1. Departamento de Matemáticas, Universidad Carlos III de Madrid, Avda. de la Universidad 30, 28911, Madrid, Spain, and Department of Mathematics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 12000, Prague 2, Czechia.
2. Dipartimento di Fisica, Università di Bari Aldo Moro, and Dipartimento di Matematica, Università di Bari Aldo Moro, and Istituto Nazionale di Fisica Nucleare, Sezione di Bari, I-70125 Bari, Italy. Current address: Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Staudtstraße 7, 91058 Erlangen, Germany.
3. Departamento de Matemáticas, Universidad Carlos III de Madrid, Avda. de la Universidad 30, 28911, Madrid, Spain, and Instituto de Ciencias Matemáticas (CSIC – UAM – UC3M – UCM) ICMAT, C/ Nicolás Cabrera 13–15, 28049, Madrid, Spain.