A Randomized Algorithm for Long Directed Cycle

Abstract: Given a directed graph $G$ and a parameter $k$, the {\sc Long Directed Cycle (LDC)} problem asks whether $G$ contains a simple cycle on at least $k$ vertices, while the {\sc $k$-Path} problems asks whether $G$ contains a simple path on exactly $k$ vertices. Given a deterministic (randomized) algorithm for {\sc $k$-Path} as a black box, which runs in time $t(G,k)$, we prove that {\sc LDC} can be solved in deterministic time $O<sup>*(\max{t(G,2k),4<sup>{k+o(k)}})$ (randomized time $O<sup>*(\max{t(G,2k),4<sup>k})$). In particular, we get that {\sc LDC} can be solved in randomized time $O<sup>*(4<sup>k)$.