English

Minimal reconstructions of a coloring

Combinatorics 2024-07-17 v2

Abstract

A coloring on a finite or countable set XX is a function φ:[X]2{0,1}\varphi: [X]^{2} \to \{0,1\}, where [X]2[X]^{2} is the collection of unordered pairs of XX. The collection of homogeneous sets for φ\varphi, denoted by Hom(φ)Hom(\varphi), consist of all HXH \subseteq X such that φ\varphi is constant on [H]2[H]^2; clearly, Hom(φ)=Hom(1φ)Hom(\varphi) = Hom(1-\varphi). A coloring φ\varphi is \textit{reconstructible} up to complementation from its homogeneous sets if, for any coloring ψ\psi on XX such that Hom(φ)=Hom(ψ)Hom(\varphi) = Hom(\psi), either ψ=φ\psi = \varphi or ψ=1φ\psi = 1-\varphi. By R\mathcal{R} we denote the collection of all colorings reconstructible from their homogeneous sets. Let φ\varphi and ψ\psi be colorings on XX, and set D(φ,ψ)={{x,y}[X]2:  ψ{x,y}φ{x,y}}. D(\varphi, \psi) = \{ \{x,y\} \in [X]^2: \; \psi\{x,y\} \neq \varphi\{x,y\}\}. If φ∉R\varphi\not\in \mathcal{R}, let r(φ)=min{D(φ,ψ):  Hom(φ)=Hom(ψ),ψφ,ψ1φ}. r(\varphi) = \min\{|D(\varphi, \psi)|: \; Hom(\varphi) = Hom(\psi), \, \psi \neq \varphi, \, \psi \neq 1-\varphi\}. A coloring ψ\psi such that Hom(φ)=Hom(ψ)Hom(\varphi)=Hom(\psi), φψ\varphi\neq \psi and 1φψ1-\varphi\neq \psi is called a {\em non trivial reconstruction} of φ\varphi. If, in addition, r(φ)=D(φ,ψ)r(\varphi) =|D(\varphi, \psi)|, we call ψ\psi a {\em minimal reconstruction} of φ\varphi. The purpose of this article is to study the minimal reconstructions of a coloring. We show that, for large enough XX, r(φ)r(\varphi) can only takes the values 11 or 44.

Keywords

Cite

@article{arxiv.2403.08104,
  title  = {Minimal reconstructions of a coloring},
  author = {Diego Gamboa and Carlos Uzcategui-Aylwin},
  journal= {arXiv preprint arXiv:2403.08104},
  year   = {2024}
}
R2 v1 2026-06-28T15:18:01.102Z