1. Faculty of Mathematics and Computer Science, University of Bucharest, Strada Academiei 14, 010014 Bucureşti, Romania
2. Fachbereich Mathematik, Universität Duisburg-Essen, Fakultät für Mathematik, 45117 Essen, Germany
3. Faculty of Engineering and Natural Sciences, Sabancı University, Orta Mahalle, Tuzla 34956, Istanbul, Turkey