1. Department of Mathematics, University of Pittsburgh, Pittsburgh, PA 15260, USA
2. Department of Mathematics, National Taiwan Normal University, 88, S-4, Ting Chou Road, Taipei 116, Taiwan
3. Université Aix-Marseille III, LATP (UMR CNRS 6632), Faculté des Sciences et Techniques, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
4. Department of Applied Mathematics and Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan
5. Laboratoire M.I.P. (UMR CNRS 5640) and Institut Universitaire de France, Université Paul Sabatier, 118, route de Narbonne, 31062 Toulouse Cedex 4, France