1. Generalized axially symmetric potential theory;Weinstein;Bull. Am. math. Soc.,1953
2. On certain cases of degeneration of equations of elliptic type on the boundary of a domain;Keldys;Dokl. Akad. Nauk SSSR,1951
3. Equations of mixed type;Bitsadze,1964
4. Linear equations of mathematical physics;Mikhlin,1967
5. Second order equations with nonnegative characteristic form;Oleinik,1973