1. Faculty of Mathematics, University of Duisburg-Essen , Essen, Germany.
2. Center for Applied and Computational Mathematics, School of Mathematical Sciences, Rochester Institute of Technology , Rochester, NY, USA.
3. Departamento de Matemática Aplicada, Universidad Nacional de Educación a Distancia , Madrid, Spain.
4. Institute of Mathematics, Martin-Luther-University of Halle-Wittenberg , Halle-Saale, Germany.