bessel\_i\_nu = bessel\_i\_nu(x,nu) evaluates an approximation to the modified Bessel function of the first kind I\nu/4 (x), where the order v=-3, -2, -1, 1, 2 or 3 and x is real and positive. For positive orders it may also be called with x=0, since I\nu/4 (0)=0 when v>0. For negative orders the formula I_{-v/4}(x)=I_{v/4}(x)+\frac{2}{\pi}\sin\left(\frac{\pi v}{4}\right)K_{v/4}(x) is used.

For more information please review the s18eec function in the NAG document.


x (input,double)
The argument x of the function.
x>0.0 when nu<0,
x\geq 0.0 when nu>0.
nu (input,int)
The argument v of the function.
1\leq abs(nu)\leq 3.
bessel_i_nu (output,double)
Approximation of the modified Bessel function of the first kind.