1. Dipartimento di Matematica “F. Casorati”, IMATI-CNR Pavia Università di Pavia, via Ferrata 5, 27100 Pavia, Italy
2. Gheorghe Mihoc-Caius Iacob, Institute of Mathematical Statistics and Applied Mathematics of the Romanian Academy, Calea 13, Septembrie 13, 050711 Bucharest, Romania