Positive coexistence solutions for systems of multi-kernel Hammerstein type equations

Abstract

We investigate positive solutions with both components nontrivial for systems of Hammerstein integral equations associated with a general $2\times2$ matrix of Green's kernels, thereby extending the decoupled (diagonal) case to systems with cross-coupling. Such systems arise from boundary value problems in which the unknowns may be coupled both in the differential equations and in the boundary conditions, a setting that has received considerably less attention than the decoupled case. Using a component-wise Krasnosel'ski\uı compression--expansion fixed point theorem in product cones, we derive existence and non-existence criteria. A multiplicity result guaranteeing at least two positive solutions is also obtained, by means of the fixed point index in cones. As an application, we consider radially symmetric solutions for an elliptic system on an annulus with coupled Robin boundary conditions.