Robust Algorithms for Online k-means Clustering

In the online version of the classic k-means clustering problem, the points of a dataset u(1), u(2),... arrive one after another in an arbitrary order. When the algorithm sees a point, it should either add it to the set of centers, or let go of the point. Once added, a center cannot be removed. The goal is to end up with set of roughly k centers, while competing in k-means objective value with the best set of k centers in hindsight. Online versions of k-means and other clustering problem have received significant attention in the literature. The key idea in many algorithms is that of adaptive sampling: when a new point arrives, it is added to the set of centers with a probability that depends on the distance to the centers chosen so far. Our contributions are as follows: 1. We give a modified adaptive sampling procedure that obtains a better approximation ratio (improving it from logarithmic to constant). 2. Our main result is to show how to perform adaptive sampling when data has outliers (>> k points that are potentially arbitrarily far from the actual data, thus rendering distance-based sampling prone to picking the outliers). 3. We also discuss lower bounds for k-means clustering in an online setting.