Affiliation: | (1) Department of Mechanics and Mathematics, Ivan Franko Lviv National University, Universytetska 1, Lviv, 79000, Ukraine;(2) Institute of Mathematics, Physics and Mechanics, and Faculty of Natural Sciences and Mathematics, University of Maribor, Gosposvetska 84, Maribor, 2000, Slovenia;(3) Faculty of Mathematics and Physics, and Faculty of Education, University of Ljubljana, P.O.B. 2964, Ljubljana, 1001, Slovenia |