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LemmaStatement: AI-adaptedProof: AI-generatedPipeline-generatedprecheck passjudge pass (gpt-6-sol)audited 2026-09-30
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A curvilinear triangulation gives a finite regular CW complex

Statement

Let T=(V,E,F,ϕ) be a finite face-to-face curvilinear triangulation of a compact surface M, possibly disconnected, nonorientable, or with boundary. The vertices, relative interiors of edges, and interiors of faces form a finite regular CW complex on M: each cell characteristic map is a homeomorphism from a closed ball onto its closed cell. Its 0-, 1-, and 2-cells are indexed respectively by V, E, and F.

Facts & Assumptions

Given: The compact surface M and finite face, edge, vertex, map, and incidence data of a curvilinear triangulation.

[F1]

Each edge is the image of a regular C2 embedding of [0,1]; its endpoints are two distinct vertices, its relative interior contains no vertex, and distinct edges meet only at common endpoints (Curvilinear face-to-face triangulation).

[F2]

Each face map is a homeomorphism from the closed standard triangle onto a closed face image; its open face is open in Int⁡M, its image is the closure of that open face, and each full side maps homeomorphically onto an edge (Curvilinear face-to-face triangulation).

[F3]

Face images cover M and meet pairwise only in a common full edge, a common vertex, or not at all; every edge is incident to its stated one or two faces, the boundary edges and vertices form ∂M, and each vertex link is a circle or a closed interval as specified in the triangulation definition (Curvilinear face-to-face triangulation).

[F4]

Attaching an n-cell means forming the pushout of a closed n-disk along its boundary map; the quotient map restricted to the disk is its characteristic map (Cell attachment by a characteristic map).

[F5]

A CW complex is Hausdorff, is built by disk attachments, and satisfies closure finiteness and the weak-topology condition on closed cells (CW complex with closure finiteness and weak topology).

[F6]

A topological manifold with boundary is Hausdorff and second countable (Topological manifolds with boundary).

Proof

technique · Build the skeleta from the supplied maps, then verify the finite closed-cell topology and regularity
1.1F1F2F3F6given

For each v∈V take the singleton 0-cell {v}; for each e∈E take its relative interior as a 1-cell; and for each f∈F take ϕf((Δ2)∘) as a 2-cell. The compact edge interval has closed image in the Hausdorff surface by [F1] and [F6], and its relative interior is dense in that image, so its cell closure is the interval with its two endpoint vertices. By [F2], each face image is the closure of its open face, and its boundary is the three full edge cells and their vertices. The face-edge incidences and vertex-link clauses in [F3], together with the face intersection and edge intersection clauses, ensure that these open cells are pairwise disjoint and cover M and that every cell boundary is a union of lower cells.

2.1F1F2F3F4F5F6step 1.1

Let X0=V and X1=V∪⋃e∈Ee. The finite subspace V is discrete: since M is Hausdorff, each singleton is closed, and for each v∈V the intersection of the finitely many open sets M∖{w}, w∈V∖{v}, isolates v in V. Attach one closed interval for each edge to its two endpoint vertices using its supplied embedding from [F1]. The resulting finite attachment quotient is compact and maps continuously and bijectively to X1: injectivity follows because distinct edge images meet only at common endpoints. Since X1 is a subspace of the Hausdorff space M by [F6], the map is a homeomorphism; a continuous bijection from a compact space to a Hausdorff space is closed because the image of every closed subset is compact and hence closed. Now attach one triangular disk for each face along its boundary map from [F2]. For completeness, let c be the triangle's centroid; each ray from c meets the polygonal boundary once, at a positive continuous radial distance R(u) for unit direction u, so the map c+rR(u)u↦ru for 0<r≤1, extended by c↦0, is a homeomorphism from the closed triangle to the closed unit disk (continuity at c follows as the image norm is r). Thus the supplied face map gives a disk attachment as in [F4]. The face quotient is compact and maps continuously and bijectively onto M: [F2] gives injectivity on each face, while [F3] and step 1.1 show distinct face interiors are disjoint from each other and from X1, with all remaining identifications exactly along their common full edges or vertices already in X1. The compact-to-Hausdorff argument using [F6] makes this map a homeomorphism. The open interval and face interiors map homeomorphically onto the cells of step 1.1; taking Xn=M for n≥2 completes the filtration required in [F5].

3.1F1F2F3F5F6step 1.1step 2.1

There are finitely many cells, so every closed cell meets only finitely many cells. Each face image is closed by [F2]; each edge image is compact by [F1] and hence closed in the Hausdorff space M by [F6]; each vertex is closed as well. These finitely many closed cells cover M. If A⊆M has closed intersection with every closed cell, each such intersection is closed in M because the closed cell is closed in M, and their finite union is A; hence A is closed. The converse follows by restriction to each closed cell. This is the weak topology required in [F5].

4.1F1F2F4F5step 1.1step 2.1step 3.1given∎

The vertex characteristic map is the point inclusion, each edge characteristic map is its supplied interval embedding, and each face characteristic map is the composition of the fixed disk-to-triangle homeomorphism in step 2.1 with its supplied face homeomorphism. Thus every characteristic map is a homeomorphism onto its closed cell, which is the regularity asserted in the statement. The open cells are indexed once each by V, E, and F, giving the stated counts. If M is empty, the covering clause forces all three finite index sets to be empty and the empty CW complex satisfies the same clauses. The supplied finite data require no choice principle.

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