diff --git a/comparable-box-dimension.tex b/comparable-box-dimension.tex index 021af0dae9eed11df5592f59b3a3d103bde9234f..b05640aeba11f6bc5c29103d5c1346faaebecbf1 100644 --- a/comparable-box-dimension.tex +++ b/comparable-box-dimension.tex @@ -34,7 +34,7 @@ \title{On comparable box dimension} \author{Zden\v{e}k Dvo\v{r}\'ak\thanks{Computer Science Institute, Charles University, Prague, Czech Republic. E-mail: {\tt rakdver@iuuk.mff.cuni.cz}. Supported by the ERC-CZ project LL2005 (Algorithms and complexity within and beyond bounded expansion) of the Ministry of Education of Czech Republic.}\and -Daniel Gon\c{c}alves\thanks{...} +Daniel Gon\c{c}alves\thanks{...}\and Abhiruk Lahiri\thanks{...}\and Jane Tan\thanks{...}\and Torsten Ueckerdt\thanks{Karlsruhe Institute of Technology. E-mail: {\tt torsten.ueckerdt@kit.edu}}} @@ -53,20 +53,20 @@ comparable box dimension and explore further properties of this notion. \section{Introduction} For a system $\OO$ of subsets of $\mathbb{R}^d$, we say that a graph $G$ is a \emph{touching graph of objects from $\OO$} -if there exists a function $f:V(G)\to \OO$ (called the \emph{touching representation by objects from $\OO$}) +if there exists a function $f:V(G)\to \OO$ (called a \emph{touching representation by objects from $\OO$}) such that for distinct $u,v\in V(G)$, the interiors of $f(u)$ and $f(v)$ are disjoint and $f(u)\cap f(v)\neq\emptyset$ if and only if $uv\in E(G)$. Famously, Koebe~\cite{koebe} proved that a graph is planar if and only if it is a touching graph of balls in $\mathbb{R}^2$. -This result motivated a number of strenthenings and variations~\cite{...}; most relevantly for us, every planar graph is +This result motivated a number of strengthenings and variations~\cite{...}; most relevantly for us, every planar graph is a touching graph of cubes in $\mathbb{R}^3$~\cite{felsner2011contact}. An attractive feature of touching representation is that it makes it possible to represent graph classes that are sparse (e.g., planar graphs, or more generally, graph classes with bounded expansion theory~\cite{nesbook}), whereas in a general intersection representation, the represented class always includes arbitrarily large cliques. Of course, whether the class of touching graph of objects from $\OO$ is sparse or not depends on the system $\OO$. -For example, all complete bipartite graphs $K_{n,m}$ are touching graphs of axis-aligned boxes in $\mathbb{R}^d$, where the vertices in +For example, all complete bipartite graphs $K_{n,m}$ are touching graphs of axis-aligned boxes in $\mathbb{R}^3$, where the vertices in one part are represented by $m\times 1\times 1$ boxes and the vertices of the other part are represented by $1\times n\times 1$ -boxes (a \emph{box} is the cartesian product of intervals of non-zero length). +boxes (a \emph{box} is the Cartesian product of intervals of non-zero length). Dvo\v{r}\'ak, McCarty and Norin~\cite{subconvex} noticed that this issue disappears if we forbid such a combination of long and wide boxes: For two boxes $B_1$ and $B_2$, we write $B_1 \sqsubseteq B_2$ if a translation of $B_1$ is a subset of $B_2$. We say that $B_1$ and $B_2$ are \emph{comparable} if $B_1\sqsubseteq B_2$ or $B_2\sqsubseteq B_1$. @@ -82,7 +82,7 @@ numbers, which implies that $\GG$ has strongly sublinear separators. They also that for any function $h$, the class of graphs with strong coloring numbers bounded by $h$ has infinite comparable box dimension. Dvo\v{r}\'ak et al.~\cite{wcolig} proved that graphs of comparable box dimension $3$ have exponential weak coloring number, giving the -first natural graph class with olynomial strong coloring numbers and superpolynomial weak coloring numbers +first natural graph class with polynomial strong coloring numbers and superpolynomial weak coloring numbers (the previous example is obtained by subdividing edges of every graph suitably many times~\cite{covcol}). We show that the comparable box dimension behaves well under the operations of addition of apex vertices, @@ -94,13 +94,13 @@ The comparable box dimension of every proper minor-closed class of graphs is fin \end{theorem} Additionally, we show that classes of graphs with finite comparable box dimension are fractionally treewidth-fragile. -This gives arbitrarily precise approximation algorithms for alll monotone maximization problems that are +This gives arbitrarily precise approximation algorithms for all monotone maximization problems that are expressible in terms of distances between the solution vertices and tractable on graphs of bounded treewidth~\cite{distapx} or expressible in the first-order logic~\cite{logapx}. \section{Operations} -Let us start with a simple lemma saying that addition of a vertex increases the comparable box dimension by at most one. +Let us start with a simple lemma saying that the addition of a vertex increases the comparable box dimension by at most one. In particular, this implies that $\cbdim(G)\le |V(G)$. \begin{lemma}\label{lemma-apex} For any graph $G$ and $v\in V(G)$, we have $\cbdim(G)\le \cbdim(G-v)+1$. @@ -114,12 +114,15 @@ Then $h$ is a touching representation of $G$ by comparable boxes in $\mathbb{R}^ \end{proof} We need a bound on the clique number in terms of the comparable box dimension. -For a box $B=I_1\times \cdots I_d$ and $i\in\{1,\ldots,d\}$, let $B[i]=I_i$. +For a box $B=I_1\times \cdots \times I_d$ and $i\in\{1,\ldots,d\}$, let $B[i]=I_i$. \begin{lemma}\label{lemma-cliq} If $G$ has a touching representation $f$ by comparable boxes in $\mathbb{R}^d$, then $\omega(G)\le 2^d$. \end{lemma} \begin{proof} -... + For any clique $A = \{a_1,\ldots,a_w\}$ in $G$, the corresponding boxes $f(a_1),\ldots,f(a_w)$ have pairwise non-empty intersection. + Since axis-aligned boxes have the Helly property, there is a point $p \in \mathbb{R}^d$ contained in $f(a_1) \cap \cdots \cap f(a_w)$. + As each box is full-dimensional, its interior intersects at least one of the $2^d$ orthants at $p$. + Since $f$ is a touching representation, $f(a_1),\ldots,f(a_d)$ have pairwise disjoint interiors and hence $w \leq 2^d$. \end{proof} A \emph{tree decomposition} of a graph $G$ @@ -130,11 +133,11 @@ such that \item for each $v\in V(G)$, the set $\{x\in V(T):v\in\beta(x)\}$ is non-empty and induces a connected subtree of $T$. \end{itemize} For nodes $x,y\in V(T)$, we write $x\preceq y$ if $x=y$ or $x$ is a descendant of $y$ in $T$. -For each vertex $v\in V(G)$, let $p(v)$ be the node $x\in V(T)$ such that $v\in \beta(x)$ nearest to the root of $T$. +For each vertex $v\in V(G)$, let $p(v)$ be the node $x\in V(T)$ such that $v\in \beta(x)$ and $x$ is nearest to the root of $T$. The \emph{adhesion} of the tree decomposition is the maximum of $|\beta(x)\cap\beta(y)|$ over distinct $x,y\in V(T)$, and its \emph{width} is the maximum of the sizes of the bags minus $1$. The \emph{treewidth} of a graph is the minimum -of the widths of its tree decompositions. We will need to know that graphs of bounded treewidth have bounded dimension. -In fact, we will prove the following stronger fact (TODO: Was this published somehere before?) +of the widths of its tree decompositions. We will need to know that graphs of bounded treewidth have bounded comparable box dimension. +In fact, we will prove the following stronger fact (TODO: Was this published somewhere before? I am only aware of the upper bound of $t+2$ on the boxicity of $G$~\cite{box-treewidth}.) \begin{lemma}\label{lemma-tw} Let $(T,\beta)$ be a tree decomposition of a graph $G$ of width $t$. @@ -149,13 +152,13 @@ Moreover, the representation can be chosen so that no two hypercubes have the sa Next, let us deal with clique-sums. A \emph{clique-sum} of two graphs $G_1$ and $G_2$ is obtained from their disjoint union by identifying vertices of a clique in $G_1$ and a clique of the same size in $G_2$ and possibly deleting some of the edges of the resulting clique. The main issue to overcome in obtaining a representation for a clique-sum -is that the representations of $G_1$ and $G_2$ can be ``degenerate''. Consider e.g. the case that $G_1$ is represented -by unit squares arrangef in a grid; in this case, there is no space to attach $G_2$ at the cliques formed by four squares intersecting +is that the representations of $G_1$ and $G_2$ can be ``degenerate''. Consider e.g.\ the case that $G_1$ is represented +by unit squares arranged in a grid; in this case, there is no space to attach $G_2$ at the cliques formed by four squares intersecting in a single corner. This can be avoided by increasing the dimension, but we need to be careful so that the dimension stays bounded -even after arbitrary number of clique-sums. +even after an arbitrary number of clique-sums. It will be convenient to work in the setting of tree decompositions. -Consider a tree decompostion $(T,\beta)$ of a graphs $G$. +Consider a tree decomposition $(T,\beta)$ of a graph $G$. For each $x\in V(T)$, the \emph{torso} $G_x$ of $x$ is the graph obtained from $G[\beta(x)]$ by adding a clique on $\beta(x)\cap\beta(y)$ for each $y\in V(T)$. For a class of graphs $\GG$, we say that the tree decomposition is \emph{over $\GG$} if all the torsos belong to $\GG$. We use the following well-known fact. @@ -165,7 +168,7 @@ A graph $G$ is obtained from graphs in a class $\GG$ by repeated clique-sums if For each note $x\in V(T)$, let $\pi(x)=\{x\}\cup \{p(v):v\in\beta(x)\}$. Let $T_\beta$ be the graph with vertex set $V(T)$ such that $xy\in E(T_\beta)$ if and only if $x\in\pi(y)$ or $y\in\pi(x)$. \begin{lemma}\label{lemma-legraf} -If $(T,\beta)$ is a tree decompositon of $G$ of adhesion $a$, then $(T,\pi)$ is a tree decomposition of $T_\beta$ of width at most $a$. +If $(T,\beta)$ is a tree decomposition of $G$ of adhesion $a$, then $(T,\pi)$ is a tree decomposition of $T_\beta$ of width at most $a$. Moreover, $\pi(x)$ is a clique in $T_\beta$ and $p(x)=x$ for each $x\in V(T)$. \end{lemma} \begin{proof} diff --git a/data.bib b/data.bib index 1450d479ea92c1b7a0cb11548ba3ea294a655938..3ce675d0e15f0a5ecf9bf5b2492ce12098556234 100644 --- a/data.bib +++ b/data.bib @@ -5321,3 +5321,16 @@ note = {In Press} volume = {2103.08698}, year = {2021} } + +@article{box-treewidth, + author = {L. Sunil Chandran and Naveen Sivadasan}, + title = {Boxicity and treewidth}, + journal = {Journal of Combinatorial Theory, Series B}, + volume = {97}, + number = {5}, + pages = {733--744}, + year = {2007}, + issn = {0095-8956}, + doi = {https://doi.org/10.1016/j.jctb.2006.12.004}, + url = {https://www.sciencedirect.com/science/article/pii/S0095895607000111}, +}