Fredman's Trick Meets Dominance Product: Fine-Grained Complexity of Unweighted APSP, 3SUM Counting, and More

Abstract: In this paper we carefully combine Fredman's trick [SICOMP'76] and Matou\v{s}ek's approach for dominance product [IPL'91] to obtain powerful results in fine-grained complexity: - Under the hypothesis that APSP for undirected graphs with edge weights in ${1, 2, \ldots, n}$ requires $n<sup>{3-o(1)}$ time (when $\omega=2$), we show a variety of conditional lower bounds, including an $n<sup>{7/3-o(1)}$ lower bound for unweighted directed APSP and an $n<sup>{2.2-o(1)}$ lower bound for computing the Minimum Witness Product between two $n \times n$ Boolean matrices, even if $\omega=2$, improving upon their trivial $n<sup>2$ lower bounds. Our techniques can also be used to reduce the unweighted directed APSP problem to other problems. In particular, we show that (when $\omega = 2$), if unweighted directed APSP requires $n<sup>{2.5-o(1)}$ time, then Minimum Witness Product requires $n<sup>{7/3-o(1)}$ time. - We show that, surprisingly, many central problems in fine-grained complexity are equivalent to their natural counting versions. In particular, we show that Min-Plus Product and Exact Triangle are subcubically equivalent to their counting versions, and 3SUM is subquadratically equivalent to its counting version. - We obtain new algorithms using new variants of the Balog-Szemer\'edi-Gowers theorem from additive combinatorics. For example, we get an $O(n<sup>{3.83})$ time deterministic algorithm for exactly counting the number of shortest paths in an arbitrary weighted graph, improving the textbook $\widetilde{O}(n<sup>{4})$ time algorithm. We also get faster algorithms for 3SUM in preprocessed universes, and deterministic algorithms for 3SUM on monotone sets in ${1, 2, \ldots, n}<sup>d$.