1. Dipartimento SBAI, Sapienza Università di Roma, Rome, Italy
2. I.N.F.N.–Sez. Roma 1, Gr. IV–Mathematical Methods in NonLinear Physics, Rome, Italy
3. MOD, Mid Sweden University, Sundsvall, Sweden
4. Instytut Matematyki, Uniwersytet Jana Kochanowskiego w Kielcach, Kielce, Poland