A precise condition for independent transversals in bipartite covers

Stijn Cambie, Penny Haxell, Ross J. Kang, Ronen Wdowinski

Published 2023-08-28Version 1

Given a bipartite graph $H=(V=V_A\cup V_B,E)$ in which any vertex in $V_A$ (resp. $V_B$) has degree at most $D_A$ (resp. $D_B$), suppose there is a partition of $V$ that is a refinement of the bipartition $V_A\cup V_B$ such that the parts in $V_A$ (resp. $V_B$) have size at least $k_A$ (resp. $k_B$). We prove that the condition $D_A/k_A+D_B/k_B\le 1$ is sufficient for the existence of an independent set of vertices of $H$ that is simultaneously transversal to the partition, and show moreover that this condition is sharp. This result is a bipartite refinement of two well-known results on independent transversals, one due to the second author the other due to Szab\'o and Tardos.