1. Mathematical Institute, University of Oxford, Andrew Wiles Building, Radcliffe Observatory Quarter , Woodstock Road, Oxford OX2 6GG, UK
2. Elkem ASA , Technology, Fiskaaveien 100, 4621 Kristiansand , Norway
3. Department of Mathematics and Statistics, University of Otago , P.O. Box 56, Dunedin 9054 , New Zealand