| pages = 51–73 | title = Bigraphs versus digraphs via matrices | volume = 4 | year = 1980}}. Brualdi et al. credit the idea for this equivalence to {{citation | doi = 10.4153/CJM-1958-052-0 | last1 = Dulmage | first1 = A. L. | last2 = Mendelsohn | first2 = N. S. | author2-link = Nathan Mendelsohn | mr = 0097069 | journal = Canadian Journal of Mathematics | pages = 517–534 | title = Coverings of bipartite graphs | volume = 10 | | pages = 51–73 | title = Bigraphs versus digraphs via matrices | volume = 4 | year = 1980}}. Brualdi et al. credit the idea for this equivalence to {{citation | doi = 10.4153/CJM-1958-052-0 | last1 = Dulmage | first1 = A. L. | last2 = Mendelsohn | first2 = N. S. | author2-link = Nathan Mendelsohn | mr = 0097069 | journal = Canadian Journal of Mathematics | pages = 517–534 | title = Coverings of bipartite graphs | volume = 10 |