Affiliation: | L.M.F.N, Mathematics Department, University of Setif, Algeria; Technical University of Applied Sciences Berlin, Department of Mathematics II, Berlin, Germany;a Çankaya University, Ögretmenler Cad. No: 14 06530, Balgat — Ankara, Turkey b Institute of Space Sciences, P.O. BOX, MG-23, R 76900, Magurele-Bucharest, Romania |