Fast hierarchical clustering and other applications of dynamic closest pairs

We develop data structures for dynamic closest pair problems with arbitrary distance functions, that do not necessarily come from any geometric structure on the objects. Based on a technique previously used by the author for Euclidean closest pairs, we show how to insert and delete objects from an <i>n</i>-object set, maintaining the closest pair, in <i>O</i>(<i>n</i> log² <i>n</i>) time per update and <i>O</i>(<i>n</i>) space. With quadratic space, we can instead use a quadtree-like structure to achieve an optimal time bound, <i>O</i>(<i>n</i>) per update. We apply these data structures to hierarchical clustering, greedy matching, and TSP heuristics, and discuss other potential applications in machine learning, Gröbner bases, and local improvement algorithms for partition and placement problems. Experiments show our new methods to be faster in practice than previously used heuristics.