Rafail Ostrovsky - Publications

How Hard Is Counting Triangles in the Streaming Model?

Vladimir Braverman, Rafail Ostrovsky, Dan Vilenchik


The problem of ( approximately) counting the number of triangles in a graph is one of the basic problems in graph theory. In this paper we study the problem in the streaming model. Specifically, the amount of memory required by a randomized algorithm to solve this problem. In case the algorithm is allowed one pass over the stream , we present a best possible lower bound of Ω(m) for graphs G with m edges. If a constant number of passes is allowed, we show a lower bound with a 2-pass O(m/T1/3-memory algorithm that solves the problem of distinguishing graphs with no triangles from graphs with at least T triangles. We present a new graph parameter p(G)-the triangle density, and conjecture that the space complexity of the triangles problem is Θ(m/p(G)). We match this by a second algorithm that solves the distinguishing problem using O(m/p(G))-memory.

comment: ICALP 2013 pp: 244-254

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