A weighted symmetrization for nonlinear elliptic and parabolic equations in inhomogeneous media

In the theory of elliptic equations, the technique of Schwarz symmetriza- tion is one of the tools used to obtain a priori bounds for classical and weak solutions in terms of general information on the data. A basic result says that, in the absence of the zero-order term, the symmetric rearrangement of the solution u of an elliptic equation, that we write u⁄, can be compared pointwise with the solution of the symmetrized problem. The main ques- tion we address here is the modiflcation of the method to take into account degenerate equations posed in inhomogeneous media. Moreover, the equa- tions we want to deal with involve weights that make them non-divergent, at least when written in terms of the natural variables. We flnd comparison results covering the elliptic case and the corresponding evolution models of parabolic type, with attention to equations of porous medium type. More speciflcally, we obtain a priori bounds and decay estimates for wide classes of solutions of those equations.