1. Dipartimento di Matematica e Applicazioni, “R. Caccioppoli”, Università di Napoli “Federico II”, via Cintia, 80126 Napoli, Italy
2. Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
3. Department of Mathematics and Computer Science, University of Dschang, P.O. Box 67, Dschang, Cameroon