Deterministic PRAM Approximate Shortest Paths in Polylogarithmic Time and Slightly Super-Linear Work
We study a $(1+ε)$-approximate single-source shortest paths (henceforth, $(1+ε)$-SSSP) in $n$-vertex undirected, weighted graphs in the parallel (PRAM) model of computation. A randomized algorithm with polylogarithmic time and slightly super-linear work $\tilde{O}(|E|\cdot n^ρ)$, for an arbitrarily small $ρ>0$, was given by Cohen [Coh94] more than $25$ years ago. Exciting progress on this problem was achieved in recent years [ElkinN17,ElkinN19,Li19,AndoniSZ19], culminating in randomized polylogarithmic time and $\tilde{O}(|E|)$ work. However, the question of whether there exists a deterministic counterpart of Cohen's algorithm remained wide open. In the current paper we devise the first deterministic polylogarithmic-time algorithm for this fundamental problem, with work $\tilde{O}(|E|\cdot n^ρ)$, for an arbitrarily small $ρ>0$. This result is based on the first efficient deterministic parallel algorithm for building hopsets, which we devise in this paper.