Small $q$-kernels in digraphs

Abstract

We address three questions of Spiro [Europ. J. Combin., 133:Paper No. 104307, 2026] on smallest $q$-kernels in digraphs. All three questions are on the existence of small $q$-kernels in digraphs, where 'smallness' is measured in different ways. We answer one of these questions in the affirmative, and the other two in the negative. We further study the latter two questions in the context of bipartite digraphs. In particular, we show that in the bipartite setting, the answer to one of the two questions is affirmative if and only if $q$ is odd. For the other question, we show that the answer is positive when the digraph is bipartite and $q$ is odd (the case when the digraph is bipartite and $q$ is even remains open).

10 pages, 3 figures