Γ⁡(a,z)=Γ⁡(a)Γ⁡(a-n)⁢Γ⁡(a-n,z)+za-1⁢ⅇ-z⁢∑k=0n-1Γ⁡(a)Γ⁡(a-k)⁢z-k,