From Linear to Polylogarithmic: Dynamic Submodular Maximization over Matroids

Overview & Synopsis

We study the problem of submodular maximization under a matroid constraint. In this problem, there is a monotone submodular function $f: 2^{N} \rightarrow \mathbb{R}^+$ and a matroid $M(N, I)$, accessible via value and independence oracles and the goal is to find the independent set $S \in I$ maximizing the objective function, i.e., $f(S) = \max_{I \in I} f(I)$. In the fully dynamic setting, the active ground set $N \subseteq N$ evolves through element insertions and deletions. The goal is to maintain a solution $S \subseteq N$ with $S \in I$ and $f(S) = \max_{I \subseteq N,, I \in I} f(I)$ after each update. Two approximation algorithms have been proposed to solve this problem: Dütting et al. developed a $(4+ε)$-approximation algorithm with an expected amortized update time of $O\!\bigl(\frac{k^2}ε \log k \log n \log^3 \frac{k}ε\bigr)$, and Banihashem et al. achieved a similar approximation with a better update time $O\!\bigl(k \log k \log^3 \frac{k}ε\bigr)$. However, these methods both become inefficient when the rank $k$ of the matroid is large (e.g., when $k = n^{o(1)}$). The central open question in this line of work is whether a fully dynamic algorithm with polylogarithmic update time exists. We resolve this question by presenting the first fully dynamic algorithm for submodular maximization over matroids with polylogarithmic update time. Our algorithm maintains a $(4+ε)$-approximate solution with expected amortized update time $poly(\log n, ε^{-1})$, where $n = |N|$. In particular, the update time is independent of the matroid rank $k$, thereby eliminating the dependence on $k$ and settling the open problem.