Online Sum-Radii Clustering

TL;DR

This paper analyzes the online sum-radii clustering problem, establishing its competitive ratio bounds in various metric spaces, and introduces algorithms with provable performance guarantees.

Contribution

It provides tight bounds on the deterministic and randomized competitive ratios for online sum-radii clustering in different metric spaces, and relates the problem to the Parking Permit problem.

Findings

Deterministic competitive ratio is (log n) in general metric spaces.

A randomized (log n) algorithm is proposed.

A deterministic (log log n) fractional algorithm is developed.

Abstract

In Online Sum-Radii Clustering, n demand points arrive online and must be irrevocably assigned to a cluster upon arrival. The cost of each cluster is the sum of a fixed opening cost and its radius, and the objective is to minimize the total cost of the clusters opened by the algorithm. We show that the deterministic competitive ratio of Online Sum-Radii Clustering for general metric spaces is \Theta(\log n), where the upper bound follows from a primal-dual algorithm and holds for general metric spaces, and the lower bound is valid for ternary Hierarchically Well-Separated Trees (HSTs) and for the Euclidean plane. Combined with the results of (Csirik et al., MFCS 2010), this result demonstrates that the deterministic competitive ratio of Online Sum-Radii Clustering changes abruptly, from constant to logarithmic, when we move from the line to the plane. We also show that Online Sum-Radii Clustering in metric spaces induced by HSTs is closely related to the Parking Permit problem introduced by (Meyerson, FOCS 2005). Exploiting the relation to Parking Permit, we obtain a lower bound of \Omega(\log\log n) on the randomized competitive ratio of Online Sum-Radii Clustering in tree metrics. Moreover, we present a simple randomized O(\log n)-competitive algorithm, and a deterministic O(\log\log n)-competitive algorithm for the fractional version of the problem.