Moore and more and symmetry

In any spatially discrete model of pedestrian motion which uses a regular lattice as basis, there is the question of how the symmetry between the different directions of motion can be restored as far as possible but with limited computational effort. This question is equivalent to the question ''How important is the orientation of the axis of discretization for the result of the simulation?'' An optimization in terms of symmetry can be combined with the implementation of higher and heterogeniously distributed walking speeds by representing different walking speeds via different amounts of cells an agent may move during one round. Therefore all different possible neighborhoods for speeds up to v = 10 (cells per round) will be examined for the amount of deviation from radial symmetry. Simple criteria will be stated which will allow find an optimal neighborhood for each speed. It will be shown that following these criteria even the best mixture of steps in Moore and von Neumann neighborhoods is unable to reproduce the optimal neighborhood for a speed as low as 4.Comment: Proceedings contribution in N. Waldau et al. (editors) "Pedestrian and Evacuation Dynamics 2005" (2006) pages 297-308. Springer-Verlag Berlin Heidelberg. ISBN: 978-3-540-47062-

Spontaneous Particle-Hole Symmetry Breaking in the ν=5/2\nu=5/2 Fractional Quantum Hall Effect

The essence of the $\nu=5/2$ fractional quantum Hall effect is believed to be well captured by the Moore-Read Pfaffian (or anti-Pfaffian) description. However, an important mystery regarding the formation of the Pfaffian state is the role of the three-body interaction Hamiltonian $H_3$ that produces the Pfaffian as an exact ground state and the concomitant particle-hole symmetry breaking. We show that a two-body interaction Hamiltonian $H_2$ constructed via particle-hole symmetrization of $H_3$ produces a ground state nearly exactly approximating the Pfaffian and anti-Pfaffian states, respectively, in the spherical geometry. More importantly, the ground state energy of $H_2$ is shown to exhibit a ``Mexican-hat'' structure as a function of particle number in the vicinity of half filling for a given flux indicating spontaneous particle-hole symmetry breaking. We do not find any such Mexican-hat signature in the second Landau level Coulomb interaction at 5/2.Comment: 4 pages, 3 figures, v2 is the final versio