1. Department of Applied Mathematics and Statistics, Technological University of Cartagena, Cartagena 30203, Spain
2. Gofa Camp, Near Gofa Industrial College and German Adebabay, Nifas Silk-Lafto, Addis Ababa 26649, Ethiopia
3. Institute of Mathematical Sciences, Faculty of Science, Universiti Malaya, Kuala Lumpur 50603, Malaysia