1. Ivan Franko National University of Lviv, Ukraine, and Institute of Mathematics, Jan Kochanowski University in Kielce, Poland
2. Institute of Mathematics, Kurt Gödel Research Center,Vienna, Austria
3. Faculty of Mathematics, National University of Lviv, Universytetska 1, Lviv, 79000, Ukraine, ovgutik@yahoo.com
4. Department of Analysis, Geometry and Topology, Pidstryhach Institute for Applied Problems of Mechanics and Mathematics National Academy of Sciences of Ukraine, Naukova 3-b, Lviv, 79060, Ukraine