Affiliation: | 1. Department of Mathematics , University of Mostaganem , Box 227, Mostaganem, 27000 , Algeria sadek.gala@gmail.com;3. Department of Mathematics , Sun Yat-sen University , Guangzhou , Guangdong, 510275 , China;4. Dipartimento di Mathematica e Informatica , Università di Catania , Viale Andrea Doria, 6, 95125 Catania , Italy |