Institution: | 1. Faculty of Mathematics, University of Duisburg-Essen
, Essen, Germany.https://orcid.org/0000-0002-9948-8426;2. Departamento de Matemática Aplicada, Universidad Nacional de Educación a Distancia
, Madrid, Spain.;3. Institute of Mathematics, Martin-Luther-University of Halle-Wittenberg
, Halle-Saale, Germany. |