1. Department of Mathematics, IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA
2. Department of Mathematics, Massachusetts Institute of Technology, Cambridge Massachusetts 02139, USA
3. Education Center for Global Leaders in Molecular Systems for Devices, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan
4. Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA