1 Institut für Mathematik, Goethe Universität, D-60054 Frankfurt am Main
2 Institute for Information Transmission Problems, 19 Bolshoi Karetny, Moscow 127994, Russian Federation, National Research University Higher School of Economics, 20 Myasnitskaya Street, Moscow 101000
3 Institute of Mathematics, Czech Academy of Sciences, Žitná 25, 11567 Praha 1
4 Department of Applied Mathematics, University College Cork