Unexpected Effects of Online no-Substitution k-means Clustering

TL;DR

This paper investigates online no-substitution k-means clustering, establishing tight bounds on the number of centers needed for constant approximation depending on data order and prior knowledge, revealing new insights into online clustering limits.

Contribution

It provides the first tight bounds for online no-substitution k-means clustering, highlighting the impact of data order and prior knowledge on clustering performance.

Findings

Theta(log n) centers suffice in random order for constant approximation

Knowing n reduces centers needed to a constant

Bounds hold for any triangle-inequality distance function

Abstract

Offline k-means clustering was studied extensively, and algorithms with a constant approximation are available. However, online clustering is still uncharted. New factors come into play: the ordering of the dataset and whether the number of points, n, is known in advance or not. Their exact effects are unknown. In this paper we focus on the online setting where the decisions are irreversible: after a point arrives, the algorithm needs to decide whether to take the point as a center or not, and this decision is final. How many centers are needed and sufficient to achieve constant approximation in this setting? We show upper and lower bounds for all the different cases. These bounds are exactly the same up to a constant, thus achieving optimal bounds. For example, for k-means cost with constant k>1 and random order, Theta(log n) centers are enough to achieve a constant approximation, while the mere a priori knowledge of n reduces the number of centers to a constant. These bounds hold for any distance function that obeys a triangle-type inequality.