The Exact End-Degree Threshold for Finite-Width Directed Hexagonal Grids

Abstract

Grid theorems provide a fundamental link between the structure of ends and the existence of grid-like subgraphs in infinite graphs and digraphs. For every fixed width $n$, let $k(n)$ denote the least positive integer such that every digraph with an end of in-degree at least $k(n)$ contains a subdivision of the directed hexagonal grid of width $n$. Hamann and Heuer asked the exact vaule of $k(n)$. We solve this problem by proving that $$ k(n)=\left\lfloor\frac{3n}{2}\right\rfloor-1 \qquad\text{for every }n\geq4, $$ while $k(1)=1$, $k(2)=2$, and $k(3)=4$. For the upper bound, we establish a finite vacancy theorem for labelled token slides, convert it into a closed directed schedule on an auxiliary ray digraph, and lift the schedule through fresh clean linkages. For the matching lower bound, we use alternating orientations of products of a ray with a three-armed tree and prove a no-passing property for disjoint directed paths. We also determine the dual extremal parameter. Let $W(d)$ denote the largest width that is always forced by an end of in-degree $d$, with all branch rays remaining in that end, then $$W(d)=\left\lfloor\frac{2d}{3}\right\rfloor+1 \qquad\text{for every }d\geq4,$$ with $W(1)=1$ and $W(2)=W(3)=2$.