1. Faculty of Mathematics and Informatics, Shumen University, Shumen, Bulgaria
2. Institute of Mathematics, Faculty of Science, P.J. Šafárik University in Košice, Košice, Slovakia
3. Department of Applied Statistics, Linz Institute of Technology, Johannes Kepler University, Linz, Austria
4. Institute of Statistics, University of Valparaíso, Valparaíso, Chile