第一类修正贝塞尔函数的C语言实现

第一类修正贝塞尔函数公式

$$I_{\nu }(z)={\left(\frac{z}{2}\right)}^{\nu }\sum _{(k=0)}^{\infty }\frac{{\left(\frac{z^2}{4}\right)}^k}{k!\Gamma (\nu +k+1)}$$

其中，$\Gamma (a)$是gamma函数。

常量和结构体定义

常量和结构体定义的头文件ConstParam.h在下面文章中
MATLAB库函数hilbert(希尔伯特变换)的C语言实现(FFT采用FFTW库)

第一类修正贝塞尔函数的C语言实现

/* PURPOSE: Evaluate modified Bessel function In(x) and n=0. */
double bessi0(double x)
{
    
    
	double ax, ans;
	double y;

	if ((ax = fabs(x)) < 3.75) {
    
    
		y = x / 3.75, y = 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;
}

/* PURPOSE: Evaluate modified Bessel function In(x) and n=1. */
double bessi1(double x)
{
    
    
	double ax, ans;
	double y;

	if ((ax = fabs(x)) < 3.75) {
    
    
		y = x / 3.75, y = 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;
}

/* PURPOSE: Evaluate modified Bessel function In(x) for n >= 0*/
double bessi(int n, double x)
{
    
    
	int j;
	double bi, bim, bip, tox, ans;

	if (n < 0)
	{
    
    
		return -INFINITY;
	}
	if (n == 0)
		return(bessi0(x));
	if (n == 1)
		return(bessi1(x));


	if (x == 0.0)
		return 0.0;
	else {
    
    
		tox = 2.0 / fabs(x);
		bip = ans = 0.0;
		bi = 1.0;
		for (j = 2 * (n + (int)sqrt(ACC*n)); j > 0; j--) {
    
    
			bim = bip + j * tox*bi;
			bip = bi;
			bi = bim;
			if (fabs(bi) > BIGNO) {
    
    
				ans *= BIGNI;
				bi *= BIGNI;
				bip *= BIGNI;
			}
			if (j == n) ans = bip;
		}
		ans *= bessi0(x) / bi;
		return  x < 0.0 && n % 2 == 1 ? -ans : ans;
	}
}