Abstract 1 Introduction 2 Preliminaries 3 Whitney’s method 4 The proof of Theorem 1 5 Triangulations with dual graph of arbitrary large twin-width References

On the Twin-Width of Smooth Manifolds

Édouard Bonnet ORCID Univ Lyon, CNRS, ENS de Lyon, Université Claude Bernard Lyon 1, LIP UMR5668, France Kristóf Huszár ORCID Institute of Geometry, Graz University of Technology, Austria
Abstract

Building on Whitney’s classical method of triangulating smooth manifolds, we show that every compact d-dimensional smooth manifold admits a triangulation with dual graph of twin-width at most dO(d). In particular, it follows that every compact 3-manifold has a triangulation with dual graph of bounded twin-width. This is in sharp contrast to the case of treewidth, where for any natural number n there exists a closed 3-manifold such that every triangulation thereof has dual graph with treewidth at least n. To establish this result, we bound the twin-width of the dual graph of the d-skeleton of the second barycentric subdivision of the 2d-dimensional hypercubic honeycomb. We also show that every compact, piecewise-linear (hence smooth) d-dimensional manifold has triangulations where the dual graph has an arbitrarily large twin-width.

Keywords and phrases:
Smooth manifolds, triangulations, twin-width, Whitney embedding theorem, structural graph parameters, computational topology
Funding:
Kristóf Huszár: Partially supported by the Austrian Science Fund (FWF) grant P 33765-N.
Copyright and License:
[Uncaptioned image] © Édouard Bonnet and Kristóf Huszár; licensed under Creative Commons License CC-BY 4.0
2012 ACM Subject Classification:
Mathematics of computing Graph theory
; Mathematics of computing Geometric topology
Related Version:
Full Version: https://arxiv.org/abs/2407.10174 [10]
Acknowledgements:
We thank the anonymous reviewers for their helpful comments and for raising interesting questions for future research.
Funding:
The authors have been supported by the French National Research Agency through the project TWIN-WIDTH with reference number ANR-21-CE48-0014.
Editors:
Oswin Aichholzer and Haitao Wang

1 Introduction

Structural graph parameters have become increasingly important in computational topology in the past two decades. This is mainly due to the emergence of fixed-parameter tractable (FPT) algorithms for problems on knots, links [13, 36, 37, 39] and 3-manifolds [15, 17, 18, 19, 20],111Also see [2] for an FPT algorithm checking tightness of (weak) pseudomanifolds in arbitrary dimensions. most of which are known to be NP-hard in general. Although these FPT algorithms may have exponential worst-case running time, on inputs with bounded treewidth they are guaranteed to terminate in polynomial (or even in linear) time.222Here the term input refers either to a link diagram 𝒟, or to a 3-manifold triangulation 𝒯. In the first case the treewidth means the treewidth of 𝒟 considered as a 4-regular graph, in the second case it means the treewidth of the dual graph Γ(𝒯) of 𝒯. The running times are measured in terms of the size of the input. The size of a link diagram is defined as the number of its crossings, and the size of a 3-manifold triangulation is the number of its tetrahedra. More definitions are given in Section 2. In addition, some of these algorithms have been implemented in software packages such as Regina, providing practical tools for researchers in topology [12, 14].333See [3] for an implemented algorithm to effectively compute certain Khovanov homology groups of knots. This algorithm is conjectured to be FPT in the cutwidth of the input knot diagram, cf. [3, Section 6].

The success of the above algorithms naturally leads to the following question. Given a 3-manifold (resp. knot 𝒦), what is the smallest treewidth that the dual graph of a triangulation of (resp. a diagram of 𝒦) may have?444For links and knots, this question was respectively asked in [37, Section 4] and [16, p. 2694]. Motivated by this challenge, in recent years several results have been obtained that reveal quantitative connections between topological invariants of knots and 3-manifolds, and width parameters associated with their diagrams [22, 35] and triangulations [26, 27, 28, 29, 30, 40], respectively. It turns out that topological properties of 3-manifolds may prohibit the existence of “thin” triangulations.555For non-Haken 3-manifolds of large Heegaard genus [30] or Haken 3-manifolds with a “complicated” JSJ decomposition [29], the dual graph of any triangulation must also have large treewidth.,666For results where the treewidth of a knot diagram is bounded below by topological properties of the underlying knot, see [22, 35]. At the same time, geometric or topological descriptions of 3-manifolds can also give strong hints on how to triangulate them so that their dual graphs have constant pathwidth or treewidth, or at least bounded in terms of a topological invariant of these 3-manifolds.777This is case with Seifert fibered spaces [28] or hyperbolic 3-manifolds [27, 40].

In this work we establish similar results for another graph parameter called twin-width. Introduced in [11], this notion has been subject of growing interest and found many algorithmic applications in recent years.888For an introduction to twin-width and an overview of its applications, see [6] and the references therein. Namely, on classes of effectively999A class has effectively bounded twin-width if it has bounded twin-width, and contraction sequences of width O(1) (objects witnessing the twin-width upper bound) can be found in polynomial time; see Section 2.1 for the definitions of contraction sequences and twin-width. bounded twin-width, first-order properties can be decided efficiently101010More precisely, there is a fixed-parameter tractable algorithm that, given a first-order sentence φ and an n-vertex graph G with a contraction sequence of width d, decides if G satisfies φ in time f(φ,d)n, for some computable function f. [11] (also see [8] for improved running times on specific problems definable in first-order logic), first-order queries can be enumerated fast [24], and enhanced approximation algorithms can be designed for several graph optimization problems [4]. Besides, classes of bounded twin-width are fairly general and broad. They for instance include classes of bounded tree-width, and even its dense analogue, clique-width, classes excluding a minor, proper permutation classes, d-dimensional grid graphs [11], some classes of cubic expanders [9], segment intersection graphs without biclique subgraphs of a fixed size [7], and modifications definable in first-order logic (called first-order transductions) of all these classes [11]. We observe that the definition of twin-width can readily be lifted to binary structures (i.e., edge-colored multigraphs).

Our first result shows that a compact d-dimensional smooth manifold always has a triangulation with dual graph of twin-width bounded in terms of d.

Theorem 1.

Any compact d-dimensional smooth manifold admits a triangulation (more specifically, a Whitney triangulation111111We call a triangulation 𝒯 of a compact smooth manifold a Whitney triangulation of , if 𝒯 is obtained via Whitney’s method discussed in Section 3.) with dual graph of twin-width at most dO(d).

Let us emphasize that, in an appropriate computational model, Whitney triangulations of smooth manifolds can be constructed algorithmically [5]. As every 3-manifold is smooth [41] (see also [38]), the next corollary follows immediately from Theorem 1. We recall that tww(G) denotes the twin-width of a graph G, and Γ(𝒯) denotes the dual graph of a triangulation 𝒯.

Corollary 2.

There exists a universal constant C>0 such that every compact 3-dimensional manifold admits a triangulation 𝒯 with tww(Γ(𝒯))C.

This is in sharp contrast to the case of treewidth, for which it is known that for every n there are infinitely many 3-manifolds where the smallest treewidth of the dual graph of every triangulation is at least n [29, 30]. Complementing Theorem 1, we also show that for any fixed d3, the d-dimensional triangulations of large twin-width are abundant (Theorem 19). Moreover we show that any piecewise-linear (hence smooth) manifold of dimension at least three admits triangulations with dual graph of arbitrarily large twin-width.

Theorem 3.

Let d3 be an integer. For every compact d-dimensional piecewise-linear manifold and natural number n, there is a triangulation 𝒯 of with tww(Γ(𝒯))n.

 Remark 4.

The assumption of d3 in Theorem 3 is essential. Indeed, the dual graph of any triangulation of the genus-g surface Sg is, in particular, a graph that embeds into Sg and such graphs are known to have twin-width bounded above by c(g+1) for some universal constant c>0 [32].121212This bound is sharp up to a constant multiplicative factor [32]. For g=0, we know that planar graphs have twin-width at most eight [25], and there are planar graphs with twin-width equal to seven [31].

Outline of the paper.

In Section 2 we review the relevant notions from graph theory and topology. In Section 3 we recall Whitney’s seminal work on triangulating smooth manifolds. This is followed by a detailed proof of Theorem 1 in Section 4. Finally, in Section 5 we prove the complementary results about triangulations with dual graph of large twin-width.

2 Preliminaries

Basic notation.

For a finite set S we let |S| denote its cardinality, while for a real number x we let |x| denote its absolute value. For a positive integer k|S|, we let (Sk) denote the set of k-element subsets of S. For a positive integer n, we let [n] denote the set of all positive integers up to n, and for two real numbers ab, we use [a,b] to denote the closed interval {x:axb}. For a vector y=(y1,,yd)d we use y to denote its Euclidean norm, i.e, y2=i=1dyi2. However, if 𝒳 is a (cubical or simplicial) complex, then 𝒳 refers to its geometric realization.

2.1 Trigraphs, contraction sequences and twin-width

Following [11, Sections 3 and 4], in this section we review the graph-theoretic notions central to our work and collect some basic, yet important facts about twin-width.

Trigraphs.

A trigraph G is a triple G=(V,E,R), where V is a finite set of vertices, and E,R(V2) are two disjoint subsets of pairs of vertices called black edges and red edges, respectively. We also refer to the sets of vertices, black edges and red edges of a given trigraph G as V(G), E(G) and R(G), respectively. Any simple graph G=(V,E) may be regarded as a trigraph (V,E,R) with R=. For a vertex vV(G) the degree deg(v) of v is the number of edges incident to it, i.e., deg(v)=|{eER:ve}|. Additionally, the red degree degR(v) of v is the number of red edges incident to it, i.e., degR(v)=|{eR:ve}|. A trigraph G for which degR(G)=maxvV(G)degR(v)b is called a b-trigraph. Given two trigraphs G=(V,E,R) and G=(V,E,R), we say that G is a subtrigraph of G, if VV, EE(V2) and RR(V2).131313As usual, subtrigraphs of graphs (those without any red edges) will also be called subgraphs. In addition, if E=E(V2) and R=R(V2), then we say that G is an induced subtrigraph of G. For a trigraph G and a subset SV(G) of its vertices, GS denotes the induced subtrigraph of G with vertex set V(G)S.

Contraction sequences and twin-width.

Let G=(V,E,R) be a trigraph and u,vV be two arbitrary distinct vertices of G. We say that the trigraph G/u,v=(V,E,R) is obtained from G by contracting u and v into a new vertex w if 1. V=(V{u,v}){w}, 2. G{u,v}=(G/u,v){w} and 3. for any xV{w}=V{u,v} we have

  • {w,x}E if and only if {u,x}E and {v,x}E,

  • {w,x}ER if and only if {u,x}ER and {v,x}ER, and

  • {w,x}R otherwise.

We call the trigraph G/u,v a contraction of G. A sequence 𝔖=(G1,,Gm) of trigraphs is a contraction sequence if Gi+1 is a contraction of Gi for every 1im1. Note that |V(Gi+1)|=|V(Gi)|1. We use the notation “𝔖:G1Gm” to indicate that the trigraphs G1 and Gm are initial and terminal entries of the contraction sequence 𝔖. The width 0pt(𝔖) of a contraction sequence 𝔖=(G1,,Gm) is defined as 0pt(𝔖)=max1imdegR(Gi), i.e., the largest red degree of any vertex of any trigraph in 𝔖. Now, the twin-width tww(G) of a trigraph G is defined as the smallest width of any contraction sequence (G1,,G|V(G)|) with G1=G and G|V(G)|=, where denotes the trigraph consisting of a single vertex.

Some properties of twin-width; grid graphs

We conclude this section by collecting some properties of twin-width that we will rely on later. The first one states that twin-width is monotonic under taking induced subtrigraphs and is a simple consequence of the definitions (cf. [11, Section 4.1]).

Proposition 5.

If H is an induced subtrigraph of a trigraph G, then tww(H)tww(G).

Smallness.

An infinite class 𝒢 of graphs is small if there exists a constant c>1 such that for every n the class 𝒢 contains at most n!cn labeled graphs on n vertices. The next theorem says that every graph class of bounded twin-width – i.e., for which there exists a constant C>0, such that tww(G)C for every graph G in the class – is small.

Theorem 6 ([9, Theorem 2.5]).

Every graph class with bounded twin-width is small.

Now let s be a non-negative integer. The s-subdivision of G is the graph subds(G) obtained from G by subdividing each edge in E(G) exactly s times. A simple counting argument together with Theorem 6 yields the following:

Proposition 7.

For any fixed integers k4 and s0, the class subds(𝒢k) of s-subdivisions of k-regular141414A simple graph G=(V,E) is k-regular if every vertex vV has degree deg(v)=k. simple graphs is not small, hence has unbounded twin-width.

This proposition follows from the adaptation of an argument given in the first paragraph of [23, Section 3]. For completeness, we spell out this proof below.

Proof of Proposition 7.

Let Nk(m) be the number of labeled k-regular simple graphs on m vertices. Note that if Nk(m)>0, then km is even; which we now assume. It is known (cf. [42, Section 6.4.1]) that, asymptotically

Nk(m)exp(1k24)(km)!(km/2)!2km/2(k!)m=Ω((km/2)!(k!)m), (1)

which readily implies that k-regular graphs (for k3) do not form a small class.

Now, let Nk(s)(n) be the number of n-vertex graphs in the class subds(𝒢k) of s-subdivisions of k-regular simple graphs. If a graph Gsubds(𝒢k) has n vertices, then n=m+km2s, where m is the number of vertices of G of degree k. Note that such a graph G can be obtained by first choosing a k-regular labeled graph H on m vertices, then ordering the remaining nm=kms/2 vertices arbitrarily and evenly distributing them to the edges of H according to some fixed ordering of E(H). It follows that

Nk(s)(n)(nm)Nk(m)(nm)!=n!Nk(m)m!=(1)n!Ω((km/2)!m!(k!)m). (2)

Since k4, we have km/22m. This, together with (2m)!2m(m!)2 and (2) implies

Nk(s)(n)n!Ω(m!(2k!)m). (3)

Recall that m=2n/(2+ks). In particular, m is proportional to n. Hence m!/(2k!)m grows faster than cn for any fixed constant c>1. Thus the graph class subds(𝒢k) is not small.

Grid graphs.

The d-dimensional n-grid Pnd is the graph with vertex set V(Pnd)=[n]d, and {u,v}E(Pnd) for two vertices u=(u1,,ud) and v=(v1,,vd) if and only if i=1d|uivi|=1. The next result states that tww(Pnd)3d irrespective of the value of n.

Theorem 8 (Theorem 4 in [11]).

For every positive d and n, the d-dimensional n-grid Pnd has twin-width at most 3d.

The d-dimensional n-grid with diagonals Dn,d is the graph with V(Dn,d)=[n]d, and {u,v}E(Dn,d) for two vertices u=(u1,,ud) and v=(v1,,vd) if and only if maxi=1d|uivi|1. Now, for a given trigraph G=(V,E,R) we set red(G)=(V,,ER). In words, red(G) is the trigraph obtained from G by replacing every black edge of G by a red edge between the same vertices. With this notation red(Dn,d) is the d-dimensional red n-grid with diagonals, i.e., red(Dn,d)=([n]d,,E(Dn,d)). Clearly, tww(Dn,d)tww(red(Dn,d)).

Theorem 9 (Lemma 4.4 in [11]).

For every positive d and n, every subtrigraph of the d-dimensional red n-grid with diagonals red(Dn,d) has twin-width at most 2(3d1).

2.2 Background in topology

For general background in (combinatorial and differential) topology we refer to [43].

2.2.1 Simplicial and cubical complexes

Abstract simplicial complexes.

Given a finite ground set 𝒮, an abstract simplicial complex (or simplicial complex, for short) 𝒳 over 𝒮 is a downward closed subset of the power set 2𝒮, i.e., 𝒳 and imply 𝒳. Any element of 𝒳 is called a face or simplex of 𝒳, and for σ𝒳 the dimension of σ is defined as dimσ=|σ|1. The dimension of 𝒳, denoted by dim𝒳, is then the maximum dimension of a face of 𝒳. If dim𝒳=d, we also say that 𝒳 is a simplicial d-complex. For 0idim𝒳, we let 𝒳(i)={σ𝒳:dimσ=i} denote the set of i-dimensional faces (or i-faces, or i-simplices) of 𝒳. The i-skeleton 𝒳i=j=0i𝒳(j) of 𝒳 is the union of all faces of 𝒳 up to dimension i. Note that any simple graph G=(V,E) can be regarded as a 1-dimensional simplicial complex 𝒳G with 𝒳G(0)={{v}:vV} and 𝒳G(1)=E. For 0i3 the i-simplices of a simplicial complex 𝒳 are respectively called the vertices, edges, triangles, and tetrahedra of 𝒳.

Geometric realization of simplicial complexes.

Every abstract simplicial complex 𝒳 may be realized geometrically as follows. To each abstract i-simplex σ={v0,v1,,vi}𝒳 we associate a geometric i-simplex σ=[v0,v1,,vi]i defined as the convex hull of i+1 affinely independent points in i. We equip σ with the subspace topology inherited from i. Next, we consider the disjoint union σ𝒳σ of these geometric simplices, and perform identifications along their faces that reflect their relationship in 𝒳. The resulting space 𝒳, equipped with the quotient topology, is called the geometric realization of 𝒳, see Figure 1. The geometric realization of a simplicial complex is unique up to homeomorphism.

Cubical complexes.

Analogous to simplicial complexes, a cubical complex 𝒳 over a ground set 𝒮 is a set system 𝒳2𝒮 that consists of “cubes” instead of simplices. The terminology is the same as in the simplicial case. The only difference is that a geometric i-cube is a topological space homeomorphic to [0,1]i, where [0,1] denotes the closed unit interval.

Cubical or simplicial complexes in this paper will typically be defined geometrically, and as such they will naturally come with a geometric realization.


(a) Geometric i-simplices (i=0,1,2,3).
(b) Geometric realization of a simplicial 3-complex.

(c) Geometric i-cubes (i=0,1,2,3).
(d) Geometric realization of a cubical 3-complex.
Figure 1: The geometric perspective on simplicial and cubical complexes.
The hypercubic honeycomb.

Let n and d be positive integers and consider the d-dimensional cube [1,n]dd. The d-dimensional hypercubic honeycomb 𝐇d,n is a cubical d-complex that decomposes [1,n]d into (n1)d geometric cubes. The properties of this familiar object play an important role in this work, so we describe it for completeness. We define 𝐇d,n geometrically, in a bottom-up fashion. First, the vertex set 𝐇d,n(0) consists of precisely those points in [1,n]d, which have only integral coordinates. Next, a 1-cube (i.e., an edge) is attached along its endpoints to vertices u=(u1,,ud) and v=(v1,,vd) in 𝐇d,n(0) if and only if i=1d|uivi|=1. Thus the 1-skeleton 𝐇1d,n is just the d-dimensional grid graph Pnd encountered in the end of Section 2.1. Finally, the higher dimensional skeleta of 𝐇d,n are induced by its 1-skeleton: for each subcomplex 𝒴𝐇id,n isomorphic to the boundary of a (i+1)-cube, we attach an (i+1)-cube to 𝐇id,n along 𝒴. In words, starting from the 1-skeleton 𝐇1d,n, whenever we have the possibility to attach a cube of dimension at least two (because its boundary is already present), we attach it. See, e.g., Figure 2.

Figure 2: Constructing the complex 𝐇2,5. Its 1-skeleton 𝐇12,5 is isomorphic to the grid P52.
Pure complexes and their dual graphs.

A (cubical or simplicial) complex 𝒳 is pure if every face of 𝒳 is contained in a face of dimension dim𝒳. It follows that, for every i with 0idim𝒳, the i-skeleton 𝒳i of a pure complex 𝒳 is also pure. Examples of pure complexes include nonempty graphs without isolated vertices, or triangulations of manifolds (see Section 2.2.3). Given a pure complex 𝒳, the dual graph Γ(𝒳i)=(V,E) of its i-skeleton is defined as the graph, where the vertex set V corresponds to the set 𝒳(i) of i-faces, and {σ,τ}E if and only if σ and τ share an (i1)-dimensional face in 𝒳, see Figure 3.

(a) A pure simplicial 2-complex 𝒳 formed by six triangles τ1, τ2, τ3, τ4, τ5, and τ6, four of which meet along a single edge.
(b) The dual graph Γ(𝒳2) of 𝒳2. As τ6 shares no edge with any other triangle in 𝒳, its corresponding vertex is isolated.
Figure 3: Example of a pure simplicial 2-complex and its dual graph.

2.2.2 Barycentric subdivisions

Given a (cubical or simplicial) complex 𝒳, its barycentric subdivision is a simplicial complex 𝒳 defined abstractly as follows. For the ground set 𝒮 of 𝒳 we have 𝒮=𝒳. A (k+1)-tuple {σ0,,σk}𝒮 forms a k-simplex of 𝒳 if and only if σiσj for every 0i<jk. We denote the 2nd (resp. th) iterated barycentric subdivision of a complex 𝒳 by 𝒳′′ (resp. 𝒳()). See [10, Appendix A] for examples. The following are simple consequences of the definitions.

Observation 10.

For any 0lk, a k-simplex has (kl) l-faces.

Observation 11.

The barycentric subdivision of a k-simplex contains (k+1)! k-simplices.

Observation 12.

The barycentric subdivision of a k-cube contains 2kk! k-simplices.

See [10, Appendix A] for a proof of Observation 12.

2.2.3 Manifolds and their triangulations

Manifolds can be regarded as higher dimensional analogs of surfaces. A d-dimensional topological manifold with boundary (or d-manifold, for short) is a topological space151515As a topological space a manifold is required to be second countable [43, p. 2] and Hausdorff [43, p. 87]. , where every point has an open neighborhood homeomorphic to d, or to the closed upper half-space {(x1,,xd)d:x10}. The points of that do not have a neighborhood homeomorphic to d constitute the boundary of . If a manifold satisfies =, then is called a closed manifold. In this paper always denotes a compact manifold.

Smooth manifolds.

The main result of this paper (Theorem 1) applies for manifolds that have an additional property, namely smoothness. As we will not need to work with the actual definition of smoothness, but merely rely on it, we only give a brief definition here. For more background on smooth manifolds, we refer to [43, Chapter 5] and [33].

Given a connected and open subset U and a homeomorphism φ:Uφ(U) onto an open subset of d, the pair (U,φ) is called a chart. Given two charts (Uα,φα) and (Uβ,φβ) with UαUβ, the map τα,β:φα(UαUβ)φβ(UαUβ) defined via τα,β=φβφα1 is called a transition map. A smooth structure on a topological manifold with = is a collection 𝒜={(Uα,φα):αA} of charts that satisfies the following three properties.

  1. 1.

    The sets Uα cover , that is αAUα=.

  2. 2.

    For any α,βA with UαUβ, the transition map τα,β is smooth.161616See [43, p. 185] for a discussion of (smooth) maps of manifolds.

  3. 3.

    The collection 𝒜 is maximal in the sense that if (U,φ) is a chart and for every αA with UUα the transition maps φφα1 and φαφ1 are smooth, then (U,φ)𝒜.

A topological manifold together with a smooth structure is called a smooth manifold. By an appropriate modification of property 2 above, the definition extends to manifolds with non-empty boundary as well. We refer to [43, Section 5.1.1] for details.

Triangulations.

Let be a compact topological d-manifold. A simplicial complex 𝒯 whose geometric realization 𝒯 is homeomorphic to is called a triangulation of . It follows that 𝒯 is a pure simplicial complex of dimension d (see Section 2.2.1). For d3, every topological d-manifold admits a triangulation [41, 44], however, for d>3 this is generally not true (see [38] for an overview). Smooth manifolds can nevertheless always be triangulated, irrespective of their dimension, e.g., by work of Whitney [48, Chapter IV.B] (cf. Section 3).

Given a d-dimensional triangulation 𝒯, recall that its dual graph Γ(𝒯) is the graph with vertices corresponding to the d-simplices of 𝒯, and edges to the face gluings, i.e, those (d1)-simplices of 𝒯 that are contained in precisely two d-simplices. Note that deg(v)d+1 for any vertex v of Γ(𝒯). The following proposition is a consequence of the definitions.

Proposition 13.

Let 𝒯 be a triangulation of a d-manifold and 𝒰 be a collection of d-simplices of 𝒯 that define a submanifold of . Then Γ(𝒰) is an induced subgraph of Γ(𝒯).

3 Whitney’s method

A seminal result of Whitney states that a smooth d-dimensional manifold always admits a smooth embedding into a 2d-dimensional Euclidean space.

Theorem 14 (strong Whitney embedding theorem [47, Theorem 5], cf. [33, Theorem 6.19]).

For d>0, every smooth d-manifold admits a smooth embedding into 2d.

An important consequence of Theorem 14 is that smooth manifolds can be triangulated.

Theorem 15 (triangulation theorem [48, Chapter IV.B], cf. [5, Theorem 1.1]).

Every compact, smooth d-manifold embedded in some Euclidean space m admits a triangulation.

Next, we give a very brief and high-level overview of Whitney’s method of triangulating smooth manifolds based on [48, Chapter IV.B] sufficient for our purposes. To the interested reader we also recommend [5], where Whitney’s method is recast in a computational setting.

Triangulating smooth manifolds.

Let be a compact smooth d-manifold. By Theorem 14 there exists a smooth embedding ι:2d. Given such an embedding, we choose a sufficiently fine (with respect to ι) hypercubic honeycomb decomposition of 2d, denoted by L0. Next, we pass to the first barycentric subdivision L of L0. (Geometrically, L is obtained from L0 by subdividing each k-dimensional hypercube of L0 into (2k)!! simplices.) By slightly perturbing the vertices of L we obtain a triangulation L of 2d, which is combinatorially isomorphic to L, but is in general position with respect to ι()2d. Now, by the work of Whitney, L induces a triangulation 𝒯 of , where, importantly, 𝒯 is a subcomplex of the d-skeleton of the barycentric subdivision (L) of L.

See Figure 4 for an illustration of this procedure for d=1.

(a) The triangulation L and the image ι() (blue). The red points indicate the vertices of L contained by ι().
(b) The perturbed triangulation L, which in general position with respect to ι().
(c) The resulting triangulation 𝒯 of (dark green). 𝒯 is a subcomplex of (L).
Figure 4: Illustration of Whitney’s method of triangulating ambient submanifolds (d=1).

Similar to Proposition 13, the next proposition is a direct consequence of the definitions.

Proposition 16.

For any Whitney triangulation 𝒯 of a compact smooth d-manifold , we have that the dual graph Γ(𝒯) is an induced subgraph of Γ(((𝐇2d,n)′′)d).

4 The proof of Theorem 1

In this section we prove our main result, i.e., every compact smooth d-manifold has twin-width tww()dO(d). To streamline the notation, we let Gd,n=Γ(((𝐇2d,n)′′)d), i.e., Gd,n denotes the dual graph of the d-skeleton171717Recall that the dual graph Γ(𝒳) of a pure k-dimensional complex 𝒳 has vertices corresponding to the k-faces of 𝒳 and two vertices are connected if and only if their corresponding k-faces share a (k1)-face. of the second barycentric subdivision of the hybercubic honeycomb 𝐇2d,n. The result is based on the following property of Gd,n.

Theorem 17.

For the twin-width of the dual graph Gd,n=Γ(((𝐇2d,n)′′)d) of the d-skeleton of the second barycentric subdivision of the hypercubic honeycomb 𝐇2d,n we have

tww(Gd,n)dO(d).

Before proving Theorem 17, we show how it implies Theorem 1.

Proof of Theorem 1.

Let be a compact, smooth d-dimensional manifold. Consider a Whitney triangulation 𝒯 of . By Proposition 16, Γ(𝒯) is an induced subgraph of Gd,n and by Theorem 17, tww(Gd,n)dO(d). Hence, since twin-width is monotone under taking induced subtrigraphs (Proposition 5), we obtain tww(Γ(𝒯)))tww(Gd,n)dO(d).

To complete the proof of Theorem 1, it remains to show Theorem 17.

Proof of Theorem 17.

We establish the theorem by exhibiting a dO(d)-contraction sequence 𝔖:Gd,n. We will obtain 𝔖 by concatenating two contraction sequences 𝔖1:Gd,nGd,n and 𝔖2:Gd,n, referred to as the first and the second epoch, where Gd,n is an appropriate subtrigraph of red(Dn,2d), the 2d-dimensional red n-grid with diagonals. In the following we regard 𝐇2d,n and its subdivisions mainly as abstract complexes, but we will also take advantage of their geometric nature.

Preparations.

Consider the (2d+1)-coloring 𝒞:𝐇2d,n{0,,2d}, where we color the cubes of 𝐇2d,n by their dimension, that is, for c𝐇2d,n we set 𝒞(c)=dim(c) (Figure 5).

The coloring 𝒞 induces a (2d+1)-coloring 𝒞′′:(𝐇2d,n)′′{0,,2d} of the simplices of the second barycentric subdivision (𝐇2d,n)′′ as follows. The vertices of the first barycentric subdivision (𝐇2d,n) are in one-to-one correspondence with the cubes in 𝐇2d,n, hence 𝒞 induces a coloring 𝒞0:(𝐇2d,n)(0){0,,2d} via 𝒞0(vc)=𝒞(c), where vc denotes the vertex of (𝐇2d,n) corresponding to the cube c𝐇2d,n (Figure 5). Geometrically, vc is in the barycenter of the cube c. When we pass to the second barycentric subdivision, the vertices of (𝐇2d,n) become vertices of (𝐇2d,n)′′, thus there is a natural inclusion ι:(𝐇2d,n)(0)(𝐇2d,n)′′(0). Let 𝒱=im(ι)(𝐇2d,n)′′(0) be the image of (𝐇2d,n)(0) under this inclusion ι. We color the elements of 𝒱 identically to 𝒞0, that is, we consider the coloring 𝒞𝒱′′:𝒱{0,,2d} defined as 𝒞𝒱′′(v)=𝒞0(ι1(v)), see Figure 5. Now, pick a simplex σ(𝐇2d,n)′′ and note that v𝒱st¯(v)=(𝐇2d,n)′′, i.e., the closed stars of the vertices v𝒱 cover (𝐇2d,n)′′. Let 𝒱σ={v𝒱:σst¯(v)}. We now define 𝒞′′(σ) as

𝒞′′(σ)=min{𝒞𝒱′′(v):v𝒱σ}. (4)

In words, 𝒞′′(σ) is defined as the smallest dimension of any cube c𝐇2d,n such that σ belongs to the closed star of the vertex ι(vc) in (𝐇2d,n)′′, where vc is the vertex of (𝐇2d,n) corresponding to c. We refer to Figure 5 for an example.

(a) The coloring 𝒞 of the cubes of 𝐇2d,n by their dimension.
(b) The complex (𝐇2d,n) with its vertices colored by 𝒞0.
(c) The complex (𝐇2d,n)′′ with its vertices in 𝒱 colored by 𝒞𝒱′′.
(d) The coloring 𝒞′′ near the upper left corner of (𝐇2d,n)′′. The four larger disks represent vertices that belong to 𝒱.
(e) The coloring 𝒞′′ restricted to the d-simplices of (𝐇2d,n)′′. The color classes correspond to the families 0, 1 and 2.
(f) 𝒞′′The trigraph Gd,n obtained from Gd,n by contracting each subtrigraph Ci,c to a single node. All edges in Gd,n are red.
Figure 5: (a)–(c) Illustrations of the cubical complex 𝐇2d,n for d=1 and n=4, and of its first and second barycentric subdivisions (which are simplicial complexes) displaying the colorings described in the proof of Theorem 17. (d)–(f) Three essential steps in the proof of Theorem 17.
The first epoch.

We start the description of the first epoch 𝔖1:Gd,nGd,n by considering the restriction 𝒞d′′:(𝐇2d,n)′′(d){0,,2d} of the coloring 𝒞′′ to the d-simplices of (𝐇2d,n)′′, see Figure 5. Note that for each i{0,,2d}, the i-colored d-simplices of (𝐇2d,n)′′ form a family i={Fi,c:c𝐇2d,n(i)} of pairwise-disjoint connected subcomplexes of the d-skeleton of (𝐇2d,n)′′, where Fi,c denotes the subcomplex induced by the i-colored d-simplices of (𝐇2d,n)′′ belonging to the closed star of the vertex ι(vc), where vc is the vertex of (𝐇2d,n) corresponding to the i-cube c in 𝐇2d,n (Figure 5). We also let =i=02di.

Since Gd,n is defined as the dual graph of the d-skeleton of (𝐇2d,n)′′, the nodes of Gd,n are in one-to-one correspondence with the d-simplices of (𝐇2d,n)′′. Let γ:(𝐇2d,n)′′(d)V(Gd,n) denote the bijection realizing this correspondence. Henceforth, by a slight abuse of notation, we also consider 𝒞d′′ to be a (2d+1)-coloring of V(Gd,n) via 𝒞d′′(v)=𝒞d′′(γ1(v)). Let Ci,c denote the subtrigraph of Gd,n induced by the nodes {γ(σ):σFi,c}. Note that 𝒞d′′ assigns the color i to all nodes of Ci,c. The first epoch 𝔖1:Gd,nGd,n is merely the contraction sequence, where we contract each Ci,c (where 0i2d and c is running over 𝐇2d,n) to a single node, in any order, obtaining a trigraph Gd,n (Figure 5).

The second epoch.

𝔖2:Gd,n is defined as an optimal contraction sequence of the trigraph Gd,n to a single vertex. Since Gd,n is a subtrigraph of red(Dn,2d), the 2d-dimensional red n-grid with diagonals, by Theorem 9 the width of 𝔖2 is at most 2(32d1).

Bounding the width of the first epoch.

We now give a rough estimate to show that the first epoch 𝔖1:Gd,nGd,n is a dO(d)-contraction sequence. The estimate is based on Claim 18 below, which follows from elementary properties of the hypercubic honeycomb and barycentric subdivisions.

Claim 18.

For any color i{0,,2d} and cube c𝐇2d,n, the subcomplex Fi,c of 𝐇2d,n (resp. the subtrigraph Ci,c of Gd,n) defined above

  1. 1.

    has less than h1(d)=4d((2d)!)2(2dd) d-simplices (resp. nodes), and

  2. 2.

    less than h2(d)=9d incident subcomplexes Fi,c (resp. adjacent subtrigraphs Ci,c).

Indeed, these two facts imply that by sequentially contracting each Ci,c into a single node, the maximum red degree remains bounded by O(h1(d)3h2(d)) throughout the first epoch.

Proof of Claim 18.

To bound the number of d-simplices in Fi,c, note that

  • Fi,c is covered by an appropriate translate of a 2d-dimensional cube of 𝐇2d,

  • the barycentric subdivision of a 2d-cube contains 22d(2d)! 2d-simplices (Observation 12),

  • the barycentric subdivision of a 2d-simplex contains (2d)! 2d-simplices (Observation 11),

  • a 2d-simplex has (2dd) faces of dimension d (Observation 10).

Multiplying these numbers, we obtain the first part of the claim.

To bound the number of subcomplexes Fi,c incident to Fi,c, note that the incidences between these subcomplexes reflect those between the handles in the canonical handle decomposition of 𝐇2d,n obtained by “thickening up” its cubical cells. Thus, the number of subcomplexes in incident to Fi,c equals 32d1 if i{0,2d} and 3i+32di2 otherwise. Both of these numbers are less than 9d, so the second part of the claim also holds.

The width of 𝔖 is the maximum of the widths of 𝔖1 and 𝔖2, hence tww(Gd,n)dO(d).

5 Triangulations with dual graph of arbitrary large twin-width

In Section 4 we showed that every compact, smooth d-manifold admits a triangulation with dual graph of twin-width at most dO(d). In this section we prove complementary results, showing that triangulations with dual graphs of large twin-width are abundant. We shed light on this fact in two ways. First, we show that for any fixed dimension d3, the class of (d+1)-regular graphs that can be dual graphs of triangulated d-manifolds is not small. Second, we show that the d-dimensional ball admits triangulations with a dual graph of arbitrarily large twin-width, which extends to every piecewise-linear (hence smooth) d-manifold. Both of these results rely on counting arguments, and thus are not constructive.

5.1 The class of dual graphs of triangulations is not small

Let us fix an integer d3. Following [21], we let Md(n) denote the number of colored181818We refer to [21, Section 2.1] for the precise definitions. triangulations of closed orientable d-dimensional manifolds consisting of n d-simplices labeled from 1 to n. We assume n to be even. By [21, Theorem 1.1] we have191919Here “” denotes a comparison where exponential factors are ignored: more precisely, f(n)g(n) means that there exists some constant K>0, such that for n large enough, we have f(n)Kng(n).

n!nn/(2d)Md(n). (5)

Ignoring the colors gives an uncolored triangulation, but any such d-dimensional triangulation can be obtained from (d+1)! colored triangulations, since the d+1 colors can always be permuted. Now, any (d+1)-regular graph G with n vertices can be the dual graph of at most d!n(d+1)/2 different d-dimensional triangulations. Indeed, if G is the dual graph of some d-dimensional triangulation, then each of the n(d+1)/2 edges of G corresponds to a face gluing, i.e., an identification of two (d1)-dimensional simplices via a simplicial isomorphism, of which there are d! many.202020This is because a simplicial isomorphism between two (d1)-simplices σ and τ is determined by a perfect matching between the d vertices of σ and the d vertices of τ. Hence by (5), for the number Γd(n) of (d+1)-regular graphs on n labeled vertices that are dual graphs of some d-dimensional triangulations, we have

n!nn/(2d)(d+1)!d!n(d+1)/2Γd(n). (6)

As the left-hand side of (6) grows super-exponentially in n, the next theorem directly follows.

Theorem 19.

For every d3, the class of (d+1)-regular graphs that are dual graphs of triangulations of d-manifolds is not small. In particular, there are graphs with arbitrarily large twin-width in this class.

5.2 Complicated triangulations of the 𝒅-dimensional ball

In this section we show Theorem 3, according to which every piecewise-linear (PL) d-manifold (d3) admits triangulations with a dual graph of arbitrary large twin-width. To show the existence of such triangulations for every PL-manifold, we rely on the monotonicity of twin-width with respect to taking induced subtrigraphs (Proposition 5), the fact that the class of d-subdivisions of (d+1)-regular graphs is not small (Proposition 7) together with the following classical result from the theory of PL-manifolds. See [45] for an introduction.

Theorem 20 ([1, Corollary 1]).

Any triangulation of the boundary of a compact piecewise-linear (PL) manifold can be extended to a triangulation of the whole manifold.

We first show that already the d-dimensional ball d={xd:x1} admits triangulations with dual graph of arbitrary large twin-width. More precisely, we prove:

Theorem 21.

For every m there is a triangulation 𝒯m of the d-dimensional ball d, such that tww(Γ(𝒯m))m and 𝒯m=Δd, the boundary of the standard d-simplex Δd.

Proof.

Let Gm be a d-subdivision of a (d+1)-regular graph G such that tww(Gm)m. The existence of such a graph is guaranteed by Proposition 7. Let 𝒩 be a d-manifold homeomorphic to a closed regular neighborhood of a straight-line embedding of G in d. Informally, 𝒩 can be seen as a d-dimensional thickening of the graph G.

Construct an abstract triangulation of 𝒩 as follows. Take a d-simplex σv for each node v of G, and fix a one-to-one correspondence between the d+1 facets of σv and the d+1 arcs incident to v in G. For every arc {u,v}E(G), take a simplicial d-prism P{u,v}=σ{u,v}×[0,1], where σ{u,v} is a (d1)-simplex, and attach P{u,v} to the simplices σu and σv by identifying σ{u,v}×{0} (resp. σ{u,v}×{1}) with the facet of σu (resp. σv) that corresponds to the arc {u,v}. Now triangulate each prism P{u,v} with a minimal triangulation consisting of d d-simplices stacked onto each other, see [10, Appendix B].

Let 𝒯𝒩 denote the resulting triangulation of 𝒩.

Claim 22.

For the dual graph of the triangulation 𝒯𝒩 we have Γ(𝒯𝒩)=Gm.

Proof.

The claim follows immediately from the facts that the dual graph of the constructed triangulation of the simplicial d-prism is merely a path of length d, and Gm is the d-subdivision of the (d+1)-regular graph G on which the triangulation 𝒯𝒩 is modeled.

Pick a sufficiently large such that the th iterated barycentric subdivision 𝒯𝒩() of 𝒯𝒩 embeds linearly in d and fix a simplex-wise linear embedding :𝒯𝒩()d. Consider a large geometric d-simplex Σd that contains the image im() of in its interior. Now Σ=Σint(im()) is a PL manifold212121It is folklore that every codimension zero submanifold of a Euclidean space is a PL manifold, see, e.g., [34, p. 118] or [46, Remark 1.1.10]. with triangulated boundary, so by Theorem 20 this boundary triangulation can be extended to a triangulation 𝒯Σ of the entire manifold Σ. The boundary of 𝒯Σ has two connected components: 1𝒯Σ(𝒯𝒩()) and 2𝒯ΣΔd.

Let 𝒬 be a d-manifold homeomorphic to 𝒯𝒩×[0,1]. Take a triangulation 𝒯𝒬 of 𝒬, such that for the two boundary components we have 1𝒯𝒬𝒯𝒩 and 2𝒯𝒬(𝒯𝒩()). One way to construct such a triangulation is as follows. First, consider the decomposition 𝒫𝒬 of 𝒬 into simplicial d-prisms induced by the product structure 𝒯𝒩×[0,1]. That is, 𝒫𝒬 consists of d-prisms Pσσ×[0,1], one for each (d1)-simplex σ of 𝒯𝒩, glued together along their vertical boundary prisms the same way as the simplices of 𝒯𝒩. The boundary 𝒫𝒬 of 𝒫𝒬 has two connected components: 1𝒫𝒬 and 2𝒫𝒬, each combinatorially isomorphic to 𝒯𝒩. Next, pass to the th iterated barycentric subdivision of 2𝒫𝒬. This operation turns each prism Pσ of 𝒫𝒬 into a polyhedral cell Rσ. These cells form a polyhedral decomposition 𝒬 of 𝒬, where 1𝒬𝒯𝒩 and 2𝒬(𝒯𝒩)()=(𝒯𝒩()). Triangulate Rσ as follows. Consider an order c1c2 of the vertical cells222222These are precisely those cells that are not contained in the boundary of 𝒬. of Rσ, where cicj implies dim(ci)dim(cj). Place a new vertex vi in the barycenter of ci and, iterating over the vertical cells in the above order, triangulate ci by coning from vi over its (already triangulated) boundary ci. It is clear that the resulting triangulation of Rσ is symmetric with respect to the symmetries of its base simplex σ. Applying this procedure for each polyhedral cell Rσ of 𝒬 yields a triangulation 𝒯𝒬 of 𝒬 with the desired properties.

Now the triangulation 𝒯m of d is obtained by gluing together 𝒯𝒩, 𝒯𝒬 and 𝒯Σ via the identity maps along the isomorphic boundary-pairs 𝒯𝒩1𝒯𝒬 and 2𝒯𝒬1𝒯Σ.

To conclude, note that the d-simplicies of 𝒯𝒩 triangulate a submanifold of 𝒯m, hence by Proposition 13 the graph Γ(𝒯𝒩) is an induced subgraph of Γ(𝒯m). By Claim 22, Γ(𝒯𝒩)=Gm and by the initial assumption tww(Gm)m, hence by Proposition 5, Γ(𝒯m)m as well.

Proof of Theorem 3.

Let be an arbitrary PL-manifold possibly with non-empty boundary and 𝒯 be a simplicial triangulation of . Consider a triangulation 𝒯m of the d-ball with tww(Γ(𝒯m))m as in Theorem 21. Let Δ be a d-simplex of 𝒯 that is disjoint from (if 𝒯 does not contain such a simplex, just replace 𝒯 with its second barycentric subdivision). Since 𝒯 is simplicial, Δ is embedded in 𝒯 and is topologically a d-ball. Now replace Δ with 𝒯m by first removing Δ from 𝒯, thereby creating a boundary component isomorphic to Δ, then gluing 𝒯m to this new boundary component via a simplicial isomorphism. (Note that this is possible since 𝒯mΔ.) Let 𝒯 denote the resulting triangulation of . By Propositions 5 and 13, it follows that tww(Γ(𝒯))tww(Γ(𝒯m))m.

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