The even cycle problem for directed graphs

If each arc in a strongly connected directed graph of minimum in- degree and outdegree at least 3 is assigned a weight 0 or 1, then the resulting weighted directed graph has a directed cycle of even total weight. This proves a conjecture made by L. Lovisz in 1975 and has applications to colour-critical hypergraphs, sign-nonsingular matrices, and permanents of matrices. MATHEMATICAL INSTITUTE, TECHNICAL UNIVERSITY OF DENMARK, DK-2800 LYNGBY,