1. Conservatoire National des Arts et Métiers, M2N, x IMATH, Case 2D 5000, 292 rue Saint-Martin, 75003 Paris, France. E-mail: thierry.horsin@cnam.fr
2. Department of Differential Equations, Dnipropetrovsk National University, Gagarin av., 72, 49010 Dnipropetrovsk, Ukraine. E-mail: p.kogut@i.ua