作者机构:[1]Department of Mathematics,Shanxi Datong University,Shanxi 037009,P.R.China, [2]Department of Mathematics,Shanxi University,Shanxi 030006,P.R.China, [3]Department of Mathematics,Taiyuan University of Technology,Shanxi 030024,P.R.China
相关基金:Supported by the National Natural Science Foundation of China (Grant Nos.10771157; 10871111); Research Grant to Returned Scholars of Shanxi Province (Grant No.2007-38)Acknowledgement The authors would like to thank the referee's helpful comments which improved this paper.