Multiple solutions for an elliptic system with indefinite Robin boundary conditions
Pablo Amster · Advances in Nonlinear Analysis · 2017
Abstract Multiplicity of solutions is proved for an elliptic system with an indefinite Robin boundary condition under an assumption that links the linearisation at 0 and the eigenvalues of the associated linear scalar operator. Our result is based on a precise calculation of the topological degree of a suitable fixed point operator over large and small balls.