Effects of directional migration on prisoner’s dilemma game in a square domainHongyan Cheng, Qionglin Dai, Haihong Li, Xiaolan Qian, Mei Zhang and Junzhong YangEur. Phys. J. B, 86 4 (2013) 127DOI: https://doi.org/10.1140/epjb/e2013-40076-5