1. Department of Mathematics and Department of Computer Science; Georgetown University; Washington DC 20057 USA
2. Department of Mathematics; Fu Jen Catholic University; Taipei 242 Taiwan
3. Department of Mathematics; Linyi University; Linyi 276005 China
4. School of Mathematical Sciences; Beijing Normal University, Laboratory of Mathematics and Complex Systems, Ministry of Education; Beijing 100875 China
5. School of Mathematics and Statistics, Gansu Key Laboratory of Applied Mathematics and Complex Systems; Lanzhou University; Lanzhou 730000 Gansu China