1. Institut de Mathématiques et de Sciences Physiques, Université d’Abomey-Calavi, Bénin
2. National Higher School of Mathematics, P.O. Box 75, Mahelma, Sidi Abdellah, 16093 Algiers, Algeria
3. Department of Mathematics, Statistics and Computer Science, Purdue University Northwest, 1401 S, U.S. 421, Westville IN 46391 USA