1. School of Mathematics and Information Science, Henan Polytechnic University, Jiaozuo, Henan 454003, P. R. China
2. School of Mathematics and Statistics, Lanzhou University, Lanzhou, Gansu 730000, P. R. China
3. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, P. R. China