Institution: | a Department of Applied Mathematics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands b Faculty of Management and Organization, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands c Vologda State Pedagogical University and Vologda Scientific Coordinate Centre of CEMI RAS, S. Orlova 6, Vologda, Russia |