An $O(\log \log n)$-approximate budget feasible mechanism for subadditive valuations

TL;DR

This paper tackles budget-feasible mechanism design for subadditive valuations, where a buyer with budget B seeks to procure a high-valued subset from strategic sellers. It introduces a randomized mechanism that runs in polynomial time with demand-oracle access and achieves an $O(\log\log n)$-approximation to OPT, improving over the previous $O(\log n/\log\log n)$ bound. The key technical advance is a polytime construction of distributions over threshold vectors with bounded marginals by exploiting LP duality in a two-player item-bidding framework, enabling robust performance even when costs are concentrated. The approach also handles large-cost instances via LP-based XOS-approximation steps, strengthening the understanding of polytime budget-feasible mechanisms for subadditive valuations and suggesting connections to prophet inequalities in related settings.

Abstract

In budget-feasible mechanism design, there is a set of items $U$, each owned by a distinct seller. The seller of item $e$ incurs a private cost $\overline{c}_e$ for supplying her item. A buyer wishes to procure a set of items from the sellers of maximum value, where the value of a set $S\subseteq U$ of items is given by a valuation function $v:2^U\to \mathbb{R}_+$. The buyer has a budget of $B \in \mathbb{R}_+$ for the total payments made to the sellers. We wish to design a mechanism that is truthful, that is, sellers are incentivized to report their true costs, budget-feasible, that is, the sum of the payments made to the sellers is at most the budget $B$, and that outputs a set whose value is large compared to $\text{OPT}:=\max\{v(S):\overline{c}(S)\le B,S\subseteq U\}$. Budget-feasible mechanism design has been extensively studied, with the literature focussing on (classes of) subadditive valuation functions, and various polytime, budget-feasible mechanisms, achieving constant-factor approximation, have been devised for the special cases of additive, submodular, and XOS valuations. However, for general subadditive valuations, the best-known approximation factor achievable by a polytime budget-feasible mechanism (given access to demand oracles) was only $O(\log n / \log \log n)$, where $n$ is the number of items. We improve this state-of-the-art significantly by designing a randomized budget-feasible mechanism for subadditive valuations that achieves a substantially-improved approximation factor of $O(\log\log n)$ and runs in polynomial time, given access to demand oracles.

Theorems & Definitions (24)

• Theorem 1.1
• Theorem 1.2: NeogiPS24
• Theorem 1.3: Informal version of Theorem \ref{['thm:good_dist']}
• Lemma 1.4: Informal version of Lemma \ref{['lem:thresholds_simple']}
• Lemma 2.1: NeogiPS24
• Lemma 2.2: Badanidiyuru2018OptimizationWD
• Theorem 2.3: Truthfulness in single-parameter domains Myerson81
• Theorem 3.1
• Lemma 3.2
• proof