1. Faculty of Mathematics and Natural Science, South-West University “Neofit Rilski”, 2700 Blagoevgrad, Bulgaria
2. CMA, Departamento de Matemática, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Monte da Caparica, 2829516 Caparica, Portugal
3. Institute of Mathematics, University of Potsdam, 14476 Potsdam, Germany