1. Evans LC. Partial differential equations. Vol. 19, Graduate studies in mathematics. Providence (RI): American Mathematical Society; 1998. 668 p.
2. Koshlyakov NS, Gliner EB, Smirnov MM. Differential equations of mathematical physics. Amsterdam: North-Holland; 1964. 712 p.
3. Bouligand G, Giraud G, Delens P. Le problem de la derive oblique en theorie de potential [The problem of the oblique drifts theory of potential]. Paris: Hermann; 1935. 78 p.
4. Kellog OD. Foundations of potential theory. New York (NY): Frederick Ungar; 1970. 384 p.
5. Sobolev SL. Partial differential equations of mathematical physics. London: Pergamon Press; 1964. 427 p.