1. ETH Zürich, Department Mathematik, Zürich, Switzerland, Rätrasse 101, CH-8092 Zürich, Switzerland
2. Ecole Polytechnique Fédérale de Lausanne, Institute of Mathematics, EPFL, Station 8, CH-1015, Lausanne, Switzerland
3. ETH Zürich, Institute for Theoretical Studies, CH-8092 Zürich, Switzerland