1. Department of Mathematics, UFR-MASI FST; Hassan II University Mohammedia; Morocco
2. LAGA; Université Paris 13; 99 Av J.B. Clement 93430 Villetaneuse France
3. Department of Genie Civil, LASH EMI; Mohammed V University Rabat; Morocco
4. School of Engineering and Computing Sciences; University of Durham; South Road Durham DH1 3LE UK