Overcoming the l1 non-embeddability barrier: algorithms for product metrics

• Authors: Alexandr Andoni;Piotr Indyk;Robert Krauthgamer
• Venue: SODA '09 Proceedings of the twentieth Annual ACM-SIAM Symposium on Discrete Algorithms
• Year: 2009

A common approach for solving computational problems over a difficult metric space is to embed the "hard" metric into L1 which admits efficient algorithms and is thus considered an "easy" metric. This approach has proved successful or partially successful for important spaces such as the edit distance, but it also has inherent limitations: it is provably impossible to go below certain approximation for some metrics. We propose a new approach, of embedding the difficult space into richer host spaces, namely iterated products of standard spaces like l1 and l∞. We show that this class is rich since it contains useful metric spaces with only a constant distortion, and, at the same time, it is tractable and admits efficient algorithms. Using this approach, we obtain for example the first nearest neighbor data structure with O(log log d) approximation for edit distance in non-repetitive strings (the Ulam metric). This approximation is exponentially better than the lower bound for embedding into L1. Furthermore, we give constant factor approximation for two other computational problems. Along the way, we answer positively a question posed in [Ajtai, Jayram, Kumar, and Sivakumar, STOC 2002]. One of our algorithms has already found applications for smoothed edit distance over 0--1 strings [Andoni and Krauthgamer, ICALP 2008].