[1] Univ Bari Aldo Moro, Dipartimento Matemat, Via E Orabona 4, I-70125 Bari, Italy
[2] Univ Parma, Dept Math Phys & Comp Sci, Viale Parco Area Sci 53-A, I-43124 Parma, Italy
[3] Univ Poitiers, SP2MI, UMR CNRS 7348, Lab Math & Applicat, Blvd Marie & Pierre Curie,Teleport 2, F-86962 Futuroscope, France