Breaking quadratic time for small vertex connectivity and an approximation scheme.

Vertex connectivity a classic extensively-studied problem. Given an integer k, its goal is to decide if an n-node m-edge graph can be disconnected by removing k vertices. Although a linear-time algorithm was postulated since 1974 [Aho, Hopcroft and Ullman], and despite its sibling problem of edge connectivity being resolved over two decades ago [Karger STOC’96], so far no vertex connectivity algorithms are faster than O(n2) time even for k=4 and m=O(n). In the simplest case where m=O(n) and k=O(1), the O(n2) bound dates five decades back to [Kleitman IEEE Trans. Circuit Theory’69]. For higher m, O(m) time is known for k≤ 3 [Tarjan FOCS’71; Hopcroft, Tarjan SICOMP’73], the first O(n2) time is from [Kanevsky, Ramachandran, FOCS’87] for k=4 and from [Nagamochi, Ibaraki, Algorithmica’92] for k=O(1). For general k and m, the best bound is O(min(kn2, nω+nkω)) [Henzinger, Rao, Gabow FOCS’96; Linial, Lovasz, Wigderson FOCS’86] where O hides polylogarithmic terms and ω<2.38 is the matrix multiplication exponent. In this paper, we present a randomized Monte Carlo algorithm with O(m+k7/3n4/3) time for any k=O(√n). This gives the first subquadratic time bound for any 4≤ k ≤ o(n2/7) (subquadratic time refers to O(m)+o(n2) time.) and improves all above classic bounds for all k≤ n0.44. We also present a new randomized Monte Carlo (1+є)-approximation algorithm that is strictly faster than the previous Henzinger’s 2-approximation algorithm [J. Algorithms’97] and all previous exact algorithms. The story is the same for the directed case, where our exact O( min{km2/3n, km4/3} )-time for any k = O(√n) and (1+є)-approximation algorithms improve classic bounds for small and large k, respectively. Additionally, our algorithm is the first approximation algorithm on directed graphs. The key to our results is to avoid computing single-source connectivity, which was needed by all previous exact algorithms and is not known to admit o(n2) time. Instead, we design the first local algorithm for computing vertex connectivity; without reading the whole graph, our algorithm can find a separator of size at most k or certify that there is no separator of size at most k “near” a given seed node.