A Ramsey-type theorem for metric spaces and its applications for Metrical Task Systems and related problems

This paper gives a nearly logarithmic lower bound on the randomized competitive ratio for the Metrical Task Systems model. This implies a similar lower bound for the extensively studied K-server problem. Our proof is based on proving a Ramsey-type theorem for metric spaces. In particular we prove that in every metric space there exists a large subspace which is approximately a "hierarchically well-separated tree" (HST). This theorem may be of independent interest.