A Formula also known as the Legendre Addition Theorem which is derived by finding Green's
Functions for the Spherical Harmonic expansion and equating them to the generating function for
Legendre Polynomials. When is defined by
Arfken, G. ``The Addition Theorem for Spherical Harmonics.'' §12.8 in
Mathematical Methods for Physicists, 3rd ed. Orlando, FL: Academic Press, pp. 693-695, 1985.