Propagation of travelling waves in sub-excitable systems driven by noise and periodic forcing F. N. Si, Q. X. Liu, J. Z. Zhang and L. Q. Zhou Eur. Phys. J. B, 60 4 (2007) 507-513 Published online: 16 January 2008 DOI: 10.1140/epjb/e2008-00009-1