diff --git a/arxiv_cbd.tex b/arxiv_cbd.tex index 521b75f525cfc229920b8f384ddb06e69d87418e..829c42bb3046c07dff64b6d893314226df67655d 100644 --- a/arxiv_cbd.tex +++ b/arxiv_cbd.tex @@ -670,6 +670,12 @@ of $\cbdim(G)$ and $\chi(G)$. Therefore, (c2) holds. \end{proof} +A touching representation of axis-aligned boxes in $\mathbb{R}^d$ is said \emph{fully touching} if any two intersecting boxes intersect on a $(d-1)$-dimensional box. Note that the construction above is fully touching. +Indeed, two intersecting boxes corresponding to vertices $u,v$ of colors $c(u) < c(v)$, only touch at coordinate $2/5$ in the $(d+c(u))^\text{th}$ dimension, while they fully intersect in every other dimension. This observation with Lemma~\ref{lemma-chrom} lead to the following. +\begin{corollary} +\label{cor-fully-touching} +Any graph $G$ has a fully touching representation of comparable axis-aligned boxes in $\mathbb{R}^d$, where $d= \cbdim(G) + 3^{\cbdim(G)}$. +\end{corollary} Together, the lemmas from this section show that comparable box dimension is almost preserved by full clique-sums. @@ -778,7 +784,7 @@ $h(v_i)[j] \cap h^\varepsilon(C)[j] = [p(C)[j],p(C)[j]+\varepsilon]$ for suffici \end{proof} The \emph{treewidth} $\tw(G)$ of a graph $G$ is the minimum $k$ such that $G$ is a subgraph of a $k$-tree. Note that actually the bound on the comparable box dimension of Theorem~\ref{thm-ktree} -extends to graphs of treewidth at most $k$ (see the proof in the appendix). +extends to graphs of treewidth at most $k$. \begin{corollary}\label{cor-tw} Every graph $G$ satisfies $\cbdim(G)\le\tw(G)+1$. \end{corollary} @@ -791,8 +797,8 @@ a representation $h$ of $T-V(C^\star)$ in $\mathbb{R}^{k+1}$ such that \item the vertices are represented by hypercubes of pairwise different sizes, \item if $uv\in E(T-V(C^\star))$ and $h(u)\sqsubseteq h(v)$, then $h(u)\cap h(v)$ is a facet of $h(u)$ incident with its point with minimum coordinates, and -\item for each vertex $u$ and each facet of $h(u)$ incident with its point with minimum coordinates, there exists -at most one vertex $v$ such that $uv\in E(T-V(C^\star))$ and $h(u)\sqsubseteq h(v)$. +%\item for each vertex $u$ and each facet of $h(u)$ incident with its point with minimum coordinates, +%there exists at most one vertex $v$ such that $uv\in E(T-V(C^\star))$ and $h(u)\sqsubseteq h(v)$. \end{itemize} If for some $u,v\in V(G)$, we have $uv\in E(T)\setminus E(G)$, where without loss of generality $h(u)\sqsubseteq h(v)$, we now alter the representation by shrinking $h(u)$ slightly away from $h(v)$ (so that all other touchings are preserved). @@ -848,10 +854,44 @@ such that $\text{Pr}[v\in X]\le 1/k$ for each $v\in V(G)$). For example, the class of planar graphs is (fractionally) treewidth-fragile, since we can let $X_i$ consist of the vertices of color $i-1$ in the coloring described at the beginning of the section. + +Before going further, let us recall some notions about treewidth. + A \emph{tree decomposition} of a graph $G$ is a pair + $(T,\beta)$, where $T$ is a rooted tree and $\beta:V(T)\to 2^{V(G)}$ + assigns a \emph{bag} to each of its nodes, such that +\begin{itemize} +\item for each $uv\in E(G)$, there exists $x\in V(T)$ such that + $u,v\in\beta(x)$, and +\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 $T$. +The \emph{width} of the tree decomposition 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. Let us remark that the value of treewidth obtained via this definition coincides +with the one via $k$-trees which we used in the previous section. + Our main result is that all graph classes of bounded comparable box dimension are fractionally treewidth-fragile. We will show the result in a more general setting, motivated by concepts from~\cite{subconvex} and by applications to related representations. The argument is motivated by the idea used in the approximation algorithms for disk graphs -by Erlebach et al.~\cite{erlebach2005polynomial}. +by Erlebach et al.~\cite{erlebach2005polynomial}. Before introducing this more general setting, and as a warm-up, let us outline +how to prove that disk graphs of thickness $t$ are fractionally treewidth-fragile. Consider first unit disk graphs. +By partitionning the plane with a random grid $\HH$, having squared cells of side-length $2k$, any unit disk has probability $1/2k$ +to intersect a vertical (resp. horizontal) line of the grid. By union bound, any disk has probability at most $1/k$ to intersect +the grid. Considering this probability distribution, let us now show that removing the disks intersected by the grid leads to a +unit disk graph of bounded treewidth. Indeed, in such a graph any connected component corresponds to unit disks contained in the +same cell of the grid. Such cell having area bounded by $4k^2$, there are at most $16tk^2/\pi$ disks contained in a cell. +The size of the connected components being bounded, so is the treewidth. Note that this distribution also works if we are given +disks whose diameter lie in a certain range. If any diameter $\delta$ is such that $1/c \le \delta \le 1$, then the same process +with a random grid of $2k\times 2k$ cells, ensures that any disk is deleted with probability at most $1/k$, while now the +connected components have size at most $4tc^2k^2/\pi$. Dealing with arbitrary disk graphs (with any diameter $\delta$ being in the range +$0< \delta \le 1$) requires to delete more disks. This is why each $(2k\times 2k)$-cell is now partitionned in a quadtree-like manner. +Now a disk with diameter between $\ell /2$ and $\ell$ (with $\ell =1/2^i$ for some integer $i\ge 0$) is deleted if it is not contained +in a $(2k\ell \times 2k\ell)$-cell of a quadtree. It is not hard to see that a disk is deleted with probability at most $1/k$. +To prove that the remaining graph has bounded treewidth one should consider the following tree decomposition $(T,\beta)$. The +tree $T$ is obtained by linking the roots of the quadtrees we used (as trees) to a new common root. +Then for a $(2k\ell \times 2k\ell)$-cell $C$, $\beta(C)$ contains all the disks of diameter at least $\ell/2$ intersecting $C$. +To see that such bag is bounded consider the $((2k+1)\ell \times (2k+1)\ell)$ square $C'$ centered on $C$, and note that any +disk in $\beta(C)$ intersects $C'$ on an area at least $\pi\ell^2/16$. This implies that $|\beta(C)| \le 16t(2k+1)^2 / \pi$. For a measurable set $A\subseteq \mathbb{R}^d$, let $\vol(A)$ denote the Lebesgue measure of $A$. For two measurable subsets $A$ and $B$ of $\mathbb{R}^d$ and a positive integer $s$, we write $A\sqsubseteq_s B$ @@ -876,20 +916,6 @@ the smallest axis-aligned hypercube containing $f(v)$, then there exists a posit $(f,\omega)$ is an $s_d$-comparable envelope representation of $G$ in $\mathbb{R}^d$ of thickness at most $2$. \end{itemize} -Let us recall some notions about treewidth. - A \emph{tree decomposition} of a graph $G$ is a pair - $(T,\beta)$, where $T$ is a rooted tree and $\beta:V(T)\to 2^{V(G)}$ - assigns a \emph{bag} to each of its nodes, such that -\begin{itemize} -\item for each $uv\in E(G)$, there exists $x\in V(T)$ such that - $u,v\in\beta(x)$, and -\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 $T$. -The \emph{width} of the tree decomposition 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. Let us remark that the value of treewidth obtained via this definition coincides -with the one via $k$-trees which we used in the previous section. \begin{theorem}\label{thm-twfrag} For positive integers $t$, $s$, and $d$, the class of graphs @@ -1022,4 +1048,4 @@ has a sublinear separator of size $O_{t,s,d}\bigl(|V(G)|^{\tfrac{d}{d+1}}\bigr). %Every graph $G$ satisfies $\cbdim(G)\le\tw(G)+1$. %\end{corollary-C} -\end{document} \ No newline at end of file +\end{document}