1. School of Mathematics and Statistics, Anhui Normal University, Wuhu, 241002, P.R. China. E-mail: y.jiaojiao1025@yahoo.com
2. Department of Mathematics and Statistics, Missouri University of Science and Technology (formerly University of Missouri, Rolla), USA. E-mail: huwen@mst.edu
3. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, USA. E-mail: junchi.li.duke@gmail.com