1. ETH-Zurich, Department of Information Technology and Electrical Engineering, Gloriastrasse 35, CH-8092 Zürich, Switzerland.
2. Department of Mathematical Information Technology, P.O. Box 35 (Agora), FI-40014 University of Jyväskylä, Finland.
3. Helsinki School of Economics, Department of Business Technology, P.O. Box 1210, FI-00101 Helsinki, Finland.
4. Department of Applied Economics, University of Malaga, E-29071 Malaga, Spain.