1. Third Wave Systems, Eden Prairie, MN 55344 USA.
2. Department of Mathematics, University of British Columbia, 1984 Mathematics Road, Vancouver, BC, Canada.
3. Corresponding author. Department of Mathematics, University of British Columbia, 1984 Mathematics Road, Vancouver, BC, Canada.
4. Institute of Natural Sciences, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China.