1. Corresponding author. Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstraße 1, 39106 Magdeburg, Germany.
2. Courant Institute of Mathematical Sciences, New York University, New York, NY 10012 USA.
3. Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstraße 1, 39106 Magdeburg, Germany, and Faculty of Mathematics, Otto von Guericke University, Universitätsplatz 2, 39106 Magdeburg, Germany.