f[i] / x - f[i - 1];
        _c_exit();
	}
}