[1] Politecn Milan, Dipartimento Matemat, Via Bonardi 9, I-20133 Milan, Italy
[2] Univ Modena & Reggio Emilia, Dipartimento Sci Fis Informat & Matemat, Via Campi 213-b, I-41125 Modena, Italy
[3] Henan Normal Univ, Sch Math & Informat Sci, Xinxiang, Henan, Peoples R China
[4] Univ Poitiers, Equipe DACTIM MIS, Lab I3M & Lab Math & Applicat, Site Futuroscope Teleport 2 11 Blvd Marie & Pierre, F-86073 Poitiers 9, France