Manuscript received September 25, 2009. Revised January 29, 2010. Published online June 21, 2010. ∗School of Mathematical Sciences and Computing Technology, Central South University, Changsha 410075, China; Department of Mathematics and Computation Sciences, Hunan City University, Yiyang 413000, Hunan, China. E-mail: xiaocuie@163.com ∗∗Corresponding author. School of Mathematics and Computer Science, Guangxi University for Nationalities, Nanning 530006, China. E-mail: zhhliu100@gmail.com ∗∗∗Project supported by the National Natural Science Foundation of China (No. 10971019) and the Guangxi Provincial Natural Science Foundation of China (No. 2010GXNSFA013114).