diff --git a/comparable-box-dimension.tex b/comparable-box-dimension.tex index 94bf6043b8a866fa712b50ba46292161524e5c89..a4e7bf6c160b11f4f3c5971d1d1e7812940be9eb 100644 --- a/comparable-box-dimension.tex +++ b/comparable-box-dimension.tex @@ -5,6 +5,7 @@ \usepackage{amssymb} \usepackage{colonequals} \usepackage{epsfig} +\usepackage{tikz} \usepackage{url} \newcommand{\GG}{{\cal G}} \newcommand{\HH}{{\cal H}} @@ -23,6 +24,7 @@ \newcommand{\vol}{\brm{vol}} %%%%% +\newcommand{\note}[1]{\textcolor{blue}{#1}} \newtheorem{theorem}{Theorem} \newtheorem{corollary}[theorem]{Corollary} @@ -37,7 +39,7 @@ 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{...}\and Abhiruk Lahiri\thanks{...}\and -Jane Tan\thanks{...}\and +Jane Tan\thanks{Mathematical Institute, University of Oxford, Oxford OX2 6GG, UK. E-mail: {\tt jane.tan@maths.ox.ac.uk}}\and Torsten Ueckerdt\thanks{Karlsruhe Institute of Technology. E-mail: {\tt torsten.ueckerdt@kit.edu}}} \date{} @@ -45,9 +47,7 @@ Torsten Ueckerdt\thanks{Karlsruhe Institute of Technology. E-mail: {\tt torsten \maketitle \begin{abstract} -The comparable box dimension of a graph $G$ is the minimum integer $d$ such that $G$ can be represented -as a touching graph of comparable boxes in $\mathbb{R}^d$ (two boxes are comparable if one of them is -a subset of a translation of the other one). We show that proper minor-closed classes have bounded +Two boxes in $\mathbb{R}^d$ are comparable if one of them is a subset of a translation of the other. The comparable box dimension of a graph $G$ is the minimum integer $d$ such that $G$ can be represented as a touching graph of comparable axis-aligned boxes in $\mathbb{R}^d$. We show that proper minor-closed classes have bounded comparable box dimension and explore further properties of this notion. \end{abstract} @@ -58,18 +58,17 @@ if there exists a function $f:V(G)\to \OO$ (called a \emph{touching representati 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 strengthenings and variations~\cite{...}; most relevantly for us, every planar graph is -a touching graph of cubes in $\mathbb{R}^3$~\cite{felsner2011contact}. +This result motivated a number of strengthenings and variations (see \cite{graphsandgeom, sachs94} for some classical examples); 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. +An attractive feature of touching representations is that it is possible to represent graph classes that are sparse +(e.g., planar graphs, or more generally, graph classes with bounded expansion theory~\cite{nesbook}). +This is in contrast to general intersection representations where 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 $\mathbb{R}^3$, 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, in particular axis-aligned). 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 $B_2$. +long and wide boxes: For two boxes $B_1$ and $B_2$, we write $B_1 \sqsubseteq B_2$ if $B_2$ contains a translate of $B_1$. We say that $B_1$ and $B_2$ are \emph{comparable} if $B_1\sqsubseteq B_2$ or $B_2\sqsubseteq B_1$. A \emph{touching representation by comparable boxes} of a graph $G$ is a touching representation $f$ by boxes such that for every $u,v\in V(G)$, the boxes $f(u)$ and $f(v)$ are comparable. For a graph $G$, let the \emph{comparable box dimension} $\cbdim(G)$ @@ -102,12 +101,13 @@ or expressible in the first-order logic~\cite{logapx}. \section{Operations} 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 $\cbdim(G)\le |V(G)$. +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 $\cbdim(G)\le \cbdim(G-v)+1$. \end{lemma} \begin{proof} Let $f$ be a touching representation of $G-v$ by comparable boxes in $\mathbb{R}^d$, where $d=\cbdim(G-v)$. +We define a representation $h$ of $G$ as follows. For each $u\in V(G)\setminus\{v\}$, let $h(u)=[0,1]\times f(u)$ if $uv\in E(G)$ and $h(u)=[1/2,3/2]\times f(u)$ if $uv\not\in E(G)$. Let $h(v)=[-1,0]\times [-M,M] \times \cdots \times [-M,M]$, where $M$ is chosen large enough so that $f(u)\subseteq [-M,M] \times \cdots \times [-M,M]$ for every $u\in V(G)\setminus\{v\}$. @@ -120,7 +120,7 @@ For a box $B=I_1\times \cdots \times I_d$ and $i\in\{1,\ldots,d\}$, let $B[i]=I_ 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. +To represent 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 $w \leq 2^d$. @@ -138,11 +138,11 @@ For each vertex $v\in V(G)$, let $p(v)$ be the node $x\in V(T)$ such that $v\in 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 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}.) +In fact, we will prove the following stronger fact. \note{(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 $t$. -Then $G$ has a touching representation $h$ by axis-aligned hypercubes in $R^{t+1}$ such that +Then $G$ has a touching representation $h$ by axis-aligned hypercubes in $\mathbb{R}^{t+1}$ such that for $u,v\in V(G)$, if $p(u)\prec p(v)$, then $h(u)\sqsubset h(v)$. Moreover, the representation can be chosen so that no two hypercubes have the same size. \end{lemma} @@ -153,7 +153,7 @@ and that for each $x\in V(T)$ with parent $y$, we have $|\beta(x)\setminus\beta( we can subdivide the edge $xy$, introducing $|\beta(x)\setminus\beta(y)|-1$ new vertices, and set their bags appropriately). It is now natural to relabel the vertices of $G$ so that $V(G)=V(T)$, by giving the unique vertex in $\beta(x)\setminus\beta(y)$ -the label $x$. In particular, $p(x)=x$. Furthermore, we can assume that $y\in\beta(x)$. +the label $x$. In particular, $p(x)=x$. Furthermore, switching to this new labeling, we can assume that $y\in\beta(x)$. Otherwise, if $y$ is not the root of $T$, we can replace the edge $xy$ by the edge from $x$ to the parent of $y$. If $y$ is the root of $T$, then the subtree rooted in $x$ induces a union of connected components in $G$, and we can process this subtree separately from the @@ -211,7 +211,7 @@ Hence, $\min(h(w)[j])-2\epsilon\le \min(h''(w_1)[j])\le \max(h''(w_1)[j])\le \ma Since $h(x_i)[j]\subseteq h''(w_1)[j]$ by (a) and the length of the interval $h(x_i)[j]$ is greater than $2\varepsilon$, we conclude that $h(x_i)[j]\cap h(w)[j]\neq\emptyset$. Therefore, the boxes $h(w)$ and $h(x_i)$ touch. -Consider now two non-adjacent vertices of $G$, say $x_i$ and $w$. As we noted before, if $x_i$ and $w$ are +Finally, consider two non-adjacent vertices of $G$, say $x_i$ and $w$. As we noted before, if $x_i$ and $w$ are incomparable in $\prec$, then (a) and (b) implies that the boxes $h(x_i)$ and $h(w)$ are disjoint. Suppose now that say $x_i\prec w$, and let $j=\varphi(w)$. If $w\in\beta(x_i)$, then $h(x_i)[j]$ and $h(w)[j]$ are disjoint by (i). Otherwise, let $y$ be the last vertex on the path from $w$ to $x_i$ in $T$ such that $w\in\beta(y)$ and let $z$ be the child of $y$ @@ -310,7 +310,7 @@ $xy\in E(T_\beta)$. Hence, again we have $f(u)\cap f(v)\neq\emptyset$. We can now combine Theorem~\ref{thm-cs} with Lemma~\ref{lemma-cliq}. \begin{corollary}\label{cor-cs} If $G$ is obtained from graphs in a class $\GG$ by clique-sums, then there exists a graph $G'$ such that -$G\subseteq G'$ and $\cbdim(G')\le (\cbdim(\GG)+1)\bigl(2^{\cbdim(\GG)}+1\bigr)\le 6^{\cbdim(\GG)}$. +$G\subseteq G'$ and \[\cbdim(G')\le (\cbdim(\GG)+1)\bigl(2^{\cbdim(\GG)}+1\bigr)\le 6^{\cbdim(\GG)}.\] \end{corollary} Note that only bound the comparable box dimension of a supergraph @@ -407,7 +407,7 @@ Any graph $G$ is a subgraph of the strong product of a path, a graph of threewid \item $t=4$ and $m=\max(2g,3)$ if $G$ has Euler genus at most $g$. \end{itemize} Moreover, for every $k$, there exists $t$ such that if $K_k\not\preceq_m G$, then $G$ is a subgraph of a clique-sum -for graphs that can be obtained from the strong product of a path and a graph of treewidth at most $t$ by adding +of graphs that can be obtained from the strong product of a path and a graph of treewidth at most $t$ by adding at most $t$ apex vertices. \end{theorem} @@ -420,13 +420,10 @@ comparable box dimension at most $t+2+\lceil \log_2 m\rceil$. Without loss of generality, we can assume that $k=\log_2 m$ is an integer and the vertices of $K_m$ are elements of $\{0,1\}^k$. Moreover, we can assume that $V(P)=\{1,\ldots,n\}$ with $ij\in E(P)$ iff $|i-j|=1$. Let $h$ be the touching representation of $T$ by -hypercubes in $\mathbb{R}^{t+1}$ obtained by Lemma~\ref{lemma-tw}. - -The representation $f$ of $P\boxtimes T\boxtimes K_m$ in $\mathbb{R}^{t+2+k}$ -is obtained as follows: For $p\in V(P)$, $v\in V(T)$, and $(x_1,\ldots, x_k)\in V(K_m)$, +hypercubes in $\mathbb{R}^{t+1}$ obtained by Lemma~\ref{lemma-tw}. Then an explicit representation $f$ of $P\boxtimes T\boxtimes K_m$ in $\mathbb{R}^{t+2+k}$ can be obtained as follows. For $p\in V(P)$, $v\in V(T)$, and $(x_1,\ldots, x_k)\in V(K_m)$, we set $$f(p,v,x_1, \ldots, x_k)[j]=\begin{cases} -f(v)[j]&\text{ for $j=1,\ldots, t+1$}\\ +h(v)[j]&\text{ for $j=1,\ldots, t+1$}\\ [p,p+1]&\text{ if $j=t+2$}\\ [x_{j-t-2},x_{j-t-2}+1]&\text{ if $j>t+2$.} \end{cases}$$ @@ -444,7 +441,7 @@ Moreover, for every $k$ there exists $d$ such that if $K_k\not\preceq_m(G)$, the $\cbdim(G)\le d$. \end{corollary} Note that the graph obtained from $K_{2n}$ by deleting a perfect matching has Euler genus $\Theta(n^2)$ -and comparable box dimension $n$, showing that the dependence of the comparable box dimension cannot be +and comparable box dimension $n$. It follows that the dependence of the comparable box dimension cannot be subpolynomial (though the degree $\log_2 81$ of the polynomial established in Corollary~\ref{cor-minor} certainly can be improved). The dependence of the comparable box dimension on the size of the forbidden minor that we established is not explicit, as Theorem~\ref{thm-prod} is based on the structure theorem of Robertson and Seymour~\cite{robertson2003graph}. diff --git a/data.bib b/data.bib index 3ce675d0e15f0a5ecf9bf5b2492ce12098556234..dfa99649d4e19ae478aa06414bbbf7c9ec954793 100644 --- a/data.bib +++ b/data.bib @@ -5254,7 +5254,7 @@ note="In Press" @article{mohar2002coloring, title={Coloring Eulerian triangulations of the projective plane}, author={Mohar, Bojan}, - journal={Discrete mathematics}, + journal={Discrete Mathematics}, volume=244, pages={339--344}, year=2002 @@ -5334,3 +5334,19 @@ note = {In Press} doi = {https://doi.org/10.1016/j.jctb.2006.12.004}, url = {https://www.sciencedirect.com/science/article/pii/S0095895607000111}, } + +@book{graphsandgeom, + author= {L. Lov\'asz}, + title={Graphs and Geometry}, + publisher={American Mathematical Society}, + year=2019, + address = "Providence"} + +@article{sachs94, + author = {Horst Sachs}, + title = {Coin graphs, polyhedra, and conformal mapping }, + journal = {Discrete Mathematics}, + volume = {134}, + year=1994, + pages={133--138}} +}