Affiliation: | a School of Mathematics and Information Sciences, North China University of Water Resources and Electric Power, Zhengzhou 450011, Chinab Department of Mathematics, Hangzhou Normal University, Hangzhou 310036, Chinac Department of Mathematics, Gyeongsang National University, Jinju 660-701, Republic of Koread Department of Mathematics and the RINS, Gyeongsang National University, Jinju 660-701, Republic of Korea |