1. National Institute of Informatics, Hitotsubashi, Chiyoda-ku, Tokyo, Japan
2. Kawarabayashi Large Graph Project, ERATO, JST, c/o Global Research Center for Big Data Mathematics, NII, Chiyoda-ku, Tokyo, Japan
3. Institute of Cognitive Neuroscience, University College London, 17 Queen Square, London WC1N 3AZ, UK
4. Graduate School of Information Sciences, Tohoku University, Sendai 980-8579, Japan
5. Department of Engineering Mathematics, University of Bristol, Bristol BS8 1UB, UK