1. African Institute for Mathematical Sciences (AIMS), P.O. Box 608, Limbe Crystal Gardens, South West Region, Cameroon, Institut für Analysis, Fakultät Mathematik, Technische Universität Dresden D-01062 Dresden, Germany
2. Laboratoire L.A.M.I.A., Département de Mathématiques et Informatique, Université des Antilles, Campus Fouillole, 97159 Pointe-à-Pitre, (FWI), Guadeloupe, France
3. Department of Mathematical Sciences, Center for Mathematics and Artificial Intelligence (CMAI), George Mason University, 4400 University Dr, Fairfax, VA 22030, USA