1. Department of Mathematics, University of Rostock, Ulmenstraße 69, Rostock 18057, Germany
2. Department of Mathematics and NTIS, Faculty of Applied Sciences, University of West Bohemia, Univerzitní 8, Plzeň 301 00, Czech Republic
3. Institute of Mathematics, Ufa Federal Research Centre, RAS, Chernyshevsky str. 112, Ufa 450008, Russia