1. Department of Mathematics and Physics, Uman National University of Horticulture, 20300 Uman, Ukraine
2. Department of Algebra and Geometry, Faculty of Science, Palacky University in Olomouc, 771 46 Olomouc, Czech Republic
3. Institute of Mathematics and Descriptive Geometry, Faculty of Civil Engineering, Brno University of Technology, 602 00 Brno, Czech Republic
4. Department of Physical and Mathematical Sciences, Odesa National University of Technology, Kanatnaya 112, 65039 Odesa, Ukraine