1. Département de Mathématiques , Faculté des Sciences de Tunis , Campus Universitaire, 2092 Tunis , Tunisia
2. Department of Mathematics, College of Sciences , King Saud University , Box 2455 , Riyadh 11451 , Saudi Arabia
3. Institute of Mathematics “Simion Stoilow” of the Romanian Academy , P.O. Box 1-764, 014700 Bucharest ; and Department of Mathematics, University of Craiova, Street A.I. Cuza No. 13, 200585 Craiova , Romania