1. This identity is easily proved by induction onl.
2. See, for example, N. F. Mott and H. S. W. Massey,Theory of Atomic Collisions(Clarendon Press, Oxford, England, 1949), 2nd ed., Chap. III.
3. B. Friedman,Principles and Techniques of Applied Mathematics(John Wiley & Sons, Inc., New York, 1956), Chap. 3.
4. Remembering that δ(rʺ−rʹ) = (4πrʹrʺ)−1δ(rʺ−rʹ)Σ(2l+1)Pl(cosθ).
5. E. W. Hobson,Spherical and Ellipsoidal Harmonics(Cambridge University Press, New York, 1931), p. 62.