1 Department of Mathematics, Luoyang Teachers’ College, Luoyang 471022, P.R.China
2Department of Applied Mathematics, Dalian University of Technology, Dalian 116024, P. R. China
3Department of Mathematics, Luoyang Teachers’ College, Luoyang 471022, P.R.China
4College of Mathematics and Information Science, Henan University, Kaifeng 475001, P. R. China