**Author details**

Zhengde Dai *School of Mathematics and Physics, Yunnan University, Kunming 650091, P.R.China*

Jun Liu and Gui Mu

*College of Mathematics and Information Science, Qujing Normal University, Qujing 655000, P.R.China*

Murong Jiang *School of Information Sciences and Engineering, Yunnan University, Kunming 650091, P.R.China*
