1. College of Science, Sichuan Agricultural University, Ya'an, Sichuan 625000, China
2. Johann Bernoulli Institute for Mathematics and Computer Science, University of Groningen, 9700 AK Groningen, The Netherlands
3. Faculty of Engineering, Free University of Bozen-Bolzano, 39100 Bolzano, Italy
4. School of Mathematical Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China