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Given an unweighted, undirected graph, what is the time complexity to decide if its radius is at most 2? Are there any faster algorithms than doing BFS on each node?

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Yes. Given the adjacency matrix representation of a graph, computing its second power will allow you to see what nodes are connected by a path of length at most 2. The fastest known matrix multiplication algorithm is $O(n^{2.373})$, which is better than the $O(VE)$ you would get from doing BFS on each node.

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  • $\begingroup$ Correct in general, but if the graph is sparse--$|E| \in O(|V|)$--then we do better with BFS. $\endgroup$ – Ryan Jun 6 '15 at 18:43
  • $\begingroup$ Also BFS has a worst case complexity $O(|V| + |E|)$. $\endgroup$ – Luke Mathieson Jun 7 '15 at 3:40

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