1. School of Mathematics and Physics, China University of Geosciences, Wuhan 430074, P. R. China
2. Faculty of Mathematics and Mechanics, St. Petersburg State University, Peterhof, St. Petersburg, Russia
3. Department of Mathematical Information Technology, University of Jyväskylä, Jyväskylä, Finland
4. Zhejiang Institute, China University of Geosciences, Hangzhou, Zhejiang 311305, P. R. China