基金The project supported by National Key Basic Research Program of China under Grant No. 2004CB31800, 2006CB805905 and National Natural Science Foundation of China under Grant Nos. 10231050 and 10375087 Ding thanks Prof. A. Bellen for his warm invitation and great help while Ding was staying in Trieste, where the work was partially complected. Thanks also to Prof. G. Lindi for his kindness. And the work is (partially) supported by Inistero degli Affari Esteri-Direzione Gen- erale per la Promozione la Cooperazione Culturale, and by Istituto Nazionale di Alta Matematica, francesco sev- eri (INdAM), Roma.