On the Twin-Width of Smooth Manifolds
Abstract
Building on Whitney’s classical method of triangulating smooth manifolds, we show that every compact -dimensional smooth manifold admits a triangulation with dual graph of twin-width at most . In particular, it follows that every compact -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 there exists a closed 3-manifold such that every triangulation thereof has dual graph with treewidth at least . To establish this result, we bound the twin-width of the dual graph of the -skeleton of the second barycentric subdivision of the -dimensional hypercubic honeycomb. We also show that every compact, piecewise-linear (hence smooth) -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 topologyFunding:
Kristóf Huszár: Partially supported by the Austrian Science Fund (FWF) grant P 33765-N.Copyright and License:
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2012 ACM Subject Classification:
Mathematics of computing Graph theory ; Mathematics of computing Geometric topologyAcknowledgements:
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 WangSeries and Publisher:

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 , or to a 3-manifold triangulation . In the first case the treewidth means the treewidth of considered as a -regular graph, in the second case it means the treewidth of the dual graph of . 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 ), what is the smallest treewidth that the dual graph of a triangulation of (resp. a diagram of ) 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 (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 and an -vertex graph with a contraction sequence of width , decides if satisfies in time , for some computable function . [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, -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 -dimensional smooth manifold always has a triangulation with dual graph of twin-width bounded in terms of .
Theorem 1.
Any compact -dimensional smooth manifold admits a triangulation (more specifically, a Whitney triangulation111111We call a triangulation of a compact smooth manifold a Whitney triangulation of , if is obtained via Whitney’s method discussed in Section 3.) with dual graph of twin-width at most .
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 denotes the twin-width of a graph , and denotes the dual graph of a triangulation .
Corollary 2.
There exists a universal constant such that every compact -dimensional manifold admits a triangulation with .
This is in sharp contrast to the case of treewidth, for which it is known that for every there are infinitely many 3-manifolds where the smallest treewidth of the dual graph of every triangulation is at least [29, 30]. Complementing Theorem 1, we also show that for any fixed , the -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 be an integer. For every compact -dimensional piecewise-linear manifold and natural number , there is a triangulation of with .
Remark 4.
The assumption of in Theorem 3 is essential. Indeed, the dual graph of any triangulation of the genus- surface is, in particular, a graph that embeds into and such graphs are known to have twin-width bounded above by for some universal constant [32].121212This bound is sharp up to a constant multiplicative factor [32]. For , 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 we let denote its cardinality, while for a real number we let denote its absolute value. For a positive integer , we let denote the set of -element subsets of . For a positive integer , we let denote the set of all positive integers up to , and for two real numbers , we use to denote the closed interval . For a vector we use to denote its Euclidean norm, i.e, . However, if is a (cubical or simplicial) complex, then 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 is a triple , where is a finite set of vertices, and 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 as , and , respectively. Any simple graph may be regarded as a trigraph with . For a vertex the degree of is the number of edges incident to it, i.e., . Additionally, the red degree of is the number of red edges incident to it, i.e., . A trigraph for which is called a -trigraph. Given two trigraphs and , we say that is a subtrigraph of , if , and .131313As usual, subtrigraphs of graphs (those without any red edges) will also be called subgraphs. In addition, if and , then we say that is an induced subtrigraph of . For a trigraph and a subset of its vertices, denotes the induced subtrigraph of with vertex set .
Contraction sequences and twin-width.
Let be a trigraph and be two arbitrary distinct vertices of . We say that the trigraph is obtained from by contracting and into a new vertex if 1. , 2. and 3. for any we have
-
if and only if and ,
-
if and only if and , and
-
otherwise.
We call the trigraph a contraction of . A sequence of trigraphs is a contraction sequence if is a contraction of for every . Note that . We use the notation “” to indicate that the trigraphs and are initial and terminal entries of the contraction sequence . The width of a contraction sequence is defined as , i.e., the largest red degree of any vertex of any trigraph in . Now, the twin-width of a trigraph is defined as the smallest width of any contraction sequence with and , 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 is an induced subtrigraph of a trigraph , then .
Smallness.
An infinite class of graphs is small if there exists a constant such that for every the class contains at most labeled graphs on vertices. The next theorem says that every graph class of bounded twin-width – i.e., for which there exists a constant , such that for every graph in the class – is small.
Theorem 6 ([9, Theorem 2.5]).
Every graph class with bounded twin-width is small.
Now let be a non-negative integer. The -subdivision of is the graph obtained from by subdividing each edge in exactly times. A simple counting argument together with Theorem 6 yields the following:
Proposition 7.
For any fixed integers and , the class of -subdivisions of -regular141414A simple graph is -regular if every vertex has degree . 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 be the number of labeled -regular simple graphs on vertices. Note that if , then is even; which we now assume. It is known (cf. [42, Section 6.4.1]) that, asymptotically
(1) |
which readily implies that -regular graphs (for do not form a small class.
Now, let be the number of -vertex graphs in the class of -subdivisions of -regular simple graphs. If a graph has vertices, then , where is the number of vertices of of degree . Note that such a graph can be obtained by first choosing a -regular labeled graph on vertices, then ordering the remaining vertices arbitrarily and evenly distributing them to the edges of according to some fixed ordering of . It follows that
(2) |
Since , we have . This, together with and (2) implies
(3) |
Recall that . In particular, is proportional to . Hence grows faster than for any fixed constant . Thus the graph class is not small.
Grid graphs.
The -dimensional -grid is the graph with vertex set , and for two vertices and if and only if . The next result states that irrespective of the value of .
Theorem 8 (Theorem 4 in [11]).
For every positive and , the -dimensional -grid has twin-width at most .
The -dimensional -grid with diagonals is the graph with , and for two vertices and if and only if . Now, for a given trigraph we set . In words, is the trigraph obtained from by replacing every black edge of by a red edge between the same vertices. With this notation is the -dimensional red -grid with diagonals, i.e., . Clearly, .
Theorem 9 (Lemma 4.4 in [11]).
For every positive and , every subtrigraph of the -dimensional red -grid with diagonals has twin-width at most .
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 , an abstract simplicial complex (or simplicial complex, for short) over is a downward closed subset of the power set , i.e., and imply . Any element of is called a face or simplex of , and for the dimension of is defined as . The dimension of , denoted by , is then the maximum dimension of a face of . If , we also say that is a simplicial -complex. For , we let denote the set of -dimensional faces (or -faces, or -simplices) of . The -skeleton of is the union of all faces of up to dimension . Note that any simple graph can be regarded as a -dimensional simplicial complex with and . For the -simplices of a simplicial complex are respectively called the vertices, edges, triangles, and tetrahedra of .
Geometric realization of simplicial complexes.
Every abstract simplicial complex may be realized geometrically as follows. To each abstract -simplex we associate a geometric -simplex defined as the convex hull of affinely independent points in . We equip with the subspace topology inherited from . Next, we consider the disjoint union of these geometric simplices, and perform identifications along their faces that reflect their relationship in . The resulting space , equipped with the quotient topology, is called the geometric realization of , see Figure 1. The geometric realization of a simplicial complex is unique up to homeomorphism.
Cubical complexes.
Analogous to simplicial complexes, a cubical complex over a ground set is a set system that consists of “cubes” instead of simplices. The terminology is the same as in the simplicial case. The only difference is that a geometric -cube is a topological space homeomorphic to , where 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.
The hypercubic honeycomb.
Let and be positive integers and consider the -dimensional cube . The -dimensional hypercubic honeycomb is a cubical -complex that decomposes into geometric cubes. The properties of this familiar object play an important role in this work, so we describe it for completeness. We define geometrically, in a bottom-up fashion. First, the vertex set consists of precisely those points in , which have only integral coordinates. Next, a -cube (i.e., an edge) is attached along its endpoints to vertices and in if and only if . Thus the 1-skeleton is just the -dimensional grid graph encountered in the end of Section 2.1. Finally, the higher dimensional skeleta of are induced by its 1-skeleton: for each subcomplex isomorphic to the boundary of a -cube, we attach an -cube to along . In words, starting from the 1-skeleton , 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.
Pure complexes and their dual graphs.
A (cubical or simplicial) complex is pure if every face of is contained in a face of dimension . It follows that, for every with , the -skeleton of a pure complex 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 , the dual graph of its -skeleton is defined as the graph, where the vertex set corresponds to the set of -faces, and if and only if and share an -dimensional face in , see Figure 3.
2.2.2 Barycentric subdivisions
Given a (cubical or simplicial) complex , its barycentric subdivision is a simplicial complex defined abstractly as follows. For the ground set of we have . A -tuple forms a -simplex of if and only if for every . We denote the 2nd (resp. th) iterated barycentric subdivision of a complex by (resp. ). See [10, Appendix A] for examples. The following are simple consequences of the definitions.
Observation 10.
For any , a -simplex has -faces.
Observation 11.
The barycentric subdivision of a -simplex contains -simplices.
Observation 12.
The barycentric subdivision of a -cube contains -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 -dimensional topological manifold with boundary (or -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 , or to the closed upper half-space . The points of that do not have a neighborhood homeomorphic to 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 and a homeomorphism onto an open subset of , the pair is called a chart. Given two charts and with , the map defined via is called a transition map. A smooth structure on a topological manifold with is a collection of charts that satisfies the following three properties.
-
1.
The sets cover , that is .
-
2.
For any with , the transition map is smooth.161616See [43, p. 185] for a discussion of (smooth) maps of manifolds.
-
3.
The collection is maximal in the sense that if is a chart and for every with the transition maps and are smooth, then .
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 -manifold. A simplicial complex whose geometric realization is homeomorphic to is called a triangulation of . It follows that is a pure simplicial complex of dimension (see Section 2.2.1). For , every topological -manifold admits a triangulation [41, 44], however, for 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 -dimensional triangulation , recall that its dual graph is the graph with vertices corresponding to the -simplices of , and edges to the face gluings, i.e, those -simplices of that are contained in precisely two -simplices. Note that for any vertex of . The following proposition is a consequence of the definitions.
Proposition 13.
Let be a triangulation of a -manifold and be a collection of -simplices of that define a submanifold of . Then is an induced subgraph of .
3 Whitney’s method
A seminal result of Whitney states that a smooth -dimensional manifold always admits a smooth embedding into a -dimensional Euclidean space.
Theorem 14 (strong Whitney embedding theorem [47, Theorem 5], cf. [33, Theorem 6.19]).
For , every smooth -manifold admits a smooth embedding into .
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 -manifold embedded in some Euclidean space 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 -manifold. By Theorem 14 there exists a smooth embedding . Given such an embedding, we choose a sufficiently fine (with respect to ) hypercubic honeycomb decomposition of , denoted by . Next, we pass to the first barycentric subdivision of . (Geometrically, is obtained from by subdividing each -dimensional hypercube of into simplices.) By slightly perturbing the vertices of we obtain a triangulation of , which is combinatorially isomorphic to , but is in general position with respect to . Now, by the work of Whitney, induces a triangulation of , where, importantly, is a subcomplex of the -skeleton of the barycentric subdivision of .
See Figure 4 for an illustration of this procedure for .
Similar to Proposition 13, the next proposition is a direct consequence of the definitions.
Proposition 16.
For any Whitney triangulation of a compact smooth -manifold , we have that the dual graph is an induced subgraph of .
4 The proof of Theorem 1
In this section we prove our main result, i.e., every compact smooth -manifold has twin-width . To streamline the notation, we let , i.e., denotes the dual graph of the -skeleton171717Recall that the dual graph of a pure -dimensional complex has vertices corresponding to the -faces of and two vertices are connected if and only if their corresponding -faces share a -face. of the second barycentric subdivision of the hybercubic honeycomb . The result is based on the following property of .
Theorem 17.
For the twin-width of the dual graph of the -skeleton of the second barycentric subdivision of the hypercubic honeycomb we have
Before proving Theorem 17, we show how it implies Theorem 1.
Proof of Theorem 1.
Let be a compact, smooth -dimensional manifold. Consider a Whitney triangulation of . By Proposition 16, is an induced subgraph of and by Theorem 17, . Hence, since twin-width is monotone under taking induced subtrigraphs (Proposition 5), we obtain .
To complete the proof of Theorem 1, it remains to show Theorem 17.
Proof of Theorem 17.
We establish the theorem by exhibiting a -contraction sequence . We will obtain by concatenating two contraction sequences and , referred to as the first and the second epoch, where is an appropriate subtrigraph of , the -dimensional red -grid with diagonals. In the following we regard and its subdivisions mainly as abstract complexes, but we will also take advantage of their geometric nature.
Preparations.
Consider the -coloring , where we color the cubes of by their dimension, that is, for we set (Figure 5).
The coloring induces a -coloring of the simplices of the second barycentric subdivision as follows. The vertices of the first barycentric subdivision are in one-to-one correspondence with the cubes in , hence induces a coloring via , where denotes the vertex of corresponding to the cube (Figure 5). Geometrically, is in the barycenter of the cube . When we pass to the second barycentric subdivision, the vertices of become vertices of , thus there is a natural inclusion . Let be the image of under this inclusion . We color the elements of identically to , that is, we consider the coloring defined as , see Figure 5. Now, pick a simplex and note that , i.e., the closed stars of the vertices cover . Let . We now define as
(4) |
In words, is defined as the smallest dimension of any cube such that belongs to the closed star of the vertex in , where is the vertex of corresponding to . We refer to Figure 5 for an example.
The first epoch.
We start the description of the first epoch by considering the restriction of the coloring to the -simplices of , see Figure 5. Note that for each , the -colored -simplices of form a family of pairwise-disjoint connected subcomplexes of the -skeleton of , where denotes the subcomplex induced by the -colored -simplices of belonging to the closed star of the vertex , where is the vertex of corresponding to the -cube in (Figure 5). We also let .
Since is defined as the dual graph of the -skeleton of , the nodes of are in one-to-one correspondence with the -simplices of . Let denote the bijection realizing this correspondence. Henceforth, by a slight abuse of notation, we also consider to be a -coloring of via . Let denote the subtrigraph of induced by the nodes . Note that assigns the color to all nodes of . The first epoch is merely the contraction sequence, where we contract each (where and is running over ) to a single node, in any order, obtaining a trigraph (Figure 5).
The second epoch.
is defined as an optimal contraction sequence of the trigraph to a single vertex. Since is a subtrigraph of , the -dimensional red -grid with diagonals, by Theorem 9 the width of is at most .
Bounding the width of the first epoch.
We now give a rough estimate to show that the first epoch is a -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 and cube , the subcomplex of (resp. the subtrigraph of ) defined above
-
1.
has less than -simplices (resp. nodes), and
-
2.
less than incident subcomplexes (resp. adjacent subtrigraphs ).
Indeed, these two facts imply that by sequentially contracting each into a single node, the maximum red degree remains bounded by throughout the first epoch.
Proof of Claim 18.
To bound the number of -simplices in , note that
-
is covered by an appropriate translate of a -dimensional cube of ,
-
the barycentric subdivision of a -cube contains -simplices (Observation 12),
-
the barycentric subdivision of a -simplex contains -simplices (Observation 11),
-
a -simplex has faces of dimension (Observation 10).
Multiplying these numbers, we obtain the first part of the claim.
To bound the number of subcomplexes incident to , note that the incidences between these subcomplexes reflect those between the handles in the canonical handle decomposition of obtained by “thickening up” its cubical cells. Thus, the number of subcomplexes in incident to equals if and otherwise. Both of these numbers are less than , so the second part of the claim also holds.
The width of is the maximum of the widths of and , hence .
5 Triangulations with dual graph of arbitrary large twin-width
In Section 4 we showed that every compact, smooth -manifold admits a triangulation with dual graph of twin-width at most . 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 , the class of -regular graphs that can be dual graphs of triangulated -manifolds is not small. Second, we show that the -dimensional ball admits triangulations with a dual graph of arbitrarily large twin-width, which extends to every piecewise-linear (hence smooth) -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 . Following [21], we let denote the number of colored181818We refer to [21, Section 2.1] for the precise definitions. triangulations of closed orientable -dimensional manifolds consisting of -simplices labeled from to . We assume to be even. By [21, Theorem 1.1] we have191919Here “” denotes a comparison where exponential factors are ignored: more precisely, means that there exists some constant , such that for large enough, we have .
(5) |
Ignoring the colors gives an uncolored triangulation, but any such -dimensional triangulation can be obtained from colored triangulations, since the colors can always be permuted. Now, any -regular graph with vertices can be the dual graph of at most different -dimensional triangulations. Indeed, if is the dual graph of some -dimensional triangulation, then each of the edges of corresponds to a face gluing, i.e., an identification of two -dimensional simplices via a simplicial isomorphism, of which there are many.202020This is because a simplicial isomorphism between two -simplices and is determined by a perfect matching between the vertices of and the vertices of . Hence by (5), for the number of -regular graphs on labeled vertices that are dual graphs of some -dimensional triangulations, we have
(6) |
As the left-hand side of (6) grows super-exponentially in , the next theorem directly follows.
Theorem 19.
For every , the class of -regular graphs that are dual graphs of triangulations of -manifolds is not small. In particular, there are graphs with arbitrarily large twin-width in this class.
5.2 Complicated triangulations of the -dimensional ball
In this section we show Theorem 3, according to which every piecewise-linear (PL) -manifold () 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 -subdivisions of -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 -dimensional ball admits triangulations with dual graph of arbitrary large twin-width. More precisely, we prove:
Theorem 21.
For every there is a triangulation of the -dimensional ball , such that and , the boundary of the standard -simplex .
Proof.
Let be a -subdivision of a -regular graph such that . The existence of such a graph is guaranteed by Proposition 7. Let be a -manifold homeomorphic to a closed regular neighborhood of a straight-line embedding of in . Informally, can be seen as a -dimensional thickening of the graph .
Construct an abstract triangulation of as follows. Take a -simplex for each node of , and fix a one-to-one correspondence between the facets of and the arcs incident to in . For every arc , take a simplicial -prism , where is a -simplex, and attach to the simplices and by identifying (resp. ) with the facet of (resp. ) that corresponds to the arc . Now triangulate each prism with a minimal triangulation consisting of -simplices stacked onto each other, see [10, Appendix B].
Let denote the resulting triangulation of .
Claim 22.
For the dual graph of the triangulation we have .
Proof.
The claim follows immediately from the facts that the dual graph of the constructed triangulation of the simplicial -prism is merely a path of length , and is the -subdivision of the -regular graph on which the triangulation is modeled.
Pick a sufficiently large such that the th iterated barycentric subdivision of embeds linearly in and fix a simplex-wise linear embedding . Consider a large geometric -simplex that contains the image of in its interior. Now 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 of the entire manifold . The boundary of has two connected components: and .
Let be a -manifold homeomorphic to . Take a triangulation of , such that for the two boundary components we have and . One way to construct such a triangulation is as follows. First, consider the decomposition of into simplicial -prisms induced by the product structure . That is, consists of -prisms , one for each -simplex of , glued together along their vertical boundary prisms the same way as the simplices of . The boundary of has two connected components: and , each combinatorially isomorphic to . Next, pass to the th iterated barycentric subdivision of . This operation turns each prism of into a polyhedral cell . These cells form a polyhedral decomposition of , where and . Triangulate as follows. Consider an order of the vertical cells222222These are precisely those cells that are not contained in the boundary of . of , where implies . Place a new vertex in the barycenter of and, iterating over the vertical cells in the above order, triangulate by coning from over its (already triangulated) boundary . It is clear that the resulting triangulation of is symmetric with respect to the symmetries of its base simplex . Applying this procedure for each polyhedral cell of yields a triangulation of with the desired properties.
Now the triangulation of is obtained by gluing together , and via the identity maps along the isomorphic boundary-pairs and .
To conclude, note that the -simplicies of triangulate a submanifold of , hence by Proposition 13 the graph is an induced subgraph of . By Claim 22, and by the initial assumption , hence by Proposition 5, as well.
Proof of Theorem 3.
Let be an arbitrary PL-manifold possibly with non-empty boundary and be a simplicial triangulation of . Consider a triangulation of the -ball with as in Theorem 21. Let be a -simplex of that is disjoint from (if does not contain such a simplex, just replace with its second barycentric subdivision). Since is simplicial, is embedded in and is topologically a -ball. Now replace with by first removing from , thereby creating a boundary component isomorphic to , then gluing to this new boundary component via a simplicial isomorphism. (Note that this is possible since .) Let denote the resulting triangulation of . By Propositions 5 and 13, it follows that .
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