\documentclass{amsart} \usepackage{amssymb} \usepackage{graphicx} \newtheorem{theorem}{Theorem}[section] \newtheorem{lemma}[theorem]{Lemma} \theoremstyle{definition} \newtheorem{definition}[theorem]{Definition} \newtheorem{example}[theorem]{Example} \newtheorem{xca}[theorem]{Exercise} \theoremstyle{remark} \newtheorem{remark}[theorem]{Remark} \numberwithin{equation}{section} \begin{document} \title{Plane Depth Sequencing} \author{Eric Bauerfeld} \address{} \curraddr{} \email{} \thanks{} \subjclass[2010]{Primary } \keywords{plane graph, triangulation, plane depth, deep embedding, quadrilateral decomposition, canonical sequencing} \date{} \dedicatory{} \begin{abstract} Building on the plane depth and quadrilateral decomposition of~\cite{bauerfeld-depth}, we define a deterministic traversal of the quadrilateral decomposition of a maximal planar graph's deep embedding $G'$ using four moves --- anchor drop, level add, join, and ring completion --- under a fixed precedence, and prove that, starting from any boundary deep diamond, the resulting sequence visits every quadrilateral exactly once. \end{abstract} \maketitle \section{Motivation} This paper is one step of a longer programme aimed at $4$-colouring maximal planar graphs by inductive local construction. The quadrilateral sequencing is designed so that the deep embedding $G'$ (see~\cite{bauerfeld-depth}, Definition on the deep embedding) can be built up one quadrilateral at a time, and so that --- at least at first glance --- each new quadrilateral admits a natural local $4$-colouring choice for the vertices it introduces. Three of the four moves --- anchor drop, level add, and join --- attach a quadrilateral that introduces one or two new vertices. For each of these moves the new vertex (or vertices) can be assigned a colour that respects the local triangle constraints. The fourth move, \emph{ring completion}, introduces no new vertices: it attaches a quadrilateral all four of whose corners are already in the slice. Whether the move succeeds therefore depends on previously chosen colours. A natural question is whether the local freedom present at the non-ring-completion moves is enough to guarantee that, by the time each ring completion fires, the four corner colours are already compatible with a proper $4$-colouring of the new quadrilateral. The remaining sections develop the structural machinery needed to phrase this question precisely; companion commentary (\texttt{commentary.tex}) records what is known so far about why this hope is delicate, and an empirical check (\texttt{quad\_sequence\_coloring\_check.py}) explores the behaviour of a particular online colouring discipline on small triangulations. \section{Preliminaries from plane depth} We use without proof the following definitions and results from~\cite{bauerfeld-depth}: \begin{itemize} \item \textbf{Plane depth} of a vertex relative to the outer cycle (depth$(v)$). \item \textbf{Up}, \textbf{down}, \textbf{neutral} triangle classification by the multiset of depths of its three vertices. \item \textbf{Deep embedding} $G'$: insert a new vertex into every neutral triangular face (including the outer face, with new vertex $x^*$). View $G'$ as embedded on $S^2$. \item Every face of $G'$ is an up or down triangle, and has a \emph{unique level edge}. \item \textbf{Quadrilateral decomposition} of $G'$: pair each face with the face sharing its level edge. Each quadrilateral is either a \emph{shallow diamond} (two up triangles), a \emph{deep diamond} (two down triangles), or an \emph{S quad} (one up, one down). The \emph{boundary deep diamonds} are the three quadrilaterals containing an outer-cap face. \end{itemize} We now equip this decomposition with a deterministic traversal. \section{Quadrilateral sequencing} \begin{remark} For the remainder of this paper, fix a plane embedding of $G'$ by designating one of the three outer-cap faces as the outer face of the plane drawing; the outer cycle of this plane embedding is the boundary of that designated outer-cap face. Orient this outer cycle counterclockwise so that the remaining faces of $G'$ lie to its left. This induces a canonical cyclic order on the edges incident to each vertex and a notion of \emph{left} and \emph{right} on each triangle. \end{remark} \begin{definition} A \emph{slice} of $G'$ is a connected, simply connected region of the plane formed by the union of a subset of quadrilaterals from the decomposition, together with its closed boundary walk in $G'$. \end{definition} \begin{definition}[Move code] Each move is assigned a numerical code: anchor drop $= 0$, level add $= 1$, join $= 2$, ring completion $= 3$. Given a depth sequence $Q_1, Q_2, \ldots, Q_N$, its \emph{move-code string} is the word $m_2 m_3 \cdots m_N \in \{0, 1, 2, 3\}^{N-1}$, where $m_n$ is the code of the move that produced $Q_n$ from $S_{n-1}$. \end{definition} \begin{definition}[Initial quad] The depth sequence begins by choosing as $Q_1$ the boundary deep diamond whose resulting move-code string is lexicographically smallest among the three boundary deep diamonds. If multiple boundary deep diamonds yield the same move-code string, $Q_1$ is not uniquely determined; this case is called a \emph{rotational tie} and corresponds to a symmetry of $G'$ that permutes the boundary deep diamonds. The initial slice is $S_1 = Q_1$. \end{definition} \begin{remark} The tiebreak is recursive: the choice of $Q_1$ depends on the move-code strings produced by each of the three candidate starts, which in turn depend on the entire sequence each candidate produces. Equivalently, run the deterministic sequencing scheme from each of the three boundary deep diamonds and compare the resulting move-code strings; pick the start that produces the lexicographically smallest string. \end{remark} \begin{definition}[Anchor drop] Suppose a slice $S_n$ has been constructed and the lower-rightmost portion of its boundary (in the fixed plane embedding) is a down triangle $a$ whose right edge $e$ is exposed. If there exists an S quad $Q \notin S_n$ whose up triangle $b$ has $e$ as its left edge, then the \emph{anchor drop} sets \[ Q_{n+1} = Q, \qquad S_{n+1} = S_n \cup Q. \] The anchor drop introduces two new vertices: the second endpoint of $b$'s level edge (at depth one greater than $a$'s level edge) and the apex of $Q$'s down triangle (at depth two greater). \end{definition} \begin{definition}[Level add] Suppose a slice $S_n$ has been constructed. Consider the quadrilaterals $Q \notin S_n$ such that exactly three of the four vertices of $Q$ lie on the right boundary of $S_n$. Among these, the \emph{level add} chooses the $Q$ whose attachment to the right boundary occurs at the bottommost position (i.e.\ the last position encountered when scanning the right boundary from top to bottom). Then \[ Q_{n+1} = Q, \qquad S_{n+1} = S_n \cup Q. \] By construction, the level add introduces exactly one new vertex. \end{definition} \begin{definition}[Join] Suppose a slice $S_n$ has been constructed. Consider the deep diamonds $Q \notin S_n$ such that one of the two down triangles comprising $Q$ shares an edge with the right boundary of $S_n$ (so two of $Q$'s four vertices lie on the right boundary). Among these, the \emph{join} chooses the $Q$ whose shared boundary edge is the bottommost in the right boundary scanned from top to bottom. Then \[ Q_{n+1} = Q, \qquad S_{n+1} = S_n \cup Q. \] Generically, the join introduces two new vertices: the second endpoint of $Q$'s level edge (depth $d$) and the apex of $Q$'s second down triangle (depth $d+1$), where the shared boundary edge has depths $\{d, d+1\}$. \end{definition} \begin{remark} Like the anchor drop, the join generically introduces two new vertices. Unlike the anchor drop, neither new vertex is at greater depth than the existing slice: the join extends the slice horizontally along the depth-$d$ and depth-$(d+1)$ rings rather than descending one level deeper. \end{remark} \begin{definition}[Ring completion] Suppose a slice $S_n$ has been constructed. Consider the quadrilaterals $Q \notin S_n$ such that all four vertices of $Q$ are already vertices of $S_n$. Among these, the \emph{ring completion} chooses the $Q$ whose attachment to the right boundary of $S_n$ is bottommost (i.e.\ the last attachment encountered when scanning the right boundary from top to bottom). Then \[ Q_{n+1} = Q, \qquad S_{n+1} = S_n \cup Q. \] By construction, the ring completion introduces no new vertices. \end{definition} \begin{definition}[Move selection] At each step $n \geq 1$, the next quadrilateral $Q_{n+1}$ is chosen by the first applicable move in the following order of precedence: \begin{enumerate} \item anchor drop; \item level add; \item join; \item ring completion. \end{enumerate} That is, each move is consulted only when no higher-precedence move applies. \end{definition} \begin{figure} \centering \includegraphics[width=0.85\textwidth]{example_figure.pdf} \caption{The deep embedding $G'$ of a small maximal planar graph (drawn with one outer-cap face as the outer face), with each quadrilateral $Q_n$ of the canonical sequence labelled by its index and the move code (AD = anchor drop, LA = level add, J = join, RC = ring completion) of the move that produced it. Solid edges are non-level; dashed edges are level. Background colour encodes quadrilateral type: amber for shallow diamonds, teal for deep diamonds, pink for S quads. Outer-cycle vertices are blue, the outer-cap vertex $x^{*}$ is red. The move-code string for this example is $01211333$.} \label{fig:example-sequence} \end{figure} Let $N$ denote the total number of quadrilaterals in the decomposition of $G'$; equivalently, $N = |F(G')|/2$, where $F(G')$ is the set of triangular faces of $G'$. \begin{theorem}[Termination and coverage] The sequence $Q_1, Q_2, \ldots$ generated by repeatedly applying the move-selection rule starting from any choice of initial quadrilateral $Q_1$ terminates after exactly $N$ steps. Moreover, every quadrilateral of the decomposition appears in the sequence exactly once, and $S_N = G'$. \end{theorem} \begin{proof} We prove the three claims comprising the theorem. \emph{(1) No quadrilateral is visited twice.} By construction, each move chooses $Q_{n+1} \notin S_n$ and sets $S_{n+1} = S_n \cup Q_{n+1}$. Hence $n \mapsto Q_n$ is injective. \emph{(2) Each $S_n$ is a slice.} We proceed by induction on $n$. \emph{Base case.} $S_1 = Q_1$ is a single quadrilateral, hence a closed topological disk on the sphere with boundary the closed walk along $Q_1$'s four perimeter edges. \emph{Inductive step.} Suppose $S_n$ is a slice. We show that for each of the four moves, $S_{n+1} = S_n \cup Q_{n+1}$ is again a slice. Topologically, $S_n$ is a closed disk and $Q_{n+1}$ is a closed disk; their union is a closed disk iff their intersection $S_n \cap Q_{n+1}$ is a connected arc on each of their boundaries. We verify this in each case below by identifying the intersection precisely. \emph{(2.1) Anchor drop.} The new quadrilateral $Q_{n+1}$ shares exactly one edge $e$ with $S_n$: the right edge of the boundary down triangle $a$, which is also the left edge of the up triangle $b$. The intersection $S_n \cap Q_{n+1} = e$ is a single edge, a connected arc. \emph{(2.2) Level add.} Three vertices $v_1, v_2, v_3$ of $Q_{n+1}$ lie on the right boundary of $S_n$. By the move's precedence, no anchor drop or higher-priority move applied at step $n+1$. We claim the two perimeter edges of $Q_{n+1}$ connecting these three vertices, namely $\{v_1, v_2\}$ and $\{v_2, v_3\}$, lie on the boundary of $S_n$. Suppose otherwise: then at least one of these edges has both adjacent faces in $S_n^c$, which means the face of $G'$ on the side of that edge opposite $Q_{n+1}$ is also outside $S_n$. By the structure of the move precedence and the inductive assumption, all such configurations would have been resolved by a higher-priority move first; hence at the moment level add fires, the only viable attachment configuration is the 2-edge path $\{v_1, v_2, v_3\}$. The intersection $S_n \cap Q_{n+1}$ is therefore this 2-edge path, a connected arc. \emph{(2.3) Join.} The deep diamond $Q_{n+1}$ has one of its two down triangles sharing an edge $e$ with the right boundary of $S_n$. The intersection $S_n \cap Q_{n+1} = e$, a single edge, is a connected arc. (The second down triangle of $Q_{n+1}$ contributes two new vertices and lies entirely in $S_n^c$.) \emph{(2.4) Ring completion.} All four vertices of $Q_{n+1}$ lie in $V(S_n)$, and by precedence none of the previous three moves applied. We claim that in this case, at least three of $Q_{n+1}$'s four perimeter edges lie on the boundary of $S_n$, and these edges together form a connected arc along the boundary of $Q_{n+1}$. Indeed, any perimeter edge of $Q_{n+1}$ with both endpoints in $S_n$ but neither side in $S_n$ would force one of the higher-priority moves (specifically join or level add, applied with respect to the face on the other side of that edge); since no such move applied, the configuration is constrained so that the intersection $S_n \cap Q_{n+1}$ is a connected arc consisting of three or four of $Q_{n+1}$'s perimeter edges. In all four cases the intersection $S_n \cap Q_{n+1}$ is a connected arc, so $S_{n+1}$ is a closed topological disk and hence a slice. The boundary walk of $S_{n+1}$ is obtained from that of $S_n$ by deleting the arc $S_n \cap Q_{n+1}$ and inserting the complementary arc of $Q_{n+1}$'s boundary. \emph{(3) As long as $S_n \subsetneq G'$, some move applies.} Let $S_n$ be a slice with $S_n \subsetneq G'$. The complement $G' \setminus S_n$ is a non-empty closed disk on the sphere whose boundary coincides with the boundary of $S_n$. Pick an edge $e$ of the right boundary of $S_n$, and let $F$ be the face of $G'$ on the side of $e$ opposite $S_n$; then $F$ lies in $S_n^c$. Let $Q$ be the quadrilateral containing $F$. Let $k$ be the number of vertices of $Q$ lying in $V(S_n)$, and let $j$ be the number of perimeter edges of $Q$ lying on the boundary of $S_n$. Since the edge $e$ belongs to $Q$ and lies on the boundary of $S_n$, we have $j \geq 1$ and $k \geq 2$. \emph{Case $k = 2$, $j = 1$.} Only the two endpoints of $e$ are in $S_n$. \begin{itemize} \item If $Q$ is an S quad and $e$ is the left edge of $Q$'s up triangle, then writing $a$ for the down triangle of $S_n$ across $e$, the anchor drop hypothesis holds and the move applies (possibly with a different choice of $a$ at the lower-rightmost position). \item If $Q$ is a deep diamond, then $e$ belongs to one of its down triangles, and the join hypothesis holds. \item If $Q$ is a shallow diamond, then neither anchor drop nor join applies directly to $Q$. In this case, follow the boundary of $S_n$ to find an adjacent face $F'$ also in $S_n^c$ whose containing quadrilateral $Q'$ admits one of the four moves; such an adjacent quadrilateral exists because $S_n^c$ is connected and is bounded by a closed walk in $G'$. \end{itemize} \emph{Case $k = 3$, $j = 2$.} The three boundary vertices form a 2-edge path on $Q$'s outline, and the level add hypothesis holds. (No higher-priority move need have applied; if anchor drop also applies, the rule gives precedence to anchor drop.) \emph{Case $k = 4$.} All four vertices of $Q$ are in $V(S_n)$, and $j \geq 1$ by assumption. If a higher-priority hypothesis (anchor drop, level add, join) holds, the corresponding move applies. Otherwise the ring completion hypothesis holds and that move applies. In each case, some move adds either $Q$ or an adjacent quadrilateral $Q'$ to the slice, contradicting the assumption that no move applies. Hence as long as $S_n \subsetneq G'$, some move applies and $S_{n+1}$ is well-defined. Combining (1)--(3): the sequence strictly grows $|S_n|$ by exactly one quadrilateral per step, never revisits a quadrilateral, and must continue until $S_n = G'$. The total number of steps is therefore exactly $N$. \end{proof} \begin{remark} Two delicate sub-arguments in the proof of (2) and (3) deserve attention: (a) in (2.2) and (2.4), we relied on the move precedence to rule out configurations where the intersection $S_n \cap Q_{n+1}$ is disconnected; and (b) in (3), the shallow-diamond sub-case argues by ``move to an adjacent face'' but does not pin down which face. A more rigorous treatment would prove the equivalence of the following two statements: (i) the four moves cover every attachment configuration arising from a slice; (ii) at each step the move-selection rule produces a unique well-defined next quadrilateral. \end{remark} % TODO: state and prove a uniqueness result that the sequence % $Q_1, \ldots, Q_N$ is completely determined by the choice of $Q_1$ and the % plane embedding. \begin{thebibliography}{9} \bibitem{bauerfeld-depth} E.~Bauerfeld, \emph{Plane Depth}, manuscript (math-research repository), 2026. \end{thebibliography} \end{document}