A Deterministic Parallel Reduction from Weighted Matroid Intersection Search to Decision

Abstract

Given two matroids on the same ground set, the matroid intersection problem asks for a common base, i.e., a subset of the ground set that is a base in both the matroids. The weighted version of the problem asks for a common base with maximum weight. In the general case, when the two matroids are given via rank oracles, the question of its parallel complexity is completely open. In the case of linearly representable matroids, the problem is known to have randomized parallel (RNC) algorithms, when the given weights are polynomially bounded. Finding a deterministic parallel (NC) algorithm in this case, even for the decision question, has been a long standing open question. We make some progress towards understanding the parallel complexity of matroid intersection by showing that the weighted matroid intersection (WMI) search problem is equivalent to its decision version, in a parallel model of computation. More precisely, we give an NC algorithm for WMI-search using an oracle access to WMI-decision. This resolves an open question posed by Anari and Vazirani (ITCS 2020).