1. Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A.; and Jefferson Physical Laboratory, Harvard University, Cambridge, Mass., U.S.A.
2. Center of Mathematical Sciences and Applications, Harvard University, Cambridge, Massachusetts, U.S.A.; and Jefferson Physical Laboratory, Harvard University, Cambridge, Mass., U.S.A.
3. Department of Mathematical Sciences, Tsinghua University, Beijing, China