Nearly Optimal Time Bounds for kPath in Hypergraphs

Abstract

We give almost tight conditional lower bounds on the running time of the kHyperPath problem. Given an $r$-uniform hypergraph for some integer $r$, kHyperPath seeks a tight path of length $k$. That is, a sequence of $k$ nodes such that every consecutive $r$ of them constitute a hyperedge in the graph. This problem is a natural generalization of the extensively-studied kPath problem in graphs. We show that solving kHyperPath in time $O^*(2^{(1-\gamma)k})$ where $\gamma>0$ is independent of $r$ is probably impossible. Specifically, it implies that Set Cover on $n$ elements can be solved in time $O^*(2^{(1 - \delta)n})$ for some $\delta>0$. The only known lower bound for the kPath problem is $2^{\Omega(k)} poly(n)$ where $n$ is the number of nodes assuming the Exponential Time Hypothesis (ETH), and finding any conditional lower bound with an explicit constant in the exponent has been an important open problem. We complement our lower bound with an almost tight upper bound. Formally, for every integer $r\geq 3$ we give algorithms that solve kHyperPath and kHyperCycle on $r$-uniform hypergraphs with $n$ nodes and $m$ edges in time $2^k m \cdot poly(n)$ and $2^k m^2 poly(n)$ respectively, and that is even for the directed version of these problems. To the best of our knowledge, this is the first algorithm for kHyperPath. The fastest algorithms known for kPath run in time $2^k poly(n)$ for directed graphs (Williams, 2009), and in time $1.66^k poly(n)$ for undirected graphs (Bj\"orklund \etal, 2014).