1. Faculty of Mathematics and Computer Science, Konstantin Preslavsky University of Shumen, Shumen, Bulgaria
2. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Sofia, Bulgaria
3. Faculty of Mathematics and Informatics, Sofia University, Sofia, Bulgaria
4. Linz Institute of Technology, Johannes Kepler University, Linz, Austria
5. Institute of Statistics, University of Valparaíso, Valparaíso, Chile