Affiliation: | a Omsk Branch of Sobolev Institute of Mathematics SD RAS, Pevtsova 13, Omsk 644099, Russia b Department of Mathematics, Brigham Young University, Provo, UT 84602, United States c Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, Ljubljana 1000, Slovenia d Faculty of Education, University of Ljubljana, Kardeljeva pl. 16, Ljubljana 1000, Slovenia |