1. Department of Differential Equations, Faculty of Mathematics and Informatics, University of Sofia, Sofia, Bulgaria
2. Department of Mathematics, College of Sciences and Arts, Qassim University, Ar-Rass, Saudi Arabia
3. Departement de Mathématiques, Faculté des sciences, Université 20 Aôut 1955, Skikda, Algérie
4. Department of Mathematics and Computer Science, Faculty of Science, Port Said University, Port Said, 42511, Egypt