1. School of Mathematical Sciences/Research Center for Image and Vision Computing, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China
2. School of Economic Mathematics, Southwestern University of Finance and Economics, Chengdu, Sichuan 611130, China
3. Johann Bernoulli Institute for Mathematics and Computer Science, University of Groningen, Nijenborgh 9, P.O. Box 407, 9700 AK Groningen, Netherlands