Graph Spanners by Sketching in Dynamic Streams and the Simultaneous Communication Model
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Graph sketching is a powerful technique introduced by the seminal work of Ahn, Guha and McGregor'12 on connectivity in dynamic graph streams that has enjoyed considerable attention in the literature since then, and has led to near optimal dynamic streaming algorithms for many fundamental problems such as connectivity, cut and spectral sparsifiers and matchings. Interestingly, however, the sketching and dynamic streaming complexity of approximating the shortest path metric of a graph is still far from well-understood. Besides a direct k-pass implementation of classical spanner constructions, the state of the art amounts to a O(log k)-pass algorithm of Ahn, Guha and McGregor'12, and a 2-pass algorithm of Kapralov and Woodruff'14. In particular, no single pass algorithm is known, and the optimal tradeoff between the number of passes, stretch and space complexity is open. In this paper we introduce several new graph sketching techniques for approximating the shortest path metric of the input graph. We give the first single pass sketching algorithm for constructing graph spanners: we show how to obtain a O(n^2/3)-spanner using O(n) space, and in general a O(n^2/3(1-α))-spanner using O(n^1+α) space for every α∈ [0, 1], a tradeoff that we think may be close optimal. We also give new spanner construction algorithms for any number of passes, simultaneously improving upon all prior work on this problem. Finally, we note that unlike the original sketching approach of Ahn, Guha and McGregor'12, none of the existing spanner constructions yield simultaneous communication protocols with low per player information. We give the first such protocols for the spanner problem that use a small number of rounds.
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