1. M. V. Keldyš, “On certain cases of degeneration of equations of elliptic type on the boundary of a domain,” Doklady Akad. Nauk SSSR (N. S.) 77 (2), 181–183 (1951).
2. M. M. Smirnov, Degenerate Elliptic and Hyperbolic Equations (Nauka, Moscow, 1966) [in Russian].
3. M. M. Smirnov, Equations of Mixed Type (Nauka, Moscow, 1970) [in Russian].
4. O. A. Oleĭnik and E. V. Radkevič, Second Order Equations with Nonnegative Characteristic Form (Plenum, New York–London, 2010).
5. J. J. Kohn and L. Nirenberg, “Degenerate elliptic-parabolic equations of second order,” Commun. Pure Appl. Math. 20, 797–872 (1967).