1. Department of Economics and Business Economics, Aarhus University, 8240 Aarhus V, Denmark;
2. Department of Mathematical Sciences, University of Copenhagen, 2100 København Ø, Denmark;
3. School of Mathematics and Natural Sciences, University of Wuppertal, 42119 Wuppertal, Germany;
4. Institute of Production and Logistics Management, Johannes Kepler University Linz, 4040 Linz, Austria