1. Monotonicity with discontinuities in partial differential equations;Collatz,1974
2. On the existence of classical solutions to an elliptic free boundary problem;Fleishman,1978
3. A strong maximum principle for weakly L-subharmonic functions;Littman;J. Math. Mech.,1959
4. Maximum and monotonicity principles for elliptic boundary value problems in partitioned domain;Meyn,1978
5. Partial Differential Equations of Elliptic Type;Miranda,1970