1. Centre of Mathematics for Applications, Department of Mathematics, University of Oslo P.O. Box 1053, Blindern, N–0316 Oslo, Norway
2. Agder University College, Department of Economics and Business Administration, Serviceboks 422, N-4604 Kristiansand, Norway
3. Centre of Mathematics for Applications, Department of Mathematics, University of Oslo, P.O. Box 1053, Blindern, N–0316 Oslo, Norway
4. Department of System Research, Klaipėda University, H. Manto 84, LT-5808 Klaipėda, Lithuania