Constant Inapproximability of Pacing Equilibria in Second-Price Auctions

In this paper, we revisit the problem of approximating a pacing equilibrium in second-price auctions, introduced by Conitzer, Kroer, Sodomka, and Moses [Oper. Res. 2022]. We show that finding a constant-factor approximation of a pacing equilibrium is PPAD-hard, thereby strengthening previous results of Chen, Kroer, and Kumar [Math. Oper. Res. 2024], which established PPAD-hardness only for inverse-polynomial approximations.