K ν ( z ) ∼ ( π 2 z ) 1 2 e − z ∑ k = 0 ∞ a k ( ν ) z k K_{\nu}\left(z\right)\sim\left(\frac{\pi}{2z}\right)^{\frac{1}{2}}e^{-z}\sum_{k=0}^{\infty}\frac{a_{k}(\nu)}{z^{k}}