作者机构:[1]Department of Mathematics,Chaohu University,Anhui 238000,P.R.China, [2]Department of Mathematics,Anhui Normal University,Anhui 241000,P.R.China
相关基金:Supported by the National Natural Science Foundation of China (Grant No.10371087); Natural Science Foundation of Anhui Province (Grant No.07021019); Education Committee of Anhui Province (Grant Nos.KJ2011A138; KJ2009B097; KJ2010B127)Acknowledgements The authors are grateful to the referees for their helpful comments and suggestions.