“…A large number of papers have been devoted to this type of results, using several methods. Without any claim for completeness, we mention [1, 3, 8-15, 23-25, 31, 38, 42, 43]; observe that also similar questions have been addressed also for the stationary equation (see, e.g., [29,36,39,40]) and for systems (see, e.g., [6,33]). For a comprehensive account of literature on this subject, also containing the description of main results obtained and various methods used, we refer the reader to [43] and references therein.…”