1. Division of Mathematics and Physics, UKK, Mälardalen University, Box 883, 721 23 Västerås, Sweden
2. Department of Computer Science, Electrical Engineering and Mathematical Sciences, Faculty of Engineering and Science, Western Norway University of Applied Sciences, Postbox 7030, 5020 Bergen, Norway
3. Department of Economics, Norwegian Business School (BI), Kong Christian Frederiks plass 5, 5006 Bergen, Norway