Small Space Stream Summary for Matroid Center

In the matroid center problem, which generalizes the $k$-center problem, we need to pick a set of centers that is an independent set of a matroid with rank $r$. We study this problem in streaming, where elements of the ground set arrive in the stream. We first show that any randomized one-pass streaming algorithm that computes a better than $Δ$-approximation for partition-matroid center must use $Ω(r^2)$ bits of space, where $Δ$ is the aspect ratio of the metric and can be arbitrarily large. This shows a quadratic separation between matroid center and $k$-center, for which the Doubling algorithm gives an $8$-approximation using $O(k)$-space and one pass. To complement this, we give a one-pass algorithm for matroid center that stores at most $O(r^2\log(1/\varepsilon)/\varepsilon)$ points (viz., stream summary) among which a $(7+\varepsilon)$-approximate solution exists, which can be found by brute force, or a $(17+\varepsilon)$-approximation can be found with an efficient algorithm. If we are allowed a second pass, we can compute a $(3+\varepsilon)$-approximation efficiently; this also achieves almost the known-best approximation ratio (of $3+\varepsilon$) with total running time of $O((nr + r^{3.5})\log(1/\varepsilon)/\varepsilon + r^2(\log Δ)/\varepsilon)$, where $n$ is the number of input points. We also consider the problem of matroid center with $z$ outliers and give a one-pass algorithm that outputs a set of $O((r^2+rz)\log(1/\varepsilon)/\varepsilon)$ points that contains a $(15+\varepsilon)$-approximate solution. Our techniques extend to knapsack center and knapsack center with outliers in a straightforward way, and we get algorithms that use space linear in the size of a largest feasible set (as opposed to quadratic space for matroid center).