“…Many authors have studied (P ) by different methods like degree theory, sub-super solutions, a priori estimates, etc. We refer, for instance, to [5,7,9,16,21,25,26,30]. Some special cases have been studied by using variational arguments and we refer, for instance, to [1,13,18,19] for the case p = 2 and [15,17] for p > 1.…”