1. Faculté des Sciences, URMI Laboratory, Département de Mathématiques et Informatique, Université des Sciences et Techniques de Masuku, Franceville, Gabon.
2. Faculty of Sciences Semlalia, LIBMA Laboratory, Department of Mathmatics, Cadi Ayyad University, Marrakech, Morocco.
3. Hassan II Academy of Sciences and Technologies, Rabat, Morocco.
4. Department of Mathematical Sciences and Research Institute of Mathematics of Seoul National University, Seoul, South Korea.