1. STAR-UBB Institute, Bebes-Bolyai University, Cluj-Napoca, Romania
2. Romanian Institute of Science and Technology, Cluj-Napoca, Romania
3. Department of Mathematical Analysis and Numerical Mathematics, Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Bratislava, Slovakia
4. Mathematical Institute of Slovak Academy of Sciences, Bratislava, Slovakia