Breaking the quadratic barrier for matroid intersection

The matroid intersection problem is a fundamental problem that has been extensively studied for half a century. In the classic version of this problem, we are given two matroids M1 = (V, I1) and M2 = (V, I2) on a comment ground set V of n elements, and then we have to find the largest common independent set S ∈ I1 ∩ I2 by making independence oracle queries of the form ”Is S ∈ I1?” or ”Is S ∈ I2?” for S ⊆ V. The goal is to minimize the number of queries.

Beating the existing Õ(n2) bound, known as the quadratic barrier, is an open problem that captures the limits of techniques from two lines of work. The first one is the classic Cunningham’s algorithm [SICOMP 1986], whose Õ(n2)-query implementations were shown by CLS+ [FOCS 2019] and Nguyen [2019] (more generally, these algorithms take Õ(nr) queries where r denotes the rank which can be as big as n). The other one is the general cutting plane method of Lee, Sidford, and Wong [FOCS 2015]. The only progress towards breaking the quadratic barrier requires either approximation algorithms or a more powerful rank oracle query [CLS+ FOCS 2019]. No exact algorithm with o(n2) independence queries was known.

In this work, we break the quadratic barrier with a randomized algorithm guaranteeing Õ(n9/5) independence queries with high probability, and a deterministic algorithm guaranteeing Õ(n11/6) independence queries. Our key insight is simple and fast algorithms to solve a graph reachability problem that arose in the standard augmenting path framework [Edmonds 1968]. Combining this with previous exact and approximation algorithms leads to our results.