Jafargholi, A. . “An Exact Solution for the Generalized Spherical Orthogonality Integral of the Legendre Functions of the First and Second Kind”. Applied Computational Electromagnetics Society Journal (ACES), vol. 30, no. 04, Aug. 2021, pp. 452-6, https://journals.riverpublishers.com/index.php/ACES/article/view/10577.