1. Professor, School of Economics and Business, North China Electric Power Univ., Beijing 102206, China.
2. Ph.D. Student, School of Economics and Business, North China Electric Power Univ., Beijing 102206, China.
3. Decision and Information Sciences Division, Argonne National Laboratory, Argonne, IL 60439.
4. Assistant Professor, Sierra Nevada Research Institute, Univ. of California at Merced, Merced, CA 95344 (corresponding author).