1. H. Alfvén and C-G. Fälthammar,Cosmical Electrodynamics, Fundamental Principles(Oxford University Press, London, 1963).
2. E. Goursat,Cours d’Analyse Mathématique(Gauthier-Villars, Paris, 1903) Tome III, 5 ed. p. 341.
3. [English transl.:Course in Mathematical Analysis(Dover Publications, Inc., New York, 1959).]