Affiliation: | a Department of Mathematics, Liaoning Normal University, P.O. Box 200, Dalian, Liaoning 116029, People's Republic of China b Kingbridge Business College, Dongbei University of Finance & Economics, Dalian, Liaoning 116600, People's Republic of China c Department of Mathematics and The Research Institute of Natural Science, Gyeongsang National University, Chinju 660-701, South Korea d The Research Institute of Natural Science, Gyeongsang National University, Chinju 660-701, South Korea |