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. Storebrand Investments, P.O. Box 1380, N–0114 Oslo, Norway
4. Department of Mathematics, University of Bergen, Johs. Brunsgt. 12, N–5008 Bergen, Norway
5. Centre of Mathematics for Applications, Department of Mathematics, University of Oslo, P.O. Box 1053, Blindern, N–0316 Oslo, Norway