1. Laboratory of Mathematics , University of Saïda P.O. Box 138, Saïda 20000, Algeria
2. Laboratory of Mathematics , Djillali Liabes University of Sidi Bel-Abbes P.O. Box 89, Sidi Bel-Abbès 22000, Algeria
3. Department of Mathematics , Babeş-Bolyai University 400084 Cluj-Napoca , Romania , and Academy of Romanian Scientists , Bucharest , Romania