1. * Bolyai Institute, University of Szeged , Aradi vértanúk tere 1 , Szeged , HUNGARY
2. ** Department of Analysis, University of Miskolc , Miskolc-Egyetemváros , HUNGARY
3. *** Department of Mathematics and Informatics, University of Novi Sad, Faculty of Sciences , Novi Sad , SERBIA
4. **** Department of Mathematics and Informatics, University of Novi Sad, Faculty of Sciences, Novi Sad, Mathematical Institute SANU , Belgrade , SERBIA