We consider the boundary value problemis a bounded domain with smooth boundary. We assume c, h ∈ L q (Ω) for some q > max{N/p, 1} with c 0 and µ ∈ L ∞ (Ω). We prove existence and uniqueness results in the coercive case λ ≤ 0 and existence and multiplicity results in the non-coercive case λ > 0. Also, considering stronger assumptions on the coefficients, we clarify the structure of the set of solutions in the non-coercive case.2010 Mathematics Subject Classification. 35J20, 35J25, 35J92.