Because of the attack on the computer network of the University of Duisburg-Essen, some content (in particular some image files) cannot be accessed because it is stored on central servers of the university.

Address:
Universität Duisburg-Essen
Fakultät für Mathematik
Thea-Leymann-Str. 9
45127 Essen

Email: guillermo_gamarra - at - outlook.es