1. African Institute for Mathematical Sciences, Summerhill Estates — East Legon Hills, Santoe, GPS: GK-0647-1372, Accra, Ghana
2. Laboratoire d’Analyse Numérique et Informatique, UFR SAT, Département de Mathématiques, Université Gaston Berger de Saint-Louis, B.P. 234, Saint-Louis, Senegal
3. Department of Mathematics, West Chester University, 25 University Avenue, West Chester, PA 19383, USA