Positive solutions for perturbations of the Robin eigenvalue problem plus an indefinite potential

We study perturbations of the eigenvalue problem for the negative Laplacian plus an indefinite and unbounded potential and Robin boundary condition. First we consider the case of a sublinear perturbation and then of a superlinear perturbation. For the first case we show that for $\lambda <{\widehat{\lambda }}_1$ (${\widehat{\lambda }}_1$ being the principal eigenvalue) there is one positive solution which is unique under additional conditions on the perturbation term. For $\lambda \ge {\widehat{\lambda }}_1$ there are no positive solutions. In the superlinear case, for $\lambda <{\widehat{\lambda }}_1$ we have at least two positive solutions and for $\lambda \ge {\widehat{\lambda }}_1$ there are no positive solutions. For both cases we establish the existence of a minimal positive solution ${\bar{u}}_{\lambda }$ and we investigate the properties of the map $\lambda \mapsto {\bar{u}}_{\lambda }$.