A faster algorithm for finding Tarski fixed points

TL;DR

This paper presents a new, faster algorithm for finding Tarski fixed points in higher dimensions, disproving previous conjectures of optimality and providing improved query complexity bounds.

Contribution

It introduces an $O( ext{log}^2 n)$ query algorithm for three-dimensional Tarski fixed points and a decomposition theorem leading to improved bounds in higher dimensions.

Findings

Disproved conjectures of optimality for the three-dimensional case

Developed an $O( ext{log}^2 n)$ query algorithm for 3D Tarski fixed points

Established an $O( ext{log}^{2 ext{ceil}(k/3)} n)$ query algorithm for k-dimensional problems

Abstract

Dang et al. have given an algorithm that can find a Tarski fixed point in a $k$-dimensional lattice of width $n$ using $O(\log^{k} n)$ queries. Multiple authors have conjectured that this algorithm is optimal [Dang et al., Etessami et al.], and indeed this has been proven for two-dimensional instances [Etessami et al.]. We show that these conjectures are false in dimension three or higher by giving an $O(\log^2 n)$ query algorithm for the three-dimensional Tarski problem. We also give a new decomposition theorem for $k$-dimensional Tarski problems which, in combination with our new algorithm for three dimensions, gives an $O(\log^{2 \lceil k/3 \rceil} n)$ query algorithm for the $k$-dimensional problem.