[1] Univ Rennes 1, Inst Rech Math Rennes, F-35042 Rennes, France
[2] Shanghai Jiao Tong Univ, Sch Math Sci, Shanghai 200240, Peoples R China
[3] Ecole Polytech, Ctr Math Appl, F-91128 Palaiseau, France
[4] Univ Maradi, Dept Math, BP 465, Maradi, Niger