1. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan.
2. Department of Mathematical Informatics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.
3. Institute of Industrial Science, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan.
4. National Institute of Informatics, Hitotsubashi 2-1-2, Chiyoda-ku, Tokyo 101-8403, Japan.
5. Division of Electrical, Electronic and Information Engineering, Osaka University, Osaka 565-0871, Japan.
6. ERATO Kawarabayashi Large Graph Project, Japan Science and Technology Agency, Hitotsubashi 2-1-2, Chiyoda-ku, Tokyo 101-8403, Japan.
7. E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA.
8. NTT Device Technology Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan.