1. LMGC, UMR-CNRS 5508, Université Montpellier II, Case courier 048, Place Eugène Bataillon, 34095 Montpellier cedex 5, France. E-mail: clicht@univ-montp2.fr
2. Department of Mathematics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
3. Centre of Excellence in Mathematics, CHE, Bangkok 10400, Thailand. E-mail: somsak.ora@mahidol.ac.th