1. Department of Mathematics and Computer Sciences, ENP-Oran, El'Mnaouar, Algeria. E-mails: kaoutar.ghomari@gmail.com, ghomari_kaoutar@yahoo.fr
2. Department of Mathematics, University of Oran Es-Senia, El'Mnaouar, Algeria. E-mail: messirdi.bekkai@univ-oran.dz
3. Mathematics Research Institute IRMAR, University of Rennes 1 and Institut Universitaire de France, Rennes, France. E-mail: san.vu-ngoc@univ-rennes1.fr