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\begin{document}

\title{Plane Depth Sequencing}

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\author{Eric Bauerfeld}
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\begin{abstract}
\end{abstract}

\maketitle

\section{Definitions}

\begin{definition}
Let $G$ be a graph with a plane embedding, and let $C$ be the outer cycle of that embedding. The \emph{plane depth} of a vertex $v \in V(G)$ relative to the embedding and $C$ is
\[
    \mathrm{depth}(v) = \min_{u \in V(C)} d(v, u),
\]
where $d(v, u)$ denotes the graph distance between $v$ and $u$ in $G$.
\end{definition}

\begin{definition}
An edge $\{u, v\} \in E(G)$ is a \emph{level edge} if $\mathrm{depth}(u) = \mathrm{depth}(v)$.
\end{definition}

\begin{definition}
Let $G$ be a maximal planar graph with a plane embedding and outer cycle $C$. The \emph{deep embedding} of $G$ is the graph $G'$ obtained from $G$ by the following operation: for every 3-cycle $\{u, v, w\} \subseteq V(G)$ such that
\[
    \mathrm{depth}(u) = \mathrm{depth}(v) = \mathrm{depth}(w),
\]
add a new vertex $x$ to $G$ adjacent to each of $u$, $v$, and $w$.
\end{definition}

\begin{lemma}
Let $G'$ be the deep embedding of a maximal planar graph $G$. For each face of $G'$, the plane depths of its three vertices are either $d, d+1, d+1$ or $d, d, d+1$ for some $d \geq 0$.
\end{lemma}

\begin{proof}
We first establish that for any edge $\{p, q\}$ in $G$, the depths of $p$ and $q$ differ by at most $1$. Suppose for contradiction that $\mathrm{depth}(p) = d$ and $\mathrm{depth}(q) = d + n$ for some $n \geq 2$. Since $\mathrm{depth}(p) = d$, there exists a path of length $d$ from $p$ to some vertex of $C$. Prepending the edge $\{q, p\}$ gives a path of length $d + 1$ from $q$ to $C$, so $\mathrm{depth}(q) \leq d + 1 < d + n$, a contradiction. The case $\mathrm{depth}(q) = d - n$ is handled identically: there exists a path of length $d - n$ from $q$ to some vertex of $C$, and prepending the edge $\{p, q\}$ gives a path of length $d - n + 1 \leq d - 1 < d$ from $p$ to $C$, contradicting $\mathrm{depth}(p) = d$.

Since $G$ is a triangulation, every interior face of $G$ is a triangle $\{u,v,w\}$ with all three pairs adjacent. By the above, each pair of vertices in a triangle differs in depth by at most $1$, so no triangle can contain vertices of depths $d$ and $d+2$ simultaneously. The possible depth patterns for a triangle in $G$ are therefore exactly $d,d,d$, or $d,d,d+1$, or $d,d+1,d+1$.

We now consider each case under the deep embedding.

\textit{Case 1: depths $d,d,d+1$ or $d,d+1,d+1$.} These triangles are not modified by the deep embedding, so they remain as faces of $G'$ with the stated depth patterns, satisfying the lemma.

\textit{Case 2: depths $d,d,d$.} The deep embedding inserts a new vertex $x$ adjacent to $u$, $v$, and $w$, replacing the face $\{u,v,w\}$ with three new faces $\{u,v,x\}$, $\{v,w,x\}$, and $\{u,w,x\}$. It remains to determine the depth of $x$ in $G'$. Since $x$ is adjacent only to $u$, $v$, and $w$, every path in $G'$ from $x$ to $C$ must pass through one of them, so $x$ has strictly greater depth than $u$, $v$, and $w$. Each of the three new faces thus has depth pattern $d,d,d+1$, satisfying the lemma.

Since every face of $G'$ falls into one of these cases, the result follows.
\end{proof}

\end{document}

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