“…where L stands here for some second-order elliptic operator, specified in the study, while Ω is either the entire R N , or a cone, or an exterior domain; we refer to the works [7,8,10,16,17,20,24,25,27,29,31,35,39,41,42,58,60,61,64,68] in which L coincides with the standard Laplacian, to [11,12,14,19,21,44,47,63] where L is its nonlinear counterpart, the p-Laplacian, and to [13,[34][35][36]43,45,48,50,54,55] where more general linear or nonlinear elliptic operators are considered. Nevertheless, the above list is by no means exhaustive and the reader who wishes to get a panoramic view of this fascinating field should also consult the extensive treatise [49], as well as the very recent surveys [28] and [37].…”