# Strategyproof Facility Location with Prediction: Minimizing the Maximum Cost

## TL;DR

This paper designs strategyproof mechanisms for facility location problems that leverage imperfect predictions to improve approximation ratios, balancing consistency and robustness.

## Contribution

It characterizes deterministic strategyproof mechanisms with better than 2 consistency on the line and introduces MinMaxP, achieving optimal approximation ratios based on prediction error.

## Key findings

- MinMaxP mechanism achieves a (1+min(1, η))-approximation.
- Any deterministic SP mechanism with better than 2 consistency must be MinMaxP.
- Extensions to multi-dimensional spaces and group strategyproofness are analyzed.

## Abstract

We study the mechanism design problem of facility location on a metric space in the learning-augmented framework, where mechanisms have access to imperfect predictions of the optimal facility locations. Our objective is to design strategyproof (SP) mechanisms that truthfully elicit agents' preferences over facility locations and, using the given prediction, select a facility location that approximately minimizes the maximum cost among all agents. In particular, we seek SP mechanisms whose approximation guarantees depend on the prediction error: they should achieve improved performance when the prediction is accurate (the property of \emph{consistency}) while still ensuring strong worst-case guarantees when the prediction is arbitrarily inaccurate (the property of \emph{robustness}). On the real line, we characterize all deterministic SP mechanisms with consistency strictly better than 2 and bounded robustness for the maximum cost. We show that any such mechanism must coincide with the MinMaxP mechanism, which returns the prediction if it lies between the two extreme agent locations and otherwise returns the agent location closest to the prediction. For any prediction error $\eta\ge 0$, we prove that MinMaxP achieves a $(1+\min(1, \eta))$-approximation and that no deterministic SP mechanism can obtain a better approximation ratio. In addition, for two-dimensional spaces with the $\ell_p$ distance, we analyze the approximation guarantees of a deterministic mechanism that applies MinMaxP independently on each coordinate, as well as a randomized mechanism that selects between two deterministic mechanisms with carefully chosen probabilities. We further extend these results to the $L_p$-norm social cost objective on the line metric and the maximum cost objective on the tree metric. Finally, we examine the group strategyproofness of the mechanisms.