Fast Algorithms via Dynamic-Oracle Matroids

We initiate the study of matroid problems in a new oracle model called dynamic oracle. Our algorithms in this model lead to new bounds for some classic problems, and a "unified" algorithm whose performance matches previous results developed in various papers for various problems. We also show a lower bound that answers some open problems from a few decades ago. Concretely, our results are as follows. Improved algorithms for matroid union and disjoint spanning trees. We show an algorithm with (O) over tilde (k)(n+r root r) dynamic-rank-query and time complexities for the matroid union problem over k matroids, where n is the input size, r is the output size, and (O) over tilde hides poly(k, log(n)). This implies the following consequences. (i) An improvement over the (O) over tilde (k)(n root r) bound implied by [Chakrabarty-Lee-Sidford-Singla-Wong FOCS'19] for matroid union in the traditional rank-query model. (ii) An (O) over tilde (k) (vertical bar E vertical bar + vertical bar V vertical bar root vertical bar V vertical bar)-time algorithm for the k-disjoint spanning tree problem. This is nearly linear for moderately dense input graphs and improves the (O) over tilde (k) (vertical bar V vertical bar + root vertical bar E vertical bar) bounds of Gabow-Westermann [STOC'88] and Gabow [STOC'91]. Consequently, this gives improved bounds for, e.g., Shannon Switching Game and Graph Irreducibility. Matroid intersection. We show a matroid intersection algorithm with (O) over tilde (n root r) dynamic-rank-query and time complexities. This implies new bounds for some problems (e.g. maximum forest with deadlines) and bounds that match the classic ones obtained in various papers for various problems, e.g. colorful spanning tree [Gabow-Stallmann ICALP'85], graphic matroid intersection [Gabow-Xu FOCS'89], simple job scheduling matroid intersection [Xu-Gabow ISAAC'94], and Hopcroft-Karp combinatorial bipartite matching. More importantly, this is done via a "unified" algorithm in the sense that an improvement over our dynamic-rank-query algorithm would imply improved bounds for all the above problems simultaneously. Lower bounds. We show simple super-linear (Omega(n log n)) query lower bounds for matroid intersection and union problems in our dynamic-rank-oracle and the traditional independence-query models; the latter improves the previous log(2) (3)n - o(n) bound by Harvey [SODA'08] and answers an open problem raised by, e.g., Welsh [1976] and CLSSW [FOCS'19].