1. School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore e-mail:
2. School of Business, Shanghai DianJi University, 1350 Ganlan Road, Lingang New City, Pudong New District, Shanghai 201306, China
3. Department of Mathematics, Babeş-Bolyai University, Cluj-Napoca 400084, Romania