Collective behavior of Vicsek particles without and with obstacles⋆ Raul Martinez, Francisco Alarcon, Diego Rogel Rodriguez, Juan Luis Aragones and Chantal Valeriani Eur. Phys. J. E, 41 8 (2018) 91 Published online: 17 August 2018 DOI: 10.1140/epje/i2018-11706-8