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arXiv · 1907.12106

A Lower Bound on Cycle-Finding in Sparse Digraphs

Xi Chen·Tim Randolph·Rocco A. Servedio·Timothy Sun
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Abstract

We consider the problem of finding a cycle in a sparse directed graph $G$ that is promised to be far from acyclic, meaning that the smallest feedback arc set in $G$ is large. We prove an information-theoretic lower bound, showing that for $N$-vertex graphs with constant outdegree any algorithm for this problem must make $\tilde{\Omega}(N^{5/9})$ queries to an adjacency list representation of $G$. In the language of property testing, our result is an $\tilde{\Omega}(N^{5/9})$ lower bound on the query complexity of one-sided algorithms for testing whether sparse digraphs with constant outdegree are far from acyclic. This is the first improvement on the $\Omega(\sqrt{N})$ lower bound, implicit in Bender and Ron, which follows from a simple birthday paradox argument.

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Xi Chen, Tim Randolph, Rocco A. Servedio, Timothy Sun. 2019-07-28. A Lower Bound on Cycle-Finding in Sparse Digraphs. https://arxiv.org/abs/1907.12106

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