A Faster Algorithm for Finding Tarski Fixed Points

John Fearnley, Dömötör Pálvölgyi, Rahul Savani

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 [2]. Multiple authors have conjectured that this algorithm is optimal [2, 7], and indeed this has been proven for two-dimensional instances [7]. We show that these conjectures are false in dimension three or higher by giving an O(log2 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(log2 ⌈k/3⌉ n) query algorithm for the k-dimensional problem.