Hansjörg Geiges, born in Basel in 1966, is Professor of Mathematics at the Universität zu Köln. He received his PhD from the University of Cambridge in 1992. Before moving to his current position in 2002, he taught at Stanford University, ETH Zürich, and the Universiteit Leiden. His other published books are An Introduction to Contact Topology and The Geometry of Celestial Mechanics. Kai Zehmisch, born in Leipzig in 1975, is Professor of Mathematics at the Ruhr-Universitat Bochum. He obtained his doctorate at the Universität Leipzig in 2009, after having lived through the failure of the second socialist experiment on German soil, and was rewarded nonetheless with a book prize on 100 Jahre Mathematisches Seminar der Karl-Marx-Universität Leipzig. Previous to his current position, he taught at the Westfälische Wilhems-Universität Münster (as it was then called) and at the Justus-Liebig-Universität Giessen.