“…Subsequently some authors have been studied the problems of existence, uniqueness and other properties of solutions of (1.1) or their special forms by using different techniques, see [1,2,3,8,12,14,15] and the references cited therein. We also refer some papers and monographs [11,12,18], [1, p. 342], [6, p.308], [10, p. 18].…”