1. Institute of Applied Mathematics and Mechanics of NAS of Ukraine; Dobrovolskogo st. 1 Slovyans'k 84100 Ukraine
2. Humboldt Research Fellow at the University of Bonn; Endenicher Allee 60 53115 Bonn Germany
3. Institut für Mathematik; Universität zu Lübeck; Ratzeburger Allee 160 23562 Lübeck Germany