1. Department of Mathematics, National Kaohsiung Normal University, Kaohsiung 82444, Taiwan
2. Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovića 6, 21125 Novi Sad, Serbia
3. Department of Mathematical Analysis, Mathematics Faculty, Chelyabinsk State University, Kashirin Brothers Str. 129, Chelyabinsk 454001, Russia
4. Departamento de Matemáticas, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Casilla 653, Santiago 78000025, Chile