1. Faculty of Pure and Applied Mathematics, Wroclaw University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wroclaw, Poland
2. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Georgi Bonchev str., bl.8, Sofia 1113, Bulgaria
3. Faculty of Mathematics and Informatics, Sofia University “St. Kliment Ohridski”, 5 James Bourchier blvd., Sofia 1164, Bulgaria