Point determining digraphs, $\{0,1\}$-matrix partitions, and dualities in full homomorphisms

Abstract

We prove that every point-determining digraph $D$ contains a vertex $v$ such that $D-v$ is also point determining. We apply this result to show that for any $\{0,1\}$-matrix $M$, with $k$ diagonal zeros and $\ell$ diagonal ones, the size of a minimal $M$-obstruction is at most $(k+1)(\ell+1)$. This extends the results of Sumner, and of Feder and Hell, from undirected graphs and symmetric matrices to digraphs and general matrices.