Tarski’s theorem states that every monotone function from a complete lattice to itself has a fixed point. We analyze the query complexity of finding such a fixed point on the k-dimensional grid of side length n under the ≤ relation. In this setting, there is an unknown monotone function f: {0,1,…, n-1}^k → {0,1,…, n-1}^k and an algorithm must query a vertex v to learn f(v). The goal is to find a fixed point of f using as few oracle queries as possible. We show that the randomized query complexity of this problem is Ω((k⋅log²n)/log k) for all n,k ≥ 2. This unifies and improves upon two prior results: a lower bound of Ω(log²n) from [Etessami et al., 2020] and a lower bound of Ω((k⋅log(n)/log(k)) from [Brânzei et al., 2024], respectively.
@InProceedings{branzei_et_al:LIPIcs.APPROX/RANDOM.2025.52, author = {Br\^{a}nzei, Simina and Phillips, Reed C. and Recker, Nicholas J.}, title = {{Tarski Lower Bounds from Multi-Dimensional Herringbones}}, booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2025)}, pages = {52:1--52:12}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-397-3}, ISSN = {1868-8969}, year = {2025}, volume = {353}, editor = {Ene, Alina and Chattopadhyay, Eshan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2025.52}, URN = {urn:nbn:de:0030-drops-244186}, doi = {10.4230/LIPIcs.APPROX/RANDOM.2025.52}, annote = {Keywords: Tarski’s theorem, monotone functions, lattices, fixed points, computational complexity, oracle model, query complexity, lower bounds} }
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