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Trees of tangles in infinite separation systems

Published online by Cambridge University Press:  22 July 2021

CHRISTIAN ELBRACHT
Affiliation:
Fachbereich Mathematik Arbeitsgruppe DM, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany. e-mails: christian.elbracht@uni-hamburg.de, jakob.kneip@uni-hamburg.de, maximilian.teegen@uni-hamburg.de
JAKOB KNEIP
Affiliation:
Fachbereich Mathematik Arbeitsgruppe DM, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany. e-mails: christian.elbracht@uni-hamburg.de, jakob.kneip@uni-hamburg.de, maximilian.teegen@uni-hamburg.de
MAXIMILIAN TEEGEN
Affiliation:
Fachbereich Mathematik Arbeitsgruppe DM, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany. e-mails: christian.elbracht@uni-hamburg.de, jakob.kneip@uni-hamburg.de, maximilian.teegen@uni-hamburg.de
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Abstract

We present infinite analogues of our splinter lemma for constructing nested sets of separations. From these we derive several tree-of-tangles-type theorems for infinite graphs and infinite abstract separation systems.

Type
Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
© The Author(s), 2021. Published by Cambridge University Press on behalf of Cambridge Philosophical Society

1. Introduction

One of the key concepts in graph minor theory is that of a tree-decomposition of a graph. These allow us to analyse a large graph by dividing it into smaller subgraphs and, working with these, gain insight into the graph as a whole. Tangles, ever since their introduction by Robertson and Seymour [Reference Robertson and Seymour20], have served as a dual object to tree-decompositions of low width in graphs. They have also been studied in their own right, as a way to express, indirectly, highly cohesive substructures such as k-blocks or large grid minors.

Over the years, this theory has been expanded and generalised; the branch of abstract tangle theory emerged. There, the focus shifted from the study of graph minors to tangles as an abstract combinatorial object on an abstract separation system, replacing the concrete separations of a graph. These separation systems are just a poset which only expresses how separations lie in relation to each other, keeping little of the information provided by the underlying graph; this notion is abstract enough to be applicable to settings other than graphs. [Reference Diestel6Reference Diestel, Hundertmark and Lemanczyk9,Reference Diestel and Oum11,Reference Diestel and Oum12,Reference Elbracht, Kneip and Teegen14,Reference Elbracht, Kneip and Teegen15].

The two central theorems of abstract tangle theory are the tangle-tree duality theorem and the tree-of-tangles theorem. The former provides a tree-like structure dual to the existence of a tangle, while the latter exposes how tangles can be arranged in a tree structure by way of a nested set of distinguishing separations. When applied to the vertex separations of a graph, such finite nested sets of separations can easily be converted back into a tree-decomposition in the original sense.

In [Reference Elbracht, Kneip and Teegen14] we introduced the ‘splinter lemma’, a unified theorem which implies the known tree-of-tangles theorems for finite separations systems. The merit of this theorem lies in the fact that, while it is strong enough to imply all these results, the proof of the theorem is simple, and its assumptions are easy to check. (See Section 2 for definitions.)

Theorem 1·1 (Splinter lemma, [Reference Elbracht, Kneip and Teegen14]). Let U be a universe of separations and $ (\mathcal{A}_i)_{i\le n} $ a family of subsets of $ U.$ If $ (\mathcal{A}_i)_{i\le n}$ splinters then we can pick an element $ a_i $ from each $ \mathcal{A}_i $ so that $ \{a_1,\dots,a_n\} $ is nested.

Theorem 1·1, in a sense, is yet another step in a series of abstractions in the theory of tangles: rather than working with the tangles themselves, it operates just on the collection of sets of separations distinguishing a given pair of these.

Theorem 1·1 is proved by induction: it finds a separation $ a_i\in\mathcal{A}_i $ which is nested with some element of every other $ \mathcal{A}_j $ , and then proceeds inductively on the remaining $ n{-}1 $ family members, restricted to those separations nested with $ a_i $ . This approach cannot deal with infinite families of sets, however.

In this paper we overcome these difficulties and present two different ways to obtain a version of Theorem 1·1 for infinite families of sets of separations. Both these versions are as abstract and therefore as widely applicable, as our original Theorem 1·1; they differ in they way they overcome the difficulties of infinite sets of separations, and therefore in the assumptions required.

The first approach is to ensure that an inductive proof adopted from the finite case converges by imposing a closedness condition in a suitable topology: in Section 3 we present a result for profinite separation systems. These constitute a large class of infinite separation systems, including the systems of vertex separations of an infinite graph. In Section 4 we will, as an example, apply this theorem to precisely these graph separations.

However, to deduce the existing tangle-tree theorems in infinite graphs (see [Reference Carmesin1,Reference Carmesin, Hamann and Miraftab3]) we need a second approach which does not rely one these profinite separation systems, as not every tangle of an infinite graph is one of the corresponding profinite separation system. Following this approach, we obtain our second main theorem, which does imply these existing theorems (Theorem 1·4 and 1·5 below) and is therefore a truly infinite analogue of Theorem 1·1. It asks more of the sets $\mathcal{A}_i$ than the profinite version; in return the set of separations we obtain will be canonical, i.e., invariant under isomorphisms:

Theorem 1·2. If $(\,{\mathcal{A}_i \mid i\in I}\,)$ thinly splinters with respect to some reflexive symmetric relation $\sim$ on $\mathcal{A}\,{:\!=}\,\bigcup_{i\in I}\mathcal{A}_i$ , then there is a set $N\subseteq \mathcal{A}$ which meets every $\mathcal{A}_i$ and is nested, i.e., $n_1\sim n_2$ for all $n_1,n_2\in N$ . Moreover, this set N can be chosen invariant under isomorphisms: if $ \varphi $ is an isomorphism between $ (\mathcal{A},\sim)$ and $(\mathcal{A}',\sim')$ , then we have $ N((\,{\varphi(\mathcal{A}_i)\mid i\in I}\,))=\varphi(N((\,{\mathcal{A}_i\mid i\in I}\,))) $ .

We prove this statement in Section 5. Like Theorem 1·1, the statement of this theorem is a bit technical, as we want it to be as widely applicable as possible. We then show the usefulness of this abstract theorem throughout Section 6, by deducing the existing theorems about distinguishing tangles in infinite graphs from it.

As a simple example, we start with applying it to tangles in locally finite graphs in Section 6·1. This application is straightforward and demonstrates a prototypical application of Theorem 1·2.

It is also possible to apply Theorem 1·2 to arbitrary infinite graphs, and we do so in Section 6·2. This application uses another new, and interesting, shift of perspective: we cannot apply Theorem 1·2 directly to the sets of separations efficiently distinguishing two profiles since, in general, these do not splinter thinly. Instead we consider a slightly different set, namely, the set of separators, to which Theorem 1·2 does apply:

Theorem 1·3. Given a set of distinguishable robust regular profiles $\mathcal{P}$ of a graph G there exists a canonical nested set of separators efficiently distinguishing any pair of profiles in $\mathcal{P}$ .

This theorem acts as an intermediary result between the existing results about tangles in arbitrary infinite graphs. On the one hand we can, waiving canonicity, transform the nested set of separators back into a nested set of separations, recovering the following result of Carmesin about distinguishing tangles in infinite graphs by way of a nested set of separations:

Theorem 1·4 ([Reference Carmesin1, Theorem 5·12]) For any graph G, there is a nested set N of separations that distinguishes efficiently any two robust principal profiles (that are not restrictions of one another).

This theorem is a cornerstone in Carmesin’s proof that every infinite graph has a tree-decomposition displaying all its topological ends. For more about the relation between ends and tangles also see [Reference Diestel4,Reference Kneip19]. We deduce Theorem 1·4 from Theorem 1·3 in Section 6·2·1.

On the other hand, if we want to keep canonicity, we can use Theorem 1·3 to deduce a result by Carmesin, Hamann and Miraftab [Reference Carmesin, Hamann and Miraftab3]. They construct a canonical object, which they call a tree of tree-decompositions, to distinguish the tangles:

Theorem 1·5 ([Reference Carmesin, Hamann and Miraftab3, Remark 8·3]) Let G be a connected graph and $\mathcal{P}$ a distinguishable set of principal robust profiles in G. There exists a canonical tree of tree-decompositions with the following properties:

  1. (i) the tree of tree-decompositions distinguishes $\mathcal{P}$ efficiently;

  2. (ii) if $t\in V(T)$ has level k, then $(T_t,\mathcal{V}_t)$ contains only separations of order k;

  3. (iii) nodes t at all levels have $|V(T_t)|$ neighbours on the next level and the graphs assigned to them are all torsos of $(T_t,\mathcal{V}_t)$ .

We deduce Theorem 1·5 from Theorem 1·3 in Section 6·2·2.

Theorem 1·3 is also an interesting result in its own right: the set of separators that it provides is a natural intermediate object between the non-canonical nested set of separations in Theorem 1·4 and the canonical tree of tree-decompositions in Theorem 1·5.

Moreover, proving Theorem 1·4 or Theorem 1·5 by first proving Theorem 1·3 and then deducing them breaks up the proof nicely and is, in total, shorter than the original proofs from [Reference Carmesin1,Reference Carmesin, Hamann and Miraftab3].

2. Terminology and basic facts

All terminology for graphs in this paper is from [Reference Diestel5]. This paper builds extensively on the framework of [Reference Diestel6,Reference Elbracht, Kneip and Teegen14]. A brief summary of their definitions follows, see [Reference Diestel6,Reference Elbracht, Kneip and Teegen14] for more in-depth exposition.

A poset $\vec{S}$ together with an order-reversing involution $^*$ is called a separation system, and its elements are (oriented) separations. Given an oriented separation $\vec{s}$ we shall denote its inverse $\vec{s}^*$ as $\stackrel{\tiny\gets}{s}$ and the pair $\{\vec{s},\stackrel{\tiny\gets}{s}\}$ as s. This s is called the underlying unoriented separation of $\vec{s}$ and $\vec{s},\stackrel{\tiny\gets}{s}$ are its orientations. We write S for the set of all underlying unoriented separations of oriented separations in $\vec{S}$ . Informally we refer to either just as ‘separations’ when the distinction is immaterial or the intended meaning is clear.

Two unoriented separation r and s are nested if $ \vec{r}\le\vec{s} $ for suitable orientations of r and s. If r and s are not nested we say that they cross. Likewise we call oriented separations $ \vec{r} $ and $ \vec{s} $ nested if r and s are nested, and crossing otherwise. A nested set of separations is one whose elements are pairwise nested.

A separation $\vec{s}$ is small if $\vec{s}\le \stackrel{\tiny\gets}{s}$ , in which case we say that $\stackrel{\tiny\gets}{s}$ is cosmall. A separation $\vec{s}$ is trivial, if there exists a separation $\vec{r}\in \vec{S}$ such that $r\neq s$ and both, $\vec{s}\le\vec{r}$ and $\vec{s}\le\stackrel{\tiny\gets}{r}$ . Note that every trivial separation is small. An unoriented separation is said to be trivial or small if one of its orientations is. A tree set in a separation system S is a nested set of separations which does not contain any trivial elements. It is regular if it does not contain any small elements.

A separation system $\vec{U}$ is a universe if there are join and meet operators $\lor,\land$ which turn the poset into a lattice. A map $ f\colon \vec{U}\to\vec{U}' $ between universes $ \vec{U} $ and $ \vec{U}' $ is an homomorphism of universes if f commutes with the involutions and the $ \lor $ and $ \land $ operations of $ \vec{U} $ and $ \vec{U}' $ , i.e. if $ f(\stackrel{\tiny\gets}{s})=f(\vec{s})^* $ , $ f(\vec{r}\lor\vec{s})=f(\vec{r})\lor f(\vec{s}) $ , and $ f(\vec{r}\land\vec{s})=f(\vec{r})\land f(\vec{s}) $ for all $ \vec{r},\vec{s}\in\vec{U} $ . Clearly, if $ f\colon\vec{U}\to\vec{U}' $ is an homomorphism of universes, then $ \vec{r}\le\vec{s} $ for $ \vec{r},\vec{s}\in\vec{U} $ implies $ f(\vec{r})\le f(\vec{s}) $ in $ \vec{U}' $ .

An isomorphism of universes is a bijective homomorphism of universes whose inverse is also a homomorphism. Two universes are isomorphic if there is an isomorphism between them.

The corner separations of two separations s and t are the four separations $\vec{s}\lor\vec{t}$ , $\vec{s}\lor\stackrel{\tiny\gets}{t}$ , $\stackrel{\tiny\gets}{s}\lor\vec{t}$ , $\stackrel{\tiny\gets}{s}\lor\stackrel{\tiny\gets}{t}$ , or their underlying unoriented separations.

An order-function on a universe $\vec{U}$ is some function $\lvert{\cdot}\rvert:\vec{U}\to \mathbb{N}_0$ such that ${\lvert{\vec{s}}\rvert=\lvert{\stackrel{\tiny\gets}{s}}\rvert\,{=\!:}\,\lvert{s}\rvert}$ for every unoriented separation $s\in U$ . Note that, unlike in the finite setting, in the infinite setting we require order-functions to take their values in $\mathbb{N}_0$ rather than in $\mathbb{R}_{\ge 0}$ . In the finite case this does not make a big difference, as there were only finitely many orders of separations, which therefore could be scaled to be ‘essentially integer-valued’. Since such an argument is not possible in the infinite set-up, we need to require our order-function to be integer-valued in the first place.

An order-function is submodular if for all $\vec{s},\vec{t}\in \vec{U}$ we have

\begin{equation*} \lvert{\vec{s}}\rvert+\lvert{\vec{t}}\rvert\ge \lvert{\vec{s}\lor\vec{t}}\rvert+\lvert{\vec{s}\land\vec{t}}\rvert\,. \end{equation*}

An orientation of some set S of unoriented separations is a set $O\subseteq \vec{S}$ containing precisely one of $ \vec{s} $ and $ \stackrel{\tiny\gets}{s} $ for each $ s\in S $ . An orientation O is said to be consistent if there are no $ \stackrel{\tiny\gets}{r}\le\vec{s} $ with $ r\ne s $ and $ \vec{r},\vec{s}\in O $ .

If $ O_1 $ and $ O_2 $ are two orientations of some (possibly distinct) sets of separations, a separation s distinguishes $ O_1 $ and $ O_2 $ if s has two distinct orientations $ \vec{s}\in O_1 $ and $ \stackrel{\tiny\gets}{s}\in O_2 $ . If an order function is given, and s is of lowest possible order among all separations that distinguish $ O_1 $ and $ O_2 $ , then s distinguishes them efficiently.

For a separation system $\vec{S}\subseteq \vec{U}$ inside some universe $\vec{U}$ , we say that a consistent orientation O of S is a profile if it satisfies the profile property:

(P) \begin{equation} \forall \,\vec{r},\vec{s}\in P \colon (\stackrel{\tiny\gets}{r}\land\stackrel{\tiny\gets}{s})\notin P \end{equation}

A profile is said to be robust if additionally:

\begin{equation*}\forall \vec{s}\in P,\vec{t}\in \vec{U}:\text{ if }|\vec{s}\lor \vec{t}|<|\vec{s}|\text{ and }|\vec{s}\lor\stackrel{\tiny\gets}{t}|<|\vec{s}|, \text{ then either }\vec{s}\lor\vec{t}\in P\text{ or }\vec{s}\lor\stackrel{\tiny\gets}{t}\in P.\end{equation*}

Note that in [Reference Diestel, Hundertmark and Lemanczyk9], Diestel, Hundertmark and Lemanczyk also defined the much more technical but slightly weaker condition of a set of profiles being robust, which only requires the condition of robustness for some separations in U. Every set of distinguishable robust profiles is a robust set of profiles in the sense of [Reference Diestel, Hundertmark and Lemanczyk9], and on the other hand, the reader familiar with this terminology may replace each appearance of a set of robust profiles in this paper by a robust set of profiles without changing the proofs.

If U comes with some order function then, for some $k\in \mathbb{N}\cup \{\aleph_0\}$ , we write $S_k$ for the set of all separations in U satisfying $|s|<k$ . A k-profile (in U) is then a profile of $S_k$ and a profile in U shall mean a k-profile in U for some k.

Given some universe U and a collection $\mathcal{B}\,{:\!=}\,(\,{\mathcal{A}_i\mid i\in I}\,)$ of subsets $\mathcal{A}_i$ of U, we say that $\mathcal{B}$ splinters if it satisfies, for any $i,j\in I$ , the following property:

\begin{equation*}\forall s\in \mathcal{A}_i, t\in \mathcal{A}_j\colon s\in \mathcal{A}_j\text{ or }t\in \mathcal{A}_i\text{ or there exists a corner of }s\text{ and }t\text{ in }\mathcal{A}_i\cup \mathcal{A}_j.\end{equation*}

One example of a separation system are the (vertex) separations of a graph: given some (infinite) graph G let us consider the separations of G, these are all pairs (A, B) of vertex sets A, B such that $A\cup B=V(G)$ and there is no edge between $A\setminus B$ and $B\setminus A$ . For historical reasons and ease of notation the corresponding unoriented separations are represented as $\{A,B\}$ instead of $\{(A,B), (B,A)\}$ , contrary to the notation in the abstract set-up. These form a poset where $(A,B) \le (C,D)$ when $A \subseteq B$ and $C \supseteq D$ . The function ${}^*: (A,B) \mapsto (B,A)$ is an order-reversing involution. If $A\cap B$ is finite, the order of (A, B) is defined as $\lvert{A\cap B}\rvert$ . Note that the set $S_{\aleph_0}$ of all separations of finite order is a universe of separations where $(A,B) \lor (C,D) = (A\cup C, B\cap D)$ and $(A,B) \land (C,D) = (A\cap C, B\cup D)$ . A profile in G shall be a profile of $S_k\subseteq S_{\aleph_0}$ for some $k\in \mathbb{N}\cup \{\aleph_0\}$ .

A profile P in G is regular if it does not contain any cosmall separation of G, i.e., it contains no separation of the form (V(G), X). Note that, in graphs, the irregular profiles are not of large interest, since they always point towards either the empty set or a single non-cut-vertex. Formally, we can summarize this statement from [Reference Diestel, Eberenz and Erde7] as follows:

Lemma 2·1 ([Reference Diestel, Eberenz and Erde7]) Let $G = (V,E)$ be a graph and P an irregular profile in G then either G is connected and $P = \{(V, \emptyset)\}$ or G has a non-cutvertex $x \in V$ such that

\begin{equation*} P = \{(A,B)\in\vec{S}_2 \mid x\in B \text{ and } (A,B)\neq (\{x\},V)\}. \end{equation*}

These irregular profiles are distinguished efficiently from each other and from all other profiles in G by the set of separations

\begin{equation*} \{\{V(G), \emptyset\}\} \cup \{ \{V(G), \{x\}\} \mid x\in V(G) \text{ and }{x} \text{is not a cutvertex of} {G}\} .\end{equation*}

Every separation in this set is nested with all separations of G. Hence, our efforts for applications in graphs will concentrate on regular profiles.

Given some set of vertices $X\subseteq V(G)$ , we say that a connected component C of $G-X$ is tight, if $N(C)=X$ .

For two vertices $x,y\in V(G)$ of a graph G, an x–y-separator of order k is a vertex set $X\subseteq V(G)\setminus \{x,y\}$ of size k such that x and y lie in different components of $G-X$ . We shall need the following basic fact about such separators in infinite graphs at various points throughout this paper.

Lemma 2·2 ([Reference Halin18, Statement 2·4]). Let G be a graph, $u,v\in V(G)$ and $k\in \mathbb{N}$ . Then there are only finitely many separators of size at most k separating u and v minimally.

Another basic tool is the so-called ‘fish lemma’:

Lemma 2·3 ([Reference Diestel, Hundertmark and Lemanczyk9, Lemma 2·1]) Let U be a universe and $ r,s\in U $ two crossing separations. Every separation t that is nested with both r and s is also nested with all four corner separations of r and s.

Additionally we shall use the following more general observation about separations nested with a corner separation:

Lemma 2·4 Let $ \vec{r} $ and $ \vec{s} $ be two separations. Every separation nested with one of r or s is also nested with at least one of $ \vec{r}\land\vec{s} $ and $ \vec{r}\lor\vec{s} $ .

Proof. Let t be a separation nested with, say, r. Then t has an orientation $ \vec{t} $ with either $ \vec{t}\le\vec{r} $ or $ \vec{t}\le\stackrel{\tiny\gets}{r} $ . In the first case t is nested with $ \vec{r}\lor\vec{s} $ by $ \vec{t}\le\vec{r}\le(\vec{r}\lor\vec{s}) $ . In the latter case t is nested with $ \vec{r}\land\vec{s} $ by $ \vec{t}\le\stackrel{\tiny\gets}{r}\le(\vec{r}\land\vec{s})^* $ .

The separations that distinguish a given pair of profiles exhibit a lattice-like structure:

Lemma 2·5 Let $ \vec{U} $ be a universe with a submodular order function and P and P $^\prime$ two profiles in $ \vec{U} $ . If $ \vec{r},\vec{s}\in P $ distinguish P and P $^\prime$ efficiently, then both $ \vec{r}\lor\vec{s} $ and $ \vec{r}\land\vec{s} $ also lie in P and distinguish P and P $^\prime$ efficiently.

Proof. If one of $ \vec{r}\lor\vec{s} $ and $ \vec{r}\land\vec{s} $ has order at most $ \lvert{r}\rvert=\lvert{s}\rvert $ , then that corner separation lies in P and distinguishes P and P $^\prime$ by their consistency and the property P. The efficiency of r and s now implies that neither of the two considered corner separations can have order strictly lower than $ \lvert{r}\rvert $ . Therefore, by submodularity, both of them have order exactly $ \lvert{r}\rvert $ , which implies the claim.

Moreover we shall need a way to transition between separations and tree-decompositions in graphs. Such a method already exists in finite graphs [Reference Carmesin, Diestel, Hundertmark and Stein2]. The ingredients of that proof together with the results of [Reference Gollin and Kneip17] are all that is needed to show an analogous result for infinite graphs, which we shall present here.

Let us start by recalling the definition of a tree-decomposition. A tree-decomposition of a G is a pair $(T,\mathcal{V})$ of a tree T together with a family $\mathcal{V} = (V_t)_{t \in T}$ of vertex sets $V_t\subseteq V(G)$ such that:

  1. (T1) $V(G) = \bigcup_{t\in T}V_t$ ;

  2. (T2) Given $e\in E[G]$ there exists a $t\in T$ such that $e\subseteq V_t$ ;

  3. (T3) Given a path P in T from $t_1$ to $t_3$ and a vertex $t_2\in P$ we have $V_{t_1}\cap V_{t_3}\subseteq V_{t_2}$ .

A separation (A, B) is induced by a tree-decomposition $(T,\mathcal{V})$ if and only if there exists an edge $tt'\in T$ such that for the components $T_t,T_{t'}$ of $T-tt'$ containing t or t $^\prime$ respectively, we have

\begin{equation*}(A,B)=\Big(\bigcup_{t^{\prime\prime}\in T_t}V_{t^{\prime\prime}},\bigcup_{t^{\prime\prime}\in T_{t^{\prime}}}V_{t^{\prime\prime}}\Big)\,.\end{equation*}

A chain of order type $\alpha$ or a $\alpha$ -chain, or a chain of length $\alpha$ , for some ordinal number $\alpha$ (we shall only use $\alpha\in \mathbb{N}$ or $\alpha=\omega$ or $\alpha=\omega+1$ ) is a collection $\{\vec{s}_i\mid 0\le i<\alpha\}$ of oriented separations, such that $\vec{s}_i<\vec{s}_j$ whenever $i<j$ . A set N of unoriented separations contains such a chain, if all the separations in that chain are orientations of separations from N.

Kneip and Gollin [Reference Gollin and Kneip17] showed the following:

Theorem 2·6 ([Reference Gollin and Kneip17, Theorem 3·9]) Every regular tree set which does not contain a chain of order type $\omega+1$ is isomorphic to the edge tree set of a suitable tree.

Here the edge tree set of a tree $T=(V,E)$ is the set of separation obtained from the set of oriented edges $\vec{E}$ by ordering them in the natural partial order (that is $vw\in \vec{E}$ is smaller than $xy\in \vec{E}$ if and only if the unique path from v to y meets w and x).

We shall need the following lemma, whose proof is inspired by [Reference Carmesin, Diestel, Hundertmark and Stein2].

Lemma 2·7 Let $G=(V,E)$ be an infinite graph and let $N\subseteq S_{\aleph_0}(G)$ be a regular tree set. If we have for every $\omega$ -chain $(A_1,B_1)<(A_2,B_2)<\dots$ which is contained in N that $\bigcap_{i\in \mathbb{N}}B_i=\emptyset$ , then there exists a tree-decomposition $(T,\mathcal{V})$ of G whose set of induced separations is $\vec{N}$ .

Moreover, this tree-decomposition can be chosen canonically: if $ \varphi\colon G\to G' $ is an isomorphism of graphs, then the tree-decomposition constructed for $ \varphi(N) $ in G $^\prime$ is precisely the image under $ \varphi $ of the tree-decomposition constructed for N in G.

Proof. Let $T=(V,E)$ be the tree from Theorem 2·6. Note that by [Reference Gollin and Kneip17, Theorem 3·9(iii)] any isomorphism between the edge tree sets of two distinct trees induces an isomorphism of the underlying trees.

Let $\alpha$ be the isomorphism from the edge tree set of T to N. Given some node $t\in T$ let us denote as $F_t$ the set of oriented separations

\begin{equation*}F_t\,{:\!=}\,\{\alpha(s,t) \mid (s,t)\in\vec{E}\}\,.\end{equation*}

We define the bags of our tree-decomposition as $V_t\,{:\!=}\,\bigcap_{(A,B)\in F_t}B$ . Let us verify that $ (T,\mathcal{V}) $ with $ \mathcal{V}=(\,{V_t}\,)_{t\in T} $ is the desired tree-decomposition.

For (T1) let $ v\in V $ be given; we need to find a $ t\in T $ with $ v\in V_t $ . If $ v\in A\cap B $ for some $ (A,B)\in\vec{N} $ then $ v\in V_t $ for t being either of the two end-vertices of the edge whose image under $ \alpha $ is (A, B). Otherwise v induces an orientation O of E(T) by orienting each edge $ \left\{{x,y}\right\}$ of T as (x, y) if $ v\in B\setminus A $ for $ (A,B)=\alpha(x,y) $ .

Observe that O is consistent. If O has a sink, that is, if there is a node t of T all of whose incident edges are oriented inwards by O, then $ v\in V_t $ by definition of O. If O does not have a sink then O contains an $ \omega $ -chain. This is impossible though, since by definition of O we would have $ v\in\bigcap_{i\in\mathbb{N}}B_i $ , where $ (A_i,B_i) $ is the image under $ \alpha $ of the ith element of that $ \omega $ -chain in O. Thus (T1) holds.

The proof that (T2) holds can be carried out in much the same way due to the fact that every edge of G is included in either A or B for each $ (A,B)\in\vec{N} $ .

Before we check that (T3) holds, let us show that $ (T,\mathcal{V}) $ indeed induces N. For this we need to show that if (x, y) is an oriented edge of T then

\begin{equation*} \alpha(x,y)=\Big(\bigcup_{z\in T_x}V_z\,,\,\bigcup_{z\in T_y}V_z\Big), \end{equation*}

where $ T_x $ and $ T_y $ are the components of $ T-xy $ containing x and y, respectively. So let $ (x,y)\in\vec{E} $ be given and $ \alpha(x,y)=(A,B) $ . Observe first that $ A\cap B\subseteq V_x\cap V_y $ by definition. It thus suffices to show that $ A\supseteq\bigcup_{z\in T_x}V_z $ and $ B\supseteq\bigcup_{z\in T_y}V_z $ to establish the desired equality.

To see this consider a vertex $ v\in V_z $ for some $ z\in T_x $ . Let $ \vec{e} $ be the first edge of the unique zx -path in T and let $ \alpha(\vec{e})=(A',B') $ . We have $ \vec{e}\le(x,y) $ by definition of an edge tree set, and hence $ (A',B')\le(A,B) $ since $ \alpha $ is an isomorphism. From this we know that $ A'\subseteq A $ . We further have $ (B',A')\in F_z $ and thus, by definition of $ V_z $ , that $ v\in A' $ . This shows $ v\in A $ . The argument that $ B\supseteq\bigcup_{z\in T_y}V_z $ is similar.

Having established that $ (T,\mathcal{V}) $ indeed induces N, we can now deduce from this that (T3) holds: if $V_{t_1}$ and $V_{t_3}$ are two bags of $ (T,\mathcal{V}) $ which both contain some vertex v, then v also needs to lie in the separator of every separation that is an image under $ \alpha $ of an edge on the path P in T from $t_1$ to $t_3$ . Therefore v lies in every $V_{t_2}$ with $t_2\in P$ .

3. The profinite splinter lemma

In this section we establish an extension of the splinter lemma, Theorem 1·1, to a large class of infinite separation systems: the profinite universes. Informally, a separation system is profinite if it is determined entirely by its finite subsystems. The most prominent, and most important, example of such a universe of separations is the separation system of an infinite graph: two separations of an infinite graph are comparable precisely if all their restrictions to finite subgraphs are comparable. Moreover, a pair (A, B) of sets of vertices of an infinite graph G is a separation if and only if the restriction of (A, B) to each finite subgraph H of G is a separation of H. We will make this relation between the separations of a profinite universe and their finite restrictions more formal now.

3·1. Introduction to profinite universes

For an in-depth introduction to profinite separation systems we refer the reader to [Reference Diestel and Kneip10], where this class of separation systems was first introduced. In this section we shall give only the definitions, terms, and tools for profinite universes relevant to our studies.

A directed set is a poset P in which every two elements have a common upper bound, i.e., in which there is an $ r\in P $ with $ p\le r $ and $ q\le r $ for all $ p,q\in P $ . Given a directed set P, an inverse system (of finite sets) is a family $ \mathcal{X}=(\,{X_p\mid p\in P}\,)$ of finite sets, together with maps $ f_{qp}\colon X_q\to X_p $ for all $ q>p $ that are compatible in the sense that $ f_{rp}=f_{qp}\circ f_{rq} $ for all $ r>q>p $ . If every set $ X_p $ is a finite universe of separations $ \vec{U}_p $ , and the maps $ f_{qp} $ are homomorphisms of universes, then the family $ \mathcal{U}=(\,{\vec{U}_p\mid p\in P}\,)$ is an inverse system (of universes of separations).

A limit of an inverse system $ \mathcal{X}=(\,{X_p\mid p\in P}\,)$ is a compatible choice of one element $ x_p $ from each $ X_p $ , that is, a family $ (\,{x_p\mid p\in P}\,)$ with $ x_p\in X_p $ and $ f_{qp}(x_q)=x_p $ for all $ q>p $ . The inverse limit $ \varprojlim\,\mathcal{X} $ of $ (\,{X_p\mid p\in P}\,)$ is the set of all limits of $ \mathcal{X} $ . It is a well-known fact that every inverse system of non-empty finite sets has a limit (cf. [Reference Diestel and Kneip10]).

Limits and the inverse limit of an inverse system of universes are defined in the same way. Then the inverse limit $ \vec{U}=\varprojlim\,\mathcal{U} $ of an inverse system of universes $\,\mathcal{U}=(\,{\vec{U}_p\mid p\in P}\,)$ is itself a universe of separations by defining involution, partial order, joins, and meets coordinate-wise. That is by, for $ \vec{r}=(\,{\vec{r}_p\mid p\in P}\,)$ and $ \vec{s}=(\,{\vec{s}_p\mid p\in P}\,)$ , letting

\begin{equation*} \stackrel{\tiny\gets}{s}\,{:\!=}\,(\,{\stackrel{\tiny\gets}{s}_p\mid p\in P}\,) \end{equation*}

as well as

\begin{equation*} \vec{r}\lor\vec{s}\,{:\!=}\,(\,{\vec{r}_p\lor\vec{s}_p\mid p\in P}\,) \end{equation*}

and

\begin{equation*} \vec{r}\land\vec{s}\,{:\!=}\,(\,{\vec{r}_p\land\vec{s}_p\mid p\in P}\,), \end{equation*}

with $ \vec{r}\le\vec{s} $ if and only if $ \vec{r}_p\le\vec{s}_p $ for all $ p\in P $ . In particular the involution, joins and meets of limits of $ \mathcal{U} $ are again limits of $ \mathcal{U} $ .

A universe of separations is then called profinite if it is isomorphic to an inverse limit of finite universes of separations. The most prominent example of a profinite universe of separations comes from infinite graphs:

Example 3·1. Let $ G=(V,E) $ be an infinite graph and $ \vec{U}=\vec{U}(G) $ the universe of all separations of G, including those (A, B) with $|A\cap B|=\infty$ . Then $ \vec{U} $ is profinite: let $ \mathcal{X} $ be the set of all finite $ Z\subseteq V $ . Then $ \mathcal{X} $ , ordered by inclusion, is a directed set. For $ Z\in\mathcal{X} $ let $ \vec{U}_Z $ be the universe of separations of G[Z]. We define maps $ f_{ZY}\colon\vec{U}_Z\to\vec{U}_Y $ for $ Y\subset Z $ by letting $ f_{ZY} $ map a separation $ (A_Z,B_Z) $ of G[Z] to $ (A_Z\cap Y\,,\,B_Z\cap Y) $ , which is easily seen to be a separation of G[Y]. These maps are clearly compatible, and thus the family $ \mathcal{U}\,{:\!=}\,(\,{\vec{U}_Z\mid Z\in\mathcal{X}}\,)$ is an inverse system of finite universes.

Let us show that $ \vec{U} $ is isomorphic to the inverse limit of $ \mathcal{U} $ . For this observe that for every separation (A, B) of G the family of its restrictions $ (\,{(A\cap Z\,,\,B\cap Z)\mid Z\in\mathcal{X}} \,)$ is a limit of $ \mathcal{U} $ , and the map $ f\colon\vec{U}\to\varprojlim\,\mathcal{U} $ given by mapping each $ (A,B)\in\vec{U}(G) $ to this family $ (\,{(A\cap Z\,,\,B\cap Z)\mid Z\in\mathcal{X}} \,)$ is a homomorphism of universes. Moreover f is clearly injective, and its inverse is a homomorphism as well. To see that f is an isomorphism between $ \vec{U} $ and $ \varprojlim\,\mathcal{U} $ it thus remains to show that f is surjective, that is, that every limit of $ \mathcal{U} $ gives rise to a separation of G.

So let $ \vec{s}=(\,{(A_Z,B_Z)\mid Z\in\mathcal{X}} \,)$ be a limit of $ \mathcal{U} $ . Let A and B be the union of the sets $ A_Z $ and $ B_Z $ , respectively, over all $ Z\in\mathcal{X} $ . We claim that (A, B) is a separation of G. If so then $ f((A,B))=\vec{s} $ , showing that f is surjective.

Note first that $ A\cup B=V $ , since $ v\in A_{\left\{{v}\right\}}\cup B_{\left\{{v}\right\}} $ for every $ v\in V $ . Suppose now that G contains an edge $ vw\in E $ with $ v\in A\setminus B $ and $ w\in B\setminus A $ . Let $ Z\,{:\!=}\,\{v,w\} $ and consider the induced subgraph G[Z] of G : by definition of (A, B) we have $ A_Z=\{v\} $ and $ B_Z=\{w\} $ , but then vw is an edge of G[Z] between $ A_Z\setminus B_Z $ and $ B_Z\setminus A_Z $ , contradicting the assumption that $ (A_Z,B_Z)\in\vec{U}_Z $ . Therefore (A, B) is indeed a separations of G.

For the remainder of this section, let $ \mathcal{U}=({\vec{U}_p\mid p \in P}\,) $ be an inverse system of universes and $ \vec{U} $ its inverse limit.

Given an element $ \vec{s}=(\,{\vec{s}_p\mid p\in P}\,)$ of $ \vec{U} $ , we write $ (\vec{s}\!\restriction\! p)\,{:\!=}\,\vec{s}_p $ for the projection of $ \vec{s} $ to $ \vec{U}_p $ . Likewise, for a set $ O\subseteq\vec{U} $ the projection $ O\!\restriction\! p $ to $ \vec{U}_p $ is the set of all $ \vec{s}\!\restriction\! p $ with $ \vec{s}\in O $ .

A family $ (\,{N_p\mid p\in P}\,)$ of finite subsets of the $ \vec{U}_p $ is a restriction of $ \mathcal{U} $ if $ f_{qp}(N_q)=N_p $ for all $ q>p $ . The inverse limit of such a restriction of $ \mathcal{U} $ is a subset of $ \vec{U}=\varprojlim\,\mathcal{U} $ .

By equipping each $ \vec{U}_p $ in $ \mathcal{U} $ with the discrete topology, the inverse limit $ \vec{U}=\varprojlim\,\mathcal{U} $ becomes a topological space as a subspace of the product space $ \prod_{p\in P}\vec{U}_p $ . Doing so makes the maps $ f_{qp} $ continuous, and it is easy to see that $ \vec{U} $ is a closed subset of the product $ \prod_{p\in P}\vec{U}_p $ and hence compact. In fact, the topology on $ \vec{U} $ can be described in terms of the sets $ \vec{U}_p $ :

Lemma 3·2 ([Reference Diestel and Kneip10, Lemma 4·1]) The following statements hold:

  1. (i) the topological closure in $\vec{U}$ of a set $O\subseteq\vec{U}$ is the set of all limits $\vec{s} = (\,{\vec{s}_p\mid p\in P}\,)$ with $\vec{s}_p\in O\!\restriction\! p$ for all p;

  2. (ii) a set $O\subseteq\vec{U}$ is closed in $\vec{U}$ if and only if there are sets $O_p\subseteq\vec{U}_p$ , with $f_{qp}(O_q)\subseteq O_p$ whenever $p<q\in P$ , such that $O = \varprojlim\,\, (\,{O_p \mid p\in P}\,)$ .

We shall also use the following lemma, which is a re-formulation of [Reference Diestel and Kneip10, Lemma 5·4]:

Lemma 3·3 A set $ N\subseteq \vec{U} $ is nested if and only if $ (N\!\restriction\! p)\subseteq \vec{U}_p $ is nested for all $ p\in P $ .

A consequence of Lemma 3·2 and Lemma 3·3 (which we will not use) is that the topological closure of a nested set is still nested.

Finally, let us show that two closed sets in $ \vec{U} $ intersect if all of their projections do:

Lemma 3·4 If $ R,S\subseteq \vec{U} $ are closed and $ (R\!\restriction\! p)\cap(S\!\restriction\! p) $ is non-empty for every $ p\in P $ , then $ R\cap S $ is non-empty.

Proof. The family $ (\,{(R\!\restriction\! p)\cap(S\!\restriction\! p)\mid p\in P}\,)$ is an inverse system of non-empty finite subsets of $ \vec{U} $ , using as maps the restrictions of the maps $ f_{qp} $ of $ (\,{\vec{U}_p\mid p\in P}\,)$ . Since both R and S are closed in $ \vec{U} $ , every limit of this family lies in $ R\cap S $ , which is therefore non-empty.

3·2 Statement and proof of the profinite splinter lemma

Using the framework of profinite separation systems, we can now extend Theorem 1·1 to infinite separation systems:

Theorem 3·5 Let $ \vec{U}=\varprojlim\,(\,{\vec{U}_p\mid p\in P}\,)$ be a profinite universe and $ \mathcal{B} $ a family of non-empty closed subsets of $ \vec{U} $ . If $ \mathcal{B} $ splinters then there is a closed nested set $ N\subseteq \vec{U} $ containing at least one element from each member of $ \mathcal{B} $ .

Proof. For $ p\in P $ let $ \mathcal{B}_p $ denote the projection $ \mathcal{B}\!\restriction\! p $ of $ \mathcal{B} $ to $ \vec{U}_p $ , that is, the family of all $ B\!\restriction\! p $ , where B is a member of $ \mathcal{B} $ . Then each projection $ \mathcal{B}_p $ splinters in $ \vec{U}_p $ : consider two members $ B_p $ and $ B_p^{\prime} $ of $ \mathcal{B}_p $ , with separations $ \vec{r}_p\in B_p $ and $ \vec{s}_p\in B_p^{\prime} $ . By definition of $ B_p $ and $ B_p^{\prime} $ there are $ \vec{r}\in B $ and $ \vec{s}\in B' $ with $ (\vec{r}\!\restriction\! p)=\vec{r}_p $ and $ (\vec{s}\!\restriction\! p)=\vec{s}_p $ . Since $ \mathcal{B} $ splinters we have either that one of $ \vec{r} $ or $ \vec{s} $ lies in $ B\cap B' $ , in which case its projection to $ \vec{U}_p $ lies in $ B_p\cap B_p^{\prime} $ , or else some corner separation $ \vec{c} $ of $ \vec{r} $ and $ \vec{s} $ lies in $ B\cup B' $ . In the latter case $ (\vec{c}\!\restriction\! p)\in B_p\cup B_p^{\prime} $ is a corner separation of $ \vec{r}_p $ and $ \vec{r}_s $ , showing that $ \mathcal{B}_p $ indeed splinters.

By the above observation and Theorem 1·1 applied to $ \mathcal{B}_p $ and $ \vec{U}_p $ there is a nested set $ N_p\subseteq \vec{U}_p $ for every $ p\in P $ which contains an element of each member of $ \mathcal{B}_p $ . Let $ \mathcal{N}_p $ be the set of all such nested sets $ N_p\subseteq \vec{U}_p $ . Observe that if $ N_q\in\mathcal{N}_q $ and $ q>p $ then $ f_{qp}(N_q)\subseteq\vec{U}_p $ is a nested set meeting each member of $ \mathcal{B}_p $ and hence lies in $ \mathcal{N}_p $ . Therefore the family $ (\,{\mathcal{N}_p\mid p\in P}\,)$ together with the maps mapping $ N_q\in\mathcal{N}_q $ to $ f_{qp}(N_q)\in\mathcal{N}_p $ for $ q>p $ is an inverse system of non-empty finite sets.

Let $ (\,{N_p\mid p\in P}\,) $ be a limit of $ (\,{\mathcal{N}_p\mid p\in P}\,) $ . Then this limit is a restriction of $ \mathcal{U} $ , and hence $ N\,{:\!=}\,\varprojlim\,(\,{N_p\mid p\in P}\,) $ is a subset of $ \vec{U} $ . In fact N is a closed nested subset of $ \vec{U} $ by Lemma 3·2 and Lemma 3·3. To see that N contains an element of each member of $ \mathcal{B} $ , let B be a member of $ \mathcal{B} $ . Then $ (N\!\restriction\! p)\cap (B\!\restriction\! p) $ is non-empty for each $ p\in P $ since $ (N\!\restriction\! p)=N_p\in\mathcal{N}_p $ , and thus by Lemma 3·4 and the assumption that B is closed in $ \vec{U} $ the sets N and B intersect.

4. Application of the profinite splinter lemma

For this section, let $ G=(V,E) $ be a connected graph and $ \mathcal{X} $ the set of finite subsets of V. As seen in Example 3·1, the universe $ \vec{U}=\vec{U}(G) $ of separations of G is profinite since it arises as the inverse limit of $ (\,{\vec{U}_Z\mid Z\in\mathcal{X}}\,) $ , where $ \vec{U}_Z $ denotes the universe of separations of G[Z]. Following the notation of Section 3, we write $ (A,B)\!\restriction\! Z $ for the projection $ (A\cap Z\,,\,B\cap Z) $ of a separation $ (A,B)\in\vec{U} $ to $ \vec{U}_Z $ .

For $ k\in\mathbb{N} $ let $ \vec{S}_k=\vec{S}_k(G) $ be the separation system of all separations of order $ <k $ of G. Using Lemma 3·2, it is easy to observe the following fact about these $ \vec{S}_k $ , which will be used throughout this section:

Observation 4·1 For every $ k\in\mathbb{N} $ the set $ \vec{S}_k $ is a closed subset of $ \vec{U} $ .

Our main goal in this section is to use Theorem 3·5 to find a nested set of separations which efficiently distinguishes a large set of profiles of G. Concretely, we will be able to distinguish the set of all regular bounded profiles in G. A profile P in G is bounded if P is a k -profile of G for some k but is not a subset of any $ \aleph_0 $ -profile of G. Recall that a profile in G is regular if it contains no separation of the form (V(G), X).

The main result of this section, then, will be the following:

Theorem 4·2 Let $\mathcal{P}$ be a set of robust regular bounded profiles in G. Then there is a nested set N of separations of G which efficiently distinguishes all distinguishable profiles in $\mathcal{P}$ .

It can be shown ([Reference Diestel4]) that every $\aleph_0$ -profile P in G corresponds to either an end of G, or a so-called ultrafilter tangle: an orientation which, for some $ X\in\mathcal{X} $ , induces a non-principal ultrafilter on the set of components of $ G-X $ . These ultrafilter tangles are studied extensively in [Reference Diestel4]. Ends and ultafilter tangles both exhibit a different behaviour than profiles of finite graphs. Bounded profiles, on the other hand, do behave similarly to profiles of finite graphs, and consequently we shall be able to utilise Theorem 3·5 to establish Theorem 4·2. The latter is a weakening of Theorem 1·4, but our proof via Theorem 3·5 will be significantly shorter and simpler than the proof of Theorem 1·4.

For the remainder of this section let $ \mathcal{P} $ be a set of robust regular bounded profiles in G. Given two profiles P and P $^\prime$ in $ \mathcal{P} $ let $\mathcal{A}_{P,P'}$ be the set of all separations of G that efficiently distinguish P and P $^\prime$ .

In order to deduce Theorem 4·2 from Theorem 3·5 we need to show that the family of the sets $ \mathcal{A}_{P,P'} $ splinters, and that each $ \mathcal{A}_{P,P'} $ is a closed subset of $ \vec{U} $ . We will start by showing the latter:

Proposition 4·3 Let P and P $^\prime$ be distinguishable regular bounded profiles in G. Then $ \mathcal{A}_{P,P'} $ is a closed subset of $ \vec{U} $ .

In order to prove Proposition 4·3 we will first need to show a series of lemmas about how bounded profiles, and their efficient distinguishers, behave. The first step is to show that a regular bounded profile, for every sufficiently small set X of vertices, points towards a component of $G-X$ . That is to say: bounded profiles do not behave like the ultrafilter tangles of [Reference Diestel4].

Lemma 4·4 Let X be a finite set of vertices and P a regular bounded profile of order at least $ \lvert{X}\rvert+1 $ in G. Then there is a unique component C of $ G-X $ with $ (V\setminus C\,,\,C\cup X)\in P $ .

Proof. Suppose that P contains for every component C of $G-X$ the separation $(C \cup X , V \setminus C)$ , we are going to construct an extension of P to a profile of $S_{\aleph_0}$ .

To determine the appropriate orientation of a separation $\left\{{A,B}\right\} \in S_{\aleph_0}$ , consider the components of $G-X$ and let $C_A$ be the union of all those components which are contained in $A \setminus B$ . Likewise let $C_B$ be the union of all components contained in $B \setminus A$ and $C_R$ the union of the remaining components, i.e., those which meet both A and B. Since each of these needs to meet $A\cap B$ there are only finitely many.

By our assumption P contains for every component C of $G-X$ the separation $(C \cup X \,,\, V \setminus C)$ . Since $C_R$ is a union of only finitely many components of $G-X$ and since P is a profile, we have $(C_R \cup X \,,\, V \setminus C_R) \in P$ .

We now want to prove that one of $(V\setminus C_B\,,\, C_B\cup N(C_B))$ and $(V\setminus C_A\,,\, C_A\cup N(C_B))$ lies in P. Indeed, if this is not the case then their respective inverses are in P, so the profile property gives us

\begin{equation*}(C_B\cup C_A\cup N(C_A\cup C_B)\,,\, V\setminus (C_A\cup C_B))\in P.\end{equation*}

This however would imply that the supremum

\begin{equation*} (C_B\cup C_A\cup N(C_A\cup C_B)\,,\, V\setminus (C_A\cup C_B)) \,\lor\, (C_R \cup X \,,\, V \setminus C_R) = (V, X)\end{equation*}

lies in P by the profile property and the fact that this is a separation of order $\lvert{X}\rvert$ . This contradicts the regularity of P.

This proves that one of $(V\setminus C_B\,,\, C_B\cup N(C_B))$ and $(V\setminus C_A\,,\, C_A\cup N(C_B))$ lies in P, and by consistency we cannot have both. We may thus define an orientation P $^\prime$ of $S_{\aleph_0}$ by declaring that (A, B) shall be in P $^\prime$ if and only if $(V\setminus C_B\,,\, C_B\cup N(C_B))$ is in P. This orientation is consistent since P is consistent and $(A,B) \le (V\setminus C_B\,,\, C_B\cup N(C_B))$ . Note that $P\subseteq P'$ and it only remains to show that P $^\prime$ is a profile.

Given $(A,B), (C,D) \in P'$ we have, by definition, $(V\setminus C_B,\, C_B\cup N(C_B))$ and $(V\setminus C_D,$ $\, C_D\cup N(C_D))$ in P. The profile property of P then gives us

\begin{equation*} (V\setminus C_B\,,\, C_B\cup N(C_B)) \,\lor\, (V\setminus C_D\,,\, C_D\cup N(C_D)) \in P \subseteq P',\end{equation*}

so by the consistency of P $^\prime$ we have $((A,B) \lor (C, D))^* \notin P'$ .

Using Lemma 4·4 we can take the next step towards showing that the sets $ \mathcal{A}_{P,P'} $ are closed by showing that for every infinite chain in $ \mathcal{A}_{P,P'} $ the sequence of the separators is eventually constant:

Lemma 4·5 Let P and P $^\prime$ be distinguishable regular bounded profiles in G and consider and infinite increasing sequence $ {(A_1,B_1)\le(A_2,B_2)\le\cdots} $ in $ \mathcal{A}_{P,P'} $ . Then the sequence of separators $ (A_i\cap B_i)_{i\in\mathbb{N}} $ is eventually constant.

Proof. By switching their roles if necessary we may assume that P $^\prime$ contains $ (B_1,A_1) $ . Then, by consistency, $ (B_i,A_i)\in P' $ and consequently $ (A_i,B_i)\in P $ for every $ i\in\mathbb{N} $ .

For $ i\in\mathbb{N} $ let us write $ X_i\,{:\!=}\, A_i\cap B_i $ . By Lemma 4·4 there is a unique component $ C_i $ of $ G-X_i $ with $ (V\setminus C_i\,,\,C_i\cup X_i)\in P $ . Observe that, just like $ (A_i,B_i) $ , the separation $ {(V\setminus C_i\,,\,C_i\cup X_i)} $ distinguishes P and P $^\prime$ efficiently, and that $C_i \subseteq B_i$ . This efficiency implies that $ {N(C_i)=X_i} $ . Observe further that the separations $ {(V\setminus C_i\,,\,C_i\cup X_i)} $ form an increasing chain in $ \mathcal{A}_{P,P'} $ whose sequence of separators is $ (A_i\cap B_i)_{i\ge\mathbb{N}} $ . In fact $ C_i\supseteq C_j $ for $ i\le j $ . It thus suffices to show that the sequence of the $ C_i $ is eventually constant. So suppose for a contradiction that the sequence of the $ C_i $ is strictly decreasing.

Let us first show that $ \bigcap_{i\in\mathbb{N}}C_i $ is empty. If not there is a vertex $ v\in\bigcap_{i\in\mathbb{N}}C_i $ . By applying Lemma 4·4 to P $^\prime$ and each $ X_i $ , we obtain components $ C_i' $ of $ G-X_i $ such that $ (V\setminus C_i'\,,\,C_i'\cup X_i)\in P' $ for all $ i\in\mathbb{N} $ . Clearly $ C_i\ne C_i' $ and $ N(C_i')=X_i $ for all $ i\in\mathbb{N} $ . Fix any $ w\in C_1' $ . Then $ w\in C_i' $ for every $ i\in\mathbb{N} $ , and each $ X_i $ is a minimal $ v - w $ -separator in G. The latter contradicts Lemma 2·2 by the assumption that all $ C_i $ , and hence all $ X_i $ , are distinct.

Thus $ \bigcap_{i\in\mathbb{N}}C_i $ is indeed empty. Let us define an orientation $ \tilde{P} $ of $ S_{\aleph_0} $ and show that $ \tilde{P} $ is a profile extending P, obtaining a contradiction. Let $ \tilde{P} $ consist of all $ (A,B)\in\vec{S}_{\aleph_0} $ for which there is a $ C_i $ with $ C_i\subseteq B\setminus A $ . Since the $ C_i $ form a decreasing sequence of connected vertex sets with $ \bigcap_{i\in\mathbb{N}}C_i=\emptyset $ , and $ A\cap B $ is finite, this defines an orientation of $ S_{\aleph_0} $ . Moreover it is easy to see that $ \tilde{P} $ is consistent and has the profile property.

To obtain the desired contradiction it is thus left to check that $ P\subseteq\tilde{P} $ . However any $ (A,B)\in P $ with $ C_i\subseteq A\setminus B $ for some $ i\in\mathbb{N} $ would be inconsistent with $ {(V\setminus C_i\,,\,C_i\cup X_i)\in P} $ . Therefore $ \tilde{P} $ is an extension of P, contrary to the assumption that P is bounded.

Moreover, we can even show that the same statement also holds not only for chains, but even for the entire set $\mathcal{A}_{P,P'}$ :

Lemma 4·6 Let P and P $^\prime$ be distinguishable regular bounded profiles in G. Then the separations $ (A,B)\in\mathcal{A}_{P,P'} $ have only finitely many distinct separators $A\cap B$ .

Proof. Suppose for a contradiction that $ \mathcal{A}_{P,P'} $ contains an infinite sequence of separations $ (A_1,B_1),(A_2,B_2),\dots $ whose separators $ A_i\cap B_i $ are pairwise distinct. We may assume without loss of generality that $ (A_i,B_i)\in P $ for every $ i\in\mathbb{N} $ . By Lemma 2·5 P contains all finite joins of these separations. For each $ i\in\mathbb{N} $ let $ X_i $ be the separator of the supremum of $ (A_1,B_1) $ up to $ (A_i,B_i) $ , and let $ C_i $ be the component of $ G-X_i $ with $ (V\setminus C_i\,,\,C_i\cup X_i)\in P $ as given by Lemma 4·4. By Lemma 4·5 the sequence of the $ X_i $ is eventually constant, and therefore so is the sequence of the $ C_i $ as P is a profile. Let $ C\,{:\!=}\,\bigcap_{i\in\mathbb{N}}C_i\ne\emptyset $ .

Analogously for P $^\prime$ let $ X_i' $ be the separator of the supremum of $ (B_1,A_1) $ up to $ (B_i,A_i) $ and let $ C_i' $ be the component of $ G-X_i' $ with $ (V\setminus C_i'\,,\,C_i'\cup X_i')\in P' $ . As before let $ C'\,{:\!=}\,\bigcap_{i\in\mathbb{N}}C_i'\ne\emptyset $ .

Since $ C_1 $ and $ C_1' $ are disjoint so are C and C $^\prime$ . Fix vertices $ v\in C $ and $ w\in C' $ . We claim that every separator $ A_i\cap B_i $ is a minimal $ v - w $ -separator in G, contradicting the assertion of Lemma 2·2. To see this, consider $ A_i\cap B_i $ for some $ i\in\mathbb{N} $ . Let $ \tilde{C}_i $ and $ \tilde{C}_i' $ be the components of $ G-(A_i\cap B_i) $ obtained by applying Lemma 4·4 with P and P $^\prime$ , respectively. Then $ C\subseteq C_i\subseteq\tilde{C}_i $ and likewise $ C'\subseteq C_i'\subseteq\tilde{C}_i' $ , giving $ v\in\tilde{C}_i $ and $ w\in\tilde{C}_i' $ . Moreover both $ \tilde{C}_i $ and $ \tilde{C}_i' $ have all of $ A_i\cap B_i $ as their neighbourhood as $(A_i,B_i)$ efficiently distinguishes P and P $^\prime$ , and hence $ A_i\cap B_i $ is indeed a minimal $ v - w $ -separator in G.

We are now ready to prove Proposition 4·3, i.e. that the sets $ \mathcal{A}_{P,P'} $ are closed subsets of $ \vec{U} $ :

Proof of Proposition 4·3. Let two distinguishable regular bounded profiles P and P $^\prime$ in G be given. By Lemma 4·6 only finitely many sets, say $ X_1,\dots,X_n $ , appear as separators of separations in $ \mathcal{A}_{P,P'} $ . For each $ X_i $ let $ C_i $ and $ C_i' $ be the two components of $ G-X_i $ given by applying Lemma 4·4 to $ X_i $ for P and P $^\prime$ , respectively.

We are now able to give a complete description of the set $ \mathcal{A}_{P,P'} $ : it is easy to check that a separation $ (A,B)\in\vec{S}_{\aleph_0} $ distinguishes P and P $^\prime$ efficiently if and only if $ A\cap B=X_i $ for some i with one of $ C_i $ and $ C_i' $ being a subset of A and the other a subset of B.

For each $ X_i $ , the set of all $ (A,B)\in\vec{U} $ with $ A\cap B=X_i $ as well as $ C_i\subseteq A $ and $ C_i'\subseteq B $ is closed by Lemma 3·2. Likewise the set of all $ (A,B)\in\vec{U} $ with separator $ X_i $ as well as $ C_i'\subseteq A $ and $ C_i\subseteq B $ is closed, too. Therefore $ \mathcal{A}_{P,P'} $ is the union of finitely many closed subsets of $ \vec{U} $ and hence closed.

Having established that the sets $ \mathcal{A}_{P,P'} $ are closed in $ \vec{U} $ , it thus remains for us to verify that the family of the sets $ \mathcal{A}_{P,P'} $ splinters in order to deduce Theorem 4·2 from Theorem 3·5. Since we shall need a slightly stronger property than splintering at a later point in Section 6, we will prove this stronger assertion here. In particular we will not make use of the assumptions that the profiles in $ \mathcal{P} $ are regular and bounded.

To show that the sets $ \mathcal{A}_{P,P'} $ splinter, we need to show that for all $ (A,B)\in\mathcal{A}_{P,P'} $ and $ (C,D)\in\mathcal{A}_{Q,Q'} $ , either some corner separation of (A, B) and (C, D) lies in $ \mathcal{A}_{P,P'} $ or in $ \mathcal{A}_{Q,Q'} $ , or one of (A, B) and (C, D) lies in both $ \mathcal{A}_{P,P'} $ and $ \mathcal{A}_{Q,Q'} $ . In fact we will show that the first option always occurs.

We will split our proof of this into two separate lemmas, distinguishing the cases of equal and of distinct order of (A, B) and (C, D).

Let us first deal with the case that (A, B) is of strictly lower order than (C, D). In this case we can say precisely which of $ \mathcal{A}_{P,P'} $ and $ \mathcal{A}_{Q,Q'} $ will contain a corner separation of (A, B) and (C, D) :

Lemma 4·7 Let $ (A,B)\in\mathcal{A}_{P,P'} $ and $ (C,D)\in\mathcal{A}_{Q,Q'} $ with $ \lvert{(A,B)}\rvert<\lvert{(C,D)}\rvert$ . Then some corner separation of (A,B) and (C,D) lies in $ \mathcal{A}_{Q,Q'} $ .

Proof. Since $|(A,B)|<|(C,D)|$ it follows that both Q and Q $^\prime$ orient $\{A,B\}$ the same, say ${(A,B)\in Q\cap Q'}$ . If ${|(A,B) \lor (C,D)|\le |(C,D)|}$ or ${|(A,B) \lor (D,C)|\le |(C,D)|}$ , it follows that this corner separation efficiently distinguishes Q and Q $^\prime$ by Lemma 2·3, so suppose that this is not the case. Then submodularity implies that ${|(B,A) \lor (C,D)|< |(A,B)|}$ and ${|(B,A) \lor (D,C)|< |(A,B)|}$ , which in turn contradicts the efficiency of (A, B), since one of $(B,A) \lor (C,D)$ and $(B,A) \lor (D,C)$ would also distinguish the two robust profiles P and P $^\prime$ .

For separations r and s the corner separations given by $ \vec{r}\lor\vec{s} $ and $ \stackrel{\tiny\gets}{r}\lor\stackrel{\tiny\gets}{s} $ (as well as their underlying unoriented separations) are referred to as opposite corner separations.

The second case is that (A, B) and (C, D) are of equal order. Here we can show that there are two opposite corner separations of (A, B) and (C, D) that lie in $ \mathcal{A}_{P,P'} $ or in $ \mathcal{A}_{Q,Q'} $ :

Lemma 4·8 Let $ (A,B)\in\mathcal{A}_{P,P'} $ and $ (C,D)\in\mathcal{A}_{Q,Q'} $ with $ \lvert{(A,B)}\rvert=\lvert{(C,D)}\rvert$ . Then there is either a pair of two opposite corner separations of (A,B) and (C,D) with one element in $ \mathcal{A}_{P,P'} $ and one in $ \mathcal{A}_{Q,Q'} $ , or else there are two pairs of opposite corner separations of (A,B) and (C,D), the first with both elements in $ \mathcal{A}_{P,P'} $ and the second with both elements in $ \mathcal{A}_{Q,Q'} $ .

Proof. From $|(A,B)|=|(C,D)|$ it follows that P and P $^\prime$ both orient $ \{C,D\} $ , and likewise that Q and Q $^\prime$ both orient $ \{A,B\} $ .

Let us first treat the case that one of P and P $^\prime$ orients both $ \{A,B\} $ and $ \{C,D\} $ in the same way as one of Q and Q $^\prime$ does. So suppose that, say, both P and Q contain (A, B) as well as (C, D).

If P $^\prime$ contains (D, C), then $ (C,D)\in\mathcal{A}_{P,P'} $ and Lemma 2·5 gives $ {(A,B)\lor(C,D)\in\mathcal{A}_{P,P'}} $ and $ {(B,A)\lor(D,C)\in\mathcal{A}_{P,P'}} $ . Thus by property P we also have $ (A,B)\lor(C,D)\in\mathcal{A}_{Q,Q'} $ , producing the desired pair of opposite corner separations. If Q $^\prime$ contains (B, A) we argue analogously.

So suppose that $ (C,D)\in P' $ and $ (A,B)\in Q' $ . Then $ (B,A)\ \lor $ $(C,D)\in P' $ and $ (A,B)\;\lor(D,C)\in Q' $ by the profile property, since by submodularity and the efficiency of (A, B) and (C, D) both of these corner separations have order exactly $ \lvert{(A,B)}\rvert$ . These two separations, then, are opposite corner separations of (A, B) and (C, D) with the first lying in $ \mathcal{A}_{P,P'} $ and the second lying in $ \mathcal{A}_{Q,Q'} $ .

The remaining case is that no two of the four profiles agree in their orientation of $ \{A,B\} $ and $ \{C,D\} $ . But then both of (A, B) and (C, D) lie in $ \mathcal{A}_{P,P'} $ as well as in $ \mathcal{A}_{Q,Q'} $ , and the existence of two pairs of opposite corner separations, one with both elements in $ \mathcal{A}_{P,P'} $ and one with both in $ \mathcal{A}_{Q,Q'} $ , follows from Lemma 2·5 and the disagreement of the four profiles on $ \{A,B\} $ and $ \{C,D\} $ .

We now have all the ingredients necessary for a proof of Theorem 4·2:

Proof of Theorem 4·2. By Proposition 4·3, we can apply Theorem 3·5. Thus we only need to show that the collection of these sets $\mathcal{A}_{P,P'}$ splinters. However, this follows from Lemma 4·7 and Lemma 4·8.

We remark that even in locally finite graphs it is not generally possible to find a tree-decomposition which efficiently distinguishes all the distinguishable robust regular bounded profiles, as witnessed by the following example:

Fig. 1. A locally finite graph where no tree-decomposition distinguishes all the robust regular bounded profiles efficiently. The green separator is the one of the only separation which efficiently distinguishes the profile induced by the $K^{64}$ from the profile induced by the $K^{128}$ .

Example 4·9. Consider the graph displayed in Figure 1. This graph is constructed as follows: for every $n\in \mathbb{N}$ pick a copy of $K^{2^{n+2}}$ together with $n+3$ vertices $w_1^n,\dots,w_{n+3}^n$ . Pick $2^{n}$ vertices of the $K^{2^{n+2}}$ and call them $u_1^n,\dots,u_{2^n}^n$ . Additionally, pick $2^{n+1}$ vertices from $K^{2^{n+2}}$ , disjoint from the set of $u_i^n$ , and call them $v_1^n,\dots,v_{2^{n+1}}^n$ . Now identify $u_i^{n+1}$ with $v_i^n$ and add edges between every $w_i^n$ and every $w_j^{n+1}$ as well as between $w_i^n$ and $v_1^n=u_1^{n+1}$ .

Finally we pick one copy of $K^{10}$ and join one vertex $v_1^0$ of this $K^{10}$ to $u_1^1$ and $u_1^2$ . Additionally we pick two vertices $w_1^0,w_2^0$ which are distinct from $v_1^0$ from this $K^{10}$ and add an edge between each $w_i^0$ and each $w_j^1$ .

Now each of the chosen $K^{2^{n+2}}$ induces a robust profile $P_n$ of order $({2}/{3})\cdot2^{n+1}$ which obviously is regular and bounded. The only separation which efficiently distinguishes $P_n$ and $P_{n+1}$ is the separation $s_n$ with separator ${\{v_1^i\mid i<n\}\cup\{u_j^{n+1}\}}$ .

Additionally, the $K^{10}$ induces a robust profile $P_0$ of order 4. However the only separation that efficiently distinguishes $P_0$ and $P_1$ has the separator $\{v_1^0,w_1^0,w_2^0\}$ . But these separations $s_1,s_2,\ldots,$ and $s_0$ can be oriented such as to form a chain of order type $ \omega+1 $ . This chain witnesses that there cannot be a tree-decomposition which distinguishes all regular bounded profiles efficiently: the separations given by such a tree-decomposition would have to contain this chain of order type $ \omega+1 $ which is not possible as every chain in the edge tree set of a tree has length at most $\omega$ , cf. Lemma 2·7.

5. The thin splinter lemma

In this section we take a different approach to generalising the finite splinter lemma into an infinite setting. Unlike Theorem 3·5, the result we are going to prove does not require our universe to be ‘closed’. Instead we will require that the separations involved do not, in a sense, cross too badly in that they cross only finitely many separations of lower order.

This will allow us to choose separations that minimise the number of separations crossing them, an idea which also appeared in Carmesin’s original proof of Theorem 1·4 in [Reference Carmesin1], as well as in [Reference Carmesin, Hamann and Miraftab3] and our proof of the canonical splinter lemma for finite separation systems in [Reference Elbracht, Kneip and Teegen14]. However, our theorem here applies to a more general setting and will allow us directly to deduce Carmesin’s theorem for locally finite graphs.

In order to also be able to deduce the full Theorem 1·4 for arbitrary graphs, we will state our theorem in more generality here: not as a theorem about nestedness and separations, but as a theorem about a general nestedness-like relation. This allows us to apply the theorem in Section 6·2 not to separations directly, where it would fail, but to substitute separators as a proxy giving our Theorem 1·3. From this result we will retrieve the separations for our proof of Theorem 1·4 in Section 6·2·1, but we will also build from this a tree of tree-decompositions to deduce Theorem 1·5 in Section 6·2·2.

The statement of our Theorem 1·2 is also inspired by our canonical splinter lemma for the finite setting in [Reference Elbracht, Kneip and Teegen14], and it will too result in a canonical nested set, a set which is invariant under isomorphisms.

So let $ \mathcal{A} $ be some set and $ \sim $ a reflexive and symmetric binary relation on $ \mathcal{A} $ . In analogy to our terminology for separation systems, we say that two elements a and b of $ \mathcal{A} $ are nested if $ a\sim b $ . Elements of $ \mathcal{A} $ that are not nested cross. As usual, a subset of $ \mathcal{A} $ is nested if all of its elements are pairwise nested, and a single element is nested with a set N if it is nested with every element of N.

In an abuse of notation, given elements a and b of $ \mathcal{A} $ , we call $ c\in\mathcal{A} $ a corner of a and b if every element of $ \mathcal{A} $ crossing c also crosses one of a and b. Observe that with this definition corners of elements of $ \mathcal{A} $ exhibit the same behaviour as was asserted by Lemma 2·3 for corner separations. However, in contrast to the terminology of separation systems, we do not insist here that a corner of a and b is itself nested with both a and b. This distinction will become relevant in Section 6·2.

Now let $ (\,{\mathcal{A}_i\mid i\in I}\,)$ be a family of non-empty subsets of $ \mathcal{A} $ and $ \lvert{\;\;}\rvert\colon I\to\mathbb{N}_0 $ some function, where I is a possibly infinite index set. We shall think of $ \lvert{i}\rvert $ as the order of the elements of $ \mathcal{A}_i $ . For an $ a\in\mathcal{A} $ and $ k\in\mathbb{N}_0 $ the k-crossing number of a is the number of elements of $ \mathcal{A} $ that cross a and lie in some $ \mathcal{A}_i $ with $ \lvert{i}\rvert=k $ . This k-crossing number is either a natural number or infinity. The family $ (\,{\mathcal{A}_i\mid i\in I}\,)$ thinly splinters if it satisfies the following three properties:

  1. (i) for every $ i\in I $ all elements of $ \mathcal{A}_i $ have finite k-crossing number for all $ k\le\lvert{i}\rvert $ ;

  2. (ii) if $ a_i\in\mathcal{A}_i $ and $ a_j\in\mathcal{A}_j $ cross with $ \lvert{i}\rvert<\lvert{j}\rvert $ , then $ \mathcal{A}_j $ contains some corner of $ a_i $ and $ a_j $ that is nested with $ a_i $ ;

  3. (iii) if $ a_i\in\mathcal{A}_i $ and $ a_j\in\mathcal{A}_j $ cross with $ \lvert{i}\rvert=\lvert{j}\rvert=k \in \mathbb{N}_0 $ , then either $ \mathcal{A}_i $ contains a corner of $ a_i $ and $ a_j $ with strictly lower k-crossing number than $ a_i $ , or else $ \mathcal{A}_j $ contains a corner of $ a_i $ and $ a_j $ with strictly lower k-crossing number than $ a_j $ .

We are now ready to state and prove the main result of this section:

Theorem 1·2. If $(\,{\mathcal{A}_i \mid i\in I}\,)$ thinly splinters with respect to some reflexive symmetric relation $\sim$ on $\mathcal{A}\,{:\!=}\,\bigcup_{i\in I}\mathcal{A}_i$ , then there is a set $N\subseteq \mathcal{A}$ which meets every $\mathcal{A}_i$ and is nested, i.e., $n_1\sim n_2$ for all $n_1,n_2\in N$ . Moreover, this set N can be chosen invariant under isomorphisms: if $ \varphi $ is an isomorphism between $ (\mathcal{A},\sim)$ and $(\mathcal{A}',\sim')$ , then we have $ N((\,{\varphi(\mathcal{A}_i)\mid i\in I}\,))=\varphi(N((\,{\mathcal{A}_i\mid i\in I}\,))) $ .

Proof. We shall construct inductively, for each $ k\in\mathbb{N}_0 $ , a nested set $ N_k\subseteq\mathcal{A} $ extending $ N_{k-1} $ and meeting every $ \mathcal{A}_i $ with $ \lvert{i}\rvert\le k $ , so that the choice of $ N_k $ is invariant under isomorphisms. The desired nested set N will then be the union of all these sets $ N_k $ .

We set $ N_{-1}\,{:\!=}\,\emptyset $ . Suppose that for some $ k\in\mathbb{N}_0 $ we have already constructed a nested set $ N_{k-1} $ so that $ N_{k-1} $ is canonical and meets every $ \mathcal{A}_i $ with $ \lvert{i}\rvert\le k-1 $ . We shall construct a canonical nested set $ N_k\supseteq N_{k-1} $ that meets every $ \mathcal{A}_i $ with $ \lvert{i}\rvert\le k $ .

Let $ N^+_k $ be the set consisting of the following: for every