1Dipartimento di Matematica e Informatica, Università degli Studi di Perugia, Via Vanvitelli 1, 06123, Perugia, Italy
2College of Science, Civil Aviation University of China, Tianjin, 300300, P.R. China
3Department of Mathematics , Heilongjiang Institute of Technology , Harbin , 150050 , P. R. China