Superposition of fundamental solutions of second order quasilinear equations

We prove a superposition principle in the spirit of Crandall-Zhang and Lindqvist-Manfredi for a class of second order quasilinear equations. Riesz potentials of nonnegative and compactly supported continuous functions are either subsolutions or supersolutions for the operators associated to the stationary form of the doubly nonlinear diffusion equation. This class of operators includes both the p-Laplacian as well as the stationary porous medium equation.