1. Département de Mathématiques, Université Hassan II, Yasmina, Mohammadia, Morocco
2. Department of Mathematics and Computer Science, Royal Flemish Academy of Belgium for Science and the Arts, University of Antwerp, Middelheimlaan, Antwerpen, Belgium
3. Department of Mathematics, Gebze Institute of Technology, Çayirova Campus, Gebze Kocaeli, Turkey