Nash Social Welfare with Submodular Valuations: Approximation Algorithms and Integrality Gaps

Abstract: We study the problem of allocating items to agents such that the (un)weighted Nash social welfare (NSW) is maximized under submodular valuations. The best-known results for unweighted and weighted problems are the $(4+\epsilon)$ approximation given by Garg, Husic, Li, Vega, and Vondrak~\cite{stoc/GargHLVV23} and the $(233+\epsilon)$ approximation given by Feng, Hu, Li, and Zhang~\cite{stoc/FHLZ25}, respectively. For the weighted NSW problem, we present a $(5.18+\epsilon)$-approximation algorithm, significantly improving the previous approximation ratio and simplifying the analysis. Our algorithm is based on the same configuration LP in~\cite{stoc/FHLZ25}, but with a modified rounding algorithm. For the unweighted NSW problem, we show that the local search-based algorithm in~\cite{stoc/GargHLVV23} is an approximation of $(3.914+\epsilon)$ by more careful analysis. On the negative side, we prove that the configuration LP for weighted NSW with submodular valuations has an integrality gap at least $2^{\ln 2}-\epsilon \approx 1.617 - \epsilon$, which is slightly larger than the current best-known $e/(e-1)-\epsilon \approx 1.582-\epsilon$ hardness of approximation~\cite{talg/GargKK23}. For the additive valuation case, we show an integrality gap of $(e^{1/e}-\epsilon)$, which proves that the ratio of $(e^{1/e}+\epsilon)$~\cite{icalp/FengLi24} is tight for algorithms based on the configuration LP. For unweighted NSW with additive valuations, we show a gap of $(2^{1/4}-\epsilon) \approx 1.189-\epsilon$, slightly larger than the current best-known $\sqrt{8/7} \approx 1.069$-hardness for the problem~\cite{mor/Garg0M24}.