Add double-contraction reductio strategy note

Records the degree-5 double-contraction proof skeleton: Kempe chains as
Heawood face-chains (flip-set + same-side sign rule), the chain-transport
hypothesis L1, the inner/outer nested reductio, H'' as per-interface
contraction along the chain, and the single emptiness Claim it reduces to,
with the Errera oracle as the stress test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

papers/heawood_restrictions_on_nested_tire_graph_duals/notes/double_contraction_reductio.aux

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papers/heawood_restrictions_on_nested_tire_graph_duals/notes/double_contraction_reductio.tex

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+\documentclass[11pt]{article}
+\usepackage{amsmath,amssymb,amsthm}
+\usepackage{graphicx}
+\usepackage{geometry}
+\usepackage{booktabs}
+\geometry{margin=1in}
+
+\title{A double-contraction reductio at a degree-5 vertex\\
+via Kempe chains as Heawood face-chains}
+\author{}
+\date{}
+
+\newtheorem*{obs}{Observation}
+\newtheorem*{prop}{Proposition}
+\newtheorem*{conj}{Conjecture}
+\newtheorem*{lem}{Lemma}
+\newtheorem*{claim}{Claim}
+\newtheorem*{remk}{Remark}
+
+\begin{document}
+\maketitle
+
+\emph{Status: exploratory strategy note. Records a proof skeleton and the
+single lemma it reduces to; nothing here is proved. Companion to
+\texttt{boundary\_restriction\_structure.tex} and \texttt{paper.tex}.}
+
+\section*{Setup: the move}
+
+Let $G$ be a minimal counterexample to the Four Colour Theorem (a smallest
+triangulation with no proper $4$-vertex-colouring). By Euler $G$ has a
+vertex $v$ of degree $\le 5$; degrees $\le 4$ are classically reducible, so
+take $\deg(v)=5$. The link of $v$ is a pentagon $a,x,b,y,z$ (consecutive
+neighbours adjacent). For a non-adjacent (diagonal) pair, say $a,b$, the
+\emph{double contraction} contracts both edges $(a,v)$ and $(b,v)$,
+identifying $a=v=b$ into a single vertex. There are $5$ diagonal pairs.
+
+The double contraction is a proper minor with two fewer vertices, hence a
+smaller planar graph $G'$; by minimality $G'$ is $4$-colourable. It stays a
+simple triangulation iff each contracted edge is non-separating (its
+endpoints have exactly two common neighbours).
+
+\section*{Kempe chains as Heawood face-chains}
+
+Work in the cubic dual $G'$ (vertices $=$ faces of $G$, so the Heawood
+$\pm1$ labels sit on dual vertices). Tait-colour the edges with the three
+nonzero Klein-4 elements $\{a,b,c\}$ (edge colour $=$ colour-difference
+across the primal edge). Then:
+
+\begin{obs}[Kempe chain $=$ Tait cycle $=$ flip-set]
+A primal Kempe chain (component of two vertex-colour classes of $G$)
+corresponds to a connected component of the two-edge-colour subgraph
+$\{a,b\}$ of the dual. That subgraph is $2$-regular, so it is a disjoint
+union of cycles; each such cycle alternates $a,b,a,b,\dots$ and is therefore
+\textbf{always even}. The cycle is literally a cyclic chain of faces of $G$.
+A Kempe swap reverses the rotational order $(a,b,c)$ at exactly the vertices
+on the cycle, so the set of faces whose Heawood sign \emph{flips} under the
+swap is precisely the Kempe chain.
+\end{obs}
+
+\begin{obs}[Sign alternation $=$ same-side rule]
+Even-ness does \emph{not} force the Heawood signs to alternate along the
+cycle. At each cycle vertex the third (off-cycle, colour-$c$) edge points
+either inside or outside the region bounded by the cycle. For consecutive
+vertices $A,B$:
+\[
+  \text{same side (both in / both out)} \Rightarrow \text{signs flip;}
+  \qquad
+  \text{opposite side} \Rightarrow \text{signs repeat.}
+\]
+(Local chirality computation: the four in/out cases give
+$+,-$ / $-,+$ / $+,+$ / $-,-$.) Hence the Heawood sign sequence around a
+Tait cycle is determined, up to one global sign, by the in/out pattern of
+the third edges; it is fully alternating iff there are zero side-switches,
+i.e.\ all third edges lie on one side.
+\end{obs}
+
+\section*{The transport hypothesis (L1)}
+
+\begin{conj}[Chain transport, claimed for all triangulations]
+The double contraction preserves the relevant (crossing) Kempe/Heawood
+chain structure as it propagates up the nested tire decomposition: the two
+chains anchored at the pentagon survive, consistently, through every tire
+interface above $v$.
+\end{conj}
+
+\section*{The reductio (two nested contradictions)}
+
+We are \emph{not} claiming $G'$ is uncolourable, and we are not claiming
+intertwined Kempe chains are impossible (they occur, e.g.\ in the Errera
+graph). The reducible object is \emph{forced} intertwining.
+
+\paragraph{Inner.} Suppose intertwining is the \emph{only} way to colour
+$G'$ (every colouring is intertwined at $v$). By transport the two crossing
+chains pass through every interface above; if uncrossing is excluded at each
+interface, the tire restriction relations $R_{\mathsf K}$ collapse to their
+``crossed-only'' sub-relations. Contract each interface \textbf{along the
+chain} to obtain a strictly smaller triangulation $H''$. If the crossed-only
+sub-relations admit no compatible gluing on $H''$, then $H''$ is
+uncolourable and smaller than $G$ --- contradicting minimality.
+
+\paragraph{Outer.} Therefore $G'$ admits a non-intertwined colouring; it
+uncrosses at $v$, frees a colour for $v$, and lifts to a colouring of $G$ ---
+contradicting that $G$ is a counterexample. \hfill$\square$ (modulo the
+lemma below)
+
+\section*{What it all reduces to}
+
+\begin{claim}[the only open content]
+Under the crossed-only collapse, the surviving Heawood boundary sequences at
+a (forced-short) tire interface have empty pointwise-negation gluing on
+$H''$ --- not merely small, genuinely empty.
+\end{claim}
+
+This is where the $2^{n-2}$ constraint floor and the mod-3 side-pattern must
+do real work: the floor guarantees that \emph{smallness alone} never empties
+a relation, so the emptiness must come from the chains pinning the
+sub-relation down, not from short interface length.
+
+\section*{Open points / to pin next}
+
+\begin{itemize}
+\item \textbf{Define $H''$ precisely.} Current candidate: contract each tire
+  interface along the transported chain (the global analogue of the local
+  double contraction). Confirm this is a definite, strictly smaller
+  triangulation.
+\item \textbf{Lift is not automatic.} ``Non-intertwined'' must be defined as
+  ``admits a swap collapsing the pentagon $a,x,b,y,z$ to $\le 3$ colours,''
+  or freeing $v$ fails even after uncrossing (with $c(a)=c(b)$ the pentagon
+  can still show $4$ colours).
+\item \textbf{Errera oracle.} Run the whole construction against the Errera
+  graph (and Fritsch / Kittell). These are colourable but Kempe-intertwined;
+  the hypothesis ``every colouring intertwined'' must \emph{fail} for them,
+  so the construction must decline. \emph{Why} it declines is exactly the
+  mechanism the Claim must deny in the counterexample case.
+\item \textbf{Prove or break L1} as a statement about all triangulations.
+\end{itemize}
+
+\end{document}