\begin{matrix} P (x; ν, λ) = \sum_{i = 0}^{\infty} s_{i} t_{i} \\ s_{0} = u_{0} = e^{- \frac{λ}{2}}, s_{i} = s_{i - 1} + u_{i}, u_{i} = u_{i - 1} \frac{λ}{2 i} \\ t_{0} = \frac{1}{Γ (\frac{ν}{2} + 1)} {(\frac{x}{2})}^{\frac{ν}{2}} e^{- \frac{x}{2}}, t_{i} = t_{i - 1} \frac{x}{ν + 2 i} \end{matrix}