“…Recently, these ideas were discussed at the Belgrade Analysis Seminar, and several recent results in this subject were obtained by the first author and some of his associates: M.Svetlik, A. Khalfallah, M. Mhamdi, B. Purtić, H.P. Li and the second author of this paper, see ( [15], [24], [22]). For more details see the introduction of paper [21] by the first author of this paper.…”