1. Department of Mathematics, Faculty of Mathematics and Natural Science, South-West University “Neofit Rilski”, 2700 Blagoevgrad, Bulgaria
2. Center for Mathematics and Applications (NOVA Math) and Department of Mathematics, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Monte da Caparica, 2829–516 Caparica, Portugal.
3. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.