Profile and neighbourhood complexity of graphs excluding a minor and tree-structured graphs
Laurent Beaudou, Jan Bok, Florent Foucaud, Daniel A. Quiroz, Jean-Florent Raymond
TL;DR
This work advances the understanding of r-neighbourhood and r-profile complexities across structured graph classes. It introduces and leverages guarding sets, least common ancestor closures, and refined colourings to derive near-optimal bounds for graphs with bounded treewidth, outerplanar graphs, and several minor- and expansion-closed classes, including chordal, interval, and treelength-bounded graphs. Key results include pc_r(G,k) = O_t(r^{t+1}k) for treewidth-bounded graphs, pc_r(G,k) = O(r^2k) for outerplanar graphs, and explicit minor-exclusion bounds pc_r(G,k) ≤ 4^h(h-3)h^{2(h-1)}(r+1)^{3(h-1)}k for K_h-minor-free graphs, among others. The paper also connects these complexities to metric dimension and locating-dominating sets, and poses several open questions on tightness and extensions to new graph classes and expansions.
Abstract
The \emph{$r$-neighbourhood complexity} of a graph $G$ is the function counting, for a given integer $k$, the largest possible number, over all vertex-subsets $A$ of size $k$, of subsets of $A$ realized as the intersection between the $r$-neighbourhood of some vertex and $A$. A~refinement of this notion is the \emph{$r$-profile complexity}, that counts the maximum number of distinct distance-vectors from any vertex to the vertices of $A$, ignoring distances larger than~$r$. Typically, in structured graph classes such as graphs of bounded VC-dimension or chordal graphs, these functions are bounded, leading to insights into their structural properties and efficient algorithms. We improve existing bounds on the $r$-profile complexity (and thus on the $r$-neighbourhood complexity) for graphs in several structured graph classes. We show that the $r$-profile complexity of graphs excluding $K_h$ as a minor is in $O_h(r^{3h-3}k)$. For graphs of treewidth at most~$t$, we give a bound in $O_t(r^{t+1}k)$, which is tight up to a function of~$t$ as a factor. These bounds improve results of Joret and Rambaud and answer a question of their paper [Combinatorica, 2024]. We also apply our methods to other classes of bounded expansion such as graphs excluding a fixed complete graph as a subdivision. For outerplanar graphs, we can improve our treewidth bound by a factor of $r$ and conjecture that a similar improvement holds for graphs with bounded simple treewidth. For graphs of treelength at most~$\ell$, we give the upper bound of $O(k(r^2(\ell+1)^k))$, which we improve to $O\left (k\cdot (r 2^k + r^2k^2) \right)$ in the case of chordal graphs and $O(k^2r)$ for interval graphs. Our bounds also imply relations between the order, diameter and metric dimension of graphs in these classes, improving results from [Beaudou et al., SIDMA 2017].