$$\gamma \left(a,x\right)=x^a{e}^{-x}\sum _{k=0}^{\infty }\frac{\Gamma \left(a\right)}{\Gamma \left(a+k+1\right)}{x}^n=x^a{e}^{-x}\sum _{k=0}^{\infty }\frac{x^n}{a^{\stackrel{-}{k+1}}}$$