“…In recent years, there has been a great deal of work done on problem (1.1), especially concerning the existence, multiplicity, uniqueness and regularity of solutions. Some important and interesting results can be found, for example, in [4,5,3,2,7,10,8,11,12,13,15,18,21,19,22,23,24,25,29,30,31,32,33,40] and references therein.…”