基金
This work was supported by the National Natural Science Foundation of China(0072058)
863 Foundation of China(2003AA624010)
the National Natural Science Foundation of Guangdong Province,China(021732,2003A 3050401)
a strategic grant at City University of Hong Kong(7000650)
E.B.G.J.acknowledges the Royal Society,U.K.and City University of Hong Kong for the award of the Kan Tong Po Visiting Professorship.