A universal Hamiltonian for motion and merging of Dirac points in a two-dimensional crystal G. Montambaux, F. Piéchon, J.-N. Fuchs and M. O. Goerbig Eur. Phys. J. B, 72 4 (2009) 509-520 Published online: 07 November 2009 DOI: 10.1140/epjb/e2009-00383-0