1. Department of Economics, Paris Dauphine University and Finance For Energy Market Research Centre, 75775 Paris CEDEX 16, France;
2. Department of Industrial Engineering and Operations Research, University of California, Berkeley, Berkeley, California 94720;
3. Department of Mathematics “Tullio Levi-Civita,” University of Padova, 35122 Padova, Italy;
4. Department of Statistics, London School of Economics and Political Science, London WC2A 2AE, United Kingdom