1. Institute of Mathematics, Academy of Sciences of the Czech Republic, Žižkova 22, 61662Brno; and Faculty of Business and Management, Brno University of Technology, Kolejni 2906/4, 61200 Brno, Czech Republic
2. Ilia State University, School of Natural Sciences and Engineering, 32 I. Chavchavadze Ave., Tbilisi0179, Georgia