Abstract

Using a fixed point theorem due to M.A. Krasnosel'skii, the upper-lower solutions method and fixed point index theory, we study existence, non-existence and multiplicity of positive solutions for a class of systems of second-order ordinary differential equations. We give a new notion of superlinearity for nonlinearities. Observe that the nonlinearities may even be, in some sense, singular. As an application, we show multiplicity results for a class of semilinear elliptic systems in both bounded annular domains and exterior domains. In addition, we apply our results to fourth-order boundary value problems. © 2006 Elsevier Inc. All rights reserved.