“…In the last decade, this model (for general λ and N ) has gained renewed interest in the mathematics literature. We refer to [6] for a review and, for instance, to [18,22,19,16,2,12,9,11,17,10] and references therein; see also [14,24]. A variant of the problem with a constant background has also been intensely studied, see, for instance, [1,4,5,3,20,8,13] and references therein.…”