[1] Univ Oxford, Merton Coll, Oxford OX1 4JD, England
[2] City Univ London, Dept Math, London EC1V 0HB, England
[3] Nankai Univ, Sch Phys, Tianjin 300071, Peoples R China
[4] Uppsala Univ, Dept Phys & Astron, Angstrom Lab, Box 516, S-75120 Uppsala, Sweden
[5] Harvard Univ, Dept Math, Jefferson Phys Lab, Ctr Math Sci & Applicat, Cambridge, MA 02138 USA