Affiliation: | a Institute of Mathematics “Simion Stoilow” of the Romanian Academy, Bucharest, Romaniab Department of Mathematics, University of Craiova, 200585 Craiova, Romaniac Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, P. O. Box 2964, 1001 Ljubljana, Sloveniad Faculty of Education, University of Ljubljana, Kardeljeva ploš?ad 16, 1000 Ljubljana, Slovenia |