1. Institut de Mathématiques de Bourgogne (IMB), UMR 5584, CNRS, Université de Bourgogne Franche-Comté, F-21000 Dijon, France
2. CENTRA, Departamento de Física, Instituto Superior Técnico IST, Universidade de Lisboa UL, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal
3. School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom