1. Institut de Mathématiques et de Sciences Physiques, Université d’Abomey-Calavi, Bénin Email address:
2. Faculty of Civil Engineering, University of Zagreb, Fra Andrije Kačića-Miošića 26, 10000 Zagreb, Croatia Email address:
3. The national higher school of mathematics, Sidi Abdallah, Algiers, Algeria Email address:
4. Department of Mathematics, Statistics, Purdue University Northwest, 1401 S, U.S. 421, Westville IN 46391 USA Email address: