1. Department of Mathematics, Technische Universität Darmstadt , Schlossgartenstr. 7, 64289 Darmstadt , Germany
2. Institute of Applied Mathematics and Mechanics, National Academy of Sciences of Ukraine , Gen. Batiouk Str. 19, 84116 Sloviansk , Ukraine
3. Dipartimento di Matematica e Informatica “Ulisse Dini” Viale G. Morgagni, Università degli Studi di Firenze , 67/a, 50134 Firenze , Italy