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arXiv · 2606.08338

Discrete-time treatment number of binary trees

Karen L. Collins·Margaret-Ellen Messinger·Ann N. Trenk
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Abstract

The discrete-time treatment number of a graph $H$, denoted by $\tau(H)$, was introduced in arXiv:2408.0531(3) and arises from a deterministic process in which each vertex is assigned a color at each time-step. The pathwidth upper bound $\tau(H)\leq \lceil\frac{1+pw(H)}{2}\rceil$, is shown in arXiv:2408.0531(3), where $pw(H)$ denotes the pathwidth of graph $H$. Equality holds when $H$ is the complete binary tree of depth $d$ (denoted by $BT(d)$) and $1 \le d \le 6$. In this paper, we characterize the sizes of all subsets of vertices of $BT(d)$ whose boundary has $3$ or fewer vertices and use this result to prove that $\tau(BT(d))= 3$ for $8\leq d\leq 10$; in these cases, equality also holds in the pathwidth upper bound. By the hereditary property of the treatment number, all larger complete binary trees have treatment number at least $3$. In contrast, we provide an explicit construction to show that $\tau(BT(7))=2$, while the pathwidth upper bound only shows $\tau(BT(7))\le 3.$ We construct an infinite family of graphs, each with a cut-vertex, whose treatment number depends on the number of components when the cut-vertex is removed. We use a combination of pathwidth and vertex cuts to prove another upper bound on the treatment number and use this to construct an infinite family of graphs whose boundary size is limited, but whose treatment number is unlimited.

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Karen L. Collins, Margaret-Ellen Messinger, Ann N. Trenk. 2026-06-06. Discrete-time treatment number of binary trees. https://arxiv.org/abs/2606.08338

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