* Lev D. Landau and Evgenii M. Lifschitz, "The Classical Theory of Fields", (1973) Pergammon Press, Oxford ISBN 0-08-018176-7 "See section 87".
* لو لانداو and Evgenii M. Lifschitz, "The Classical Theory of Fields", (1973) Pergammon Press, Oxford ISBN 0-08-018176-7 "See section 87".