Digraphs that have at most one walk of a given length with the same endpoints.

Zejun Huang Xingzhi Zhan Digraphs that have at most one walk of a given length with the same endpoints. 70-79 2011 311 Discrete Mathematics 1 http://dx.doi.org/10.1016/j.disc.2010.09.025 db/journals/dm/dm311.html#HuangZ11