On Diameter Approximation in Directed Graphs

Computing the diameter of a graph, i.e. the largest distance, is a fundamental problem that is central in fine-grained complexity. In undirected graphs, the Strong Exponential Time Hypothesis (SETH) yields a lower bound on the time vs. approximation trade-off that is quite close to the upper bounds. In directed graphs, however, where only some of the upper bounds apply, much larger gaps remain. Since d(u, v) may not be the same as d(v, u), there are multiple ways to define the problem, the two most natural being the (one-way) diameter (max_(u,v) d(u, v)) and the roundtrip diameter (maxu,v d(u, v) + d(v, u)). In this paper we make progress on the outstanding open question for each of them. We design the first algorithm for diameter in sparse directed graphs to achieve n^1.5−ε time with an approximation factor better than 2. The new upper bound trade-off makes the directed case appear more similar to the undirected case. Notably, this is the first algorithm for diameter in sparse graphs that benefits from fast matrix multiplication. We design new hardness reductions separating roundtrip diameter from directed and undirected diameter. In particular, a 1.5-approximation in subquadratic time would refute the All-Nodes k-Cycle hypothesis, and any (2 − ε)-approximation would imply a breakthrough algorithm for approximate ℓ_∞-Closest-Pair. Notably, these are the first conditional lower bounds for diameter that are not based on SETH.