1. A. N. Tikhonov and A. A. Samarskii, Equations of Mathematical Physics (Nauka, Moscow, 1977) [in Russian].
2. B. S. Svetov, Fundamentals of Geoelectrics (LKI, Moscow, 2008) [in Russian].
3. V. S. Vladimirov, Equations of Mathematical Physics (Marcel Dekker, New York, 1971; Nauka, Moscow, 1988).
4. D. V. Sivukhin, General Course of Physics, Vol. 3: Electricity, 4th ed. (Fizmatlit, Moscow, 2004) [in Russian].