Affiliation: | (1) Department of Mathematics, University of Oslo, Centre of Mathematics for Applications, Blindern, P.O. Box 1053, 0316 Oslo, Norway;(2) Present address: Department of Economics and Business Administration, Agder University College, Serviceboks 422, 4604 Kristiansand, Norway;(3) Department of Mathematics, University of Oslo, Centre of Mathematics for Applications, Blindern, P.O. Box 1053, 0316 Oslo, Norway |