1. Department of Mathematics, Faculty of Science, University of Zagreb, Bijenička Cesta 30, Zagreb, Croatia
2. Department of Mathematics, Josip Juraj Strossmayer University of Osijek, Trg Ljudevita Gaja 6, Osijek, Croatia
3. SISSA–International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, Italy