1. Institute for Applied Mathematics and Cluster of Excellence PhoenixD (Photonics, Optics, and Engineering – Innovation Across Disciplines) , Leibniz Universität Hannover , Welfengarten 1, 30167 Hannover , Germany
2. Fakultät für Mathematik , Universität Duisburg-Essen , Thea-Leymann-Straße 9, 45127 Essen , Germany