Distributed Near-Equitable Coloring in the LOCAL Model

Abstract

For an $n$-vertex graph of maximum degree $Δ$, an equitable $(Δ+1)$-coloring is a proper coloring all of whose color classes have size $σ= n/(Δ+1)$ up to rounding. Known algorithms for relaxations of this target aggregate along a spanning structure, at cost scaling with the diameter $D$. We study near-equitable coloring in the LOCAL model and prove that, for deterministic algorithms, exact balance is inherently global: already on the cycle $C_n$, exact balanced free $3$-coloring requires $Ω(n)$ rounds. More generally, additive imbalance $g$ requires $Ω(n/g)$ rounds, matching a deterministic $O((n/g)\log^* n)$-round algorithm up to the $\log^* n$ factor, and these problems realize a dense family of intermediate deterministic LOCAL complexities $\tildeΘ(n^α)$, $α\in (0,1)$, on cycles. The exact bound is sharp in two further senses: the identifier-universe threshold is exactly $N=n+1$, and the $\log^* n$ gap cannot be closed via faster ruling-set anchors. The bound then extends to twisted fiber products, an explicit family realizing, for every $Δ\ge 3$, every diameter scale from the Moore bound $Θ(\log n/\logΔ)$ up to $Θ(n/Δ)$. Exact equitable $(Δ+1)$-coloring requires $Θ(D)$ rounds deterministically on this family, so exactness costs diameter time already on bounded-degree graphs of logarithmic diameter. In contrast, coarse balance admits local algorithms. On cycles, constant multiplicative equity costs $Θ(\log^* n)$ rounds. On general graphs with large color classes, all class sizes can be kept within $(1\pmη)$ times the average in time independent of the diameter: with palette exactly $Δ+1$ for $Δ\le 2^{\sqrt{\log n}/C}$, and with palette $(1+η)(Δ+1)$ for every $Δ\le n^{1-o(1)}$, the latter in $O(\log n\,(\log\log n)^2)$ rounds.