1. Department of Mathematics and Statistics Georgetown University Washington DC USA
2. Graduate Institute of Business Administration, College of Management Fu Jen Catholic University New Taipei City 24205 Taiwan
3. School of Mathematics and Statistics, Gansu Key Laboratory of Applied Mathematics and Complex Systems Lanzhou University Lanzhou 730000 China
4. Laboratory of Mathematics and Complex Systems (Ministry of Education of China), School of Mathematical Sciences Beijing Normal University Beijing 100875 China