1. TU Wien, Institute of Discrete Mathematics and Geometry, Wiedner Hauptstrasse 8-10/104, 1040 Wien, Austria
2. Creative Science Course (Mathematics), Faculty of Science, Shizuoka University, Ohya 836, Suruga-ku, Shizuoka-shi 422-8529, Japan
3. Einstein Institute of Mathematics, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel
4. Department of Mathematics, Rutgers University, New Brunswick, NJ 08854, USA