1. Chair for Energy Trading and Finance, University of Duisburg-Essen, Universitätsstraße 12, 45141 Essen, Germany
2. Department of Mathematics, University of Oslo, P. O. Box 1053 Blindern, 0316 Oslo, Norway
3. Chair for Management Science and Energy Economics, University of Duisburg-Essen, Universitätsstraße 12, 45141 Essen, Germany