# 3.5.1.3.5 bessel_i_nu

## Definition:

$bessel\_i\_nu = bessel\_i\_nu(x,nu)$ 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.

## Parameters:

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