[1]School of Mathematics and Computer Science, Panzhihua University, Sichuan 617000, P. R. China;
[2]Department of Mathematics, Kunming University, Yunnan 650031, P. R. China;
[3]School of Essential Education, Yunnan Minzu University, Yunnan 650031, P. R. China;
[4]City College, Kunming University of Science and Technology, Yunnan 650051, P. R. China;