Small and large quasi-kernels: a weighted transfer theorem

Abstract

A quasi-kernel of a digraph $D$ is an independent set from which every vertex is reachable by a directed path of length at most two. The Small Quasi-kernel Conjecture asserts that every source-free digraph on $n$ vertices has a quasi-kernel of size at most $n/2$, whereas the Large Quasi-kernel Conjecture asks for a quasi-kernel whose closed out-neighbourhood has size at least $n/2$. Spiro proved that the former implies the latter. In this paper, we establish a weighted transfer theorem: if every nonnegative integer weighting of a source-free digraph $D$ admits a quasi-kernel whose closed out-neighbourhood contains at least a $θ$-fraction of the total weight, then $D$ has a quasi-kernel of size at most $\lfloor(1-θ)α(D)+θμ\rfloor$ for $0<θ\le1/2$, where $μ$ is the maximum matching size between a maximum independent set and its complement. Using the independent blow-up construction of Spiro, we deduce that the universal small and large quasi-kernel assertions are equivalent for every such $θ$, with no loss in the parameter. In particular, the two conjectures are equivalent. As an application, we prove the half-order bound for source-free break digraphs.