David A. Gregory Norman J. Pullman Kathryn F. Jones J. Richard Lundgren Biclique coverings of regular bigraphs and minimum semiring ranks of regular matrices. 73-89 1991 51 J. Comb. Theory, Ser. B 1 http://dx.doi.org/10.1016/0095-8956(91)90006-6 db/journals/jct/jctb51.html#GregoryPJL91