Random Cuts are Optimal for Explainable k-Medians

We show that the RANDOMCOORDINATECUT algorithm gives the optimal competitive ratio for explainable k-medians in ℓ1. The problem of explainable k-medians was introduced by Dasgupta, Frost, Moshkovitz, and Rashtchian in 2020. Several groups of authors independently proposed a simple polynomial-time randomized algorithm for the problem and showed that this algorithm is O(logkloglogk) competitive. We provide a tight analysis of the algorithm and prove that its competitive ratio is upper bounded by 2lnk +2. This bound matches the Ω(logk)lower bound by Dasgupta et al (2020).