Aleksander Malnic Dragan Marusic Norbert Seifter Primoz Sparl Boris Zgrablic Reachability relations in digraphs. 1566-1581 2008 29 Eur. J. Comb. 7 http://dx.doi.org/10.1016/j.ejc.2007.11.003 db/journals/ejc/ejc29.html#MalnicMSSZ08