1. Laboratoire de Mécanique et Génie Civil; UMR 5508 CNRS - UM2, Université Montpellier 2; c.c. 048, Place Eugène Bataillon Montpellier Cedex 5 34095 France
2. Department of Mathematics; Mahidol University; Rama VI road Bangkok 10400 Thailand
3. I3M, UMR CNRS 5149-UM2, Université Montpellier 2; c.c. 51, Place Eugène Bataillon Montpellier Cedex 5 34095 France
4. Department of Mathematics; King Mongkut's University of Technology North Bangkok; 1518 Pibulsongkram road, Bangsue Bangkok 10800 Thailand