1. Department of Mathematical Sciences, Politecnico Torino Corso Duca degli Abruzzi 24, I-10129 Torino, Italy
2. IUSS and IMATI-CNR, Via Ferrata 5, I-27100 Pavia, Italy
3. Applied Mathematics and Plasma Physics Group, Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA