Existence and localization of solutions of second order elliptic problems using lower and upper solutions in the reversed order
Patrick Habets, Pierpaolo Omari · Topological Methods in Nonlinear Analysis · 1996
where Ω is a bounded domain in R ,L is a linear second order elliptic operator for which the maximum principle holds, B is a linear first order boundary operator and f is a nonlinear Caratheodory function. We are concerned with the solvability of (1.1) in presence of lower and upper solutions. A classical basic result in this context says that if α is a lower solution and β is an upper solution satisfying