# 3.5.1.3.13 Bessel_k_nu

## Definition:

$k\_nu =bessel\_k\_nu(x,nu)$ evaluates an approximation to the modified Bessel function of the second kind $K_{\upsilon /4}(x)$, where the order =-3, -2, -1, 1, 2 or 3 and x is real and positive. For negative orders the formula

$K_{-\upsilon /4}(x)=K_{\upsilon /4}(x)$

is used.

For more information please review the s18efc function in the NAG document.

## Parameters:

x (input, double)
The argument x of the function.
Constraints:
x>0.0.
$nu$ (input, int)
The argument $\nu$ of the function.
Constraints:
$1\leq abs(nu)\leq 3$
$k\_nu$ (output, double)
Approximation of the modified Bessel function of the second kind.