Affiliation: | 1. Ivan Franko National University of Lviv, Ukraine;2. Jan Kochanowski University in Kielce, Poland;3. Department of Mathematics and Statistics, University of North Carolina at Charlotte, Charlotte, NC, USA;4. Departamento de Matemáticas, Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito ext. s/n, Ciudad Universitaria, C.P. 04510, CDMX, Mexico |