1. Institute of Geometry and Applied Mathematics, RWTH Aachen University, Templergraben 55, 52074 Aachen, Germany
2. NRW.Bank, Kavalleriestraße 22, 40213 Düsseldorf, Germany
3. Department of Mathematics "G. Castelnuovo", Sapienza University of Rome, P.le Aldo Moro 5, 00185 Roma, Italy