Greedy matroid base packings with applications to dynamic graph density and orientations

Greedy minimum weight spanning tree packings have proven to be useful in connectivity-related problems. We study the process of greedy minimum weight base packings in general matroids and explore its algorithmic applications. When specialized to bicircular matroids, our results yield an algorithm for the approximate fully-dynamic densest subgraph density $ρ$. We maintain a $(1+\varepsilon)$-approximation of the density with a worst-case update time $O((ρ\varepsilon^{-2}+\varepsilon^{-4})ρ\log^3 m)$. It improves the dependency on $\varepsilon$ from the current state-of-the-art worst-case update time complexity $O(\varepsilon^{-6}\log^3 n\logρ)$ [Chekuri, Christiansen, Holm, van der Hoog, Quanrud, Rotenberg, Schwiegelshohn, SODA'24]. We also can maintain an implicit fractional out-orientation with a guarantee that all out-degrees are at most $(1+\varepsilon)ρ$. Our algorithms above work by greedily packing pseudoforests, and require maintenance of a minimum-weight pseudoforest in a dynamically changing graph. We show that this problem can be solved in $O(\log n)$ worst-case time per edge insertion or deletion. For general matroids, we observe two characterizations of the limit of the base packings (``the vector of ideal loads''), which imply the characterizations from [Cen, Fleischmann, Li, Li, Panigrahi, FOCS'25], namely, their entropy-minimization theorem and their bottom-up cut hierarchy. Finally, we give combinatorial results on the greedy tree packings. We show that a tree packing of $O(λ^5\log m)$ trees contains a tree crossing some min-cut once, which improves the bound $O(λ^7\log^3 m)$ from [Thorup, Combinatorica'07]. We also strengthen the lower bound on the edge load convergence rate from [de Vos, Christiansen, SODA'25], showing that Thorup's upper bound is tight up to a logarithmic factor.