[1] Rudolf VÝBORNÝ: Maximum principle for non-hyperbolic equations. University of Maryland, The Institute for Fluid Dynamics and Applied Mathematics, Lecture Series, No 42, 1964, 1-33.
[2] Takasi KUSANO:
Remarks on some properties of solutions of some boundary value problems for quasi-linear parabolic and elliptic equations of the second order. Proc. Japan Acad., 39 (1963), 217-222.
MR 0164150