Joint Lower Bounds for Zeroth-Order Nonconvex Optimization on Euclidean Balls

Abstract

We prove a joint stochastic zeroth-order lower bound for Goldstein stationarity on a Euclidean query ball, even when the ball is guaranteed to contain a stationary point. In dimension $d$, let $f=\mathbb{E}[F(\cdot;ξ)]$, assume $\mathbb{E}[\operatorname{Lip}(F(\cdot;ξ))^2]\le L_0^2$, and bound the initial objective gap over the ball by $Δ$. For neighborhood radius $δ>0$ and residual tolerance $\varepsilon>0$, our smooth hard family requires $Ω(dL_0^2Δ/(δ\varepsilon^3))$ scalar evaluations for success probability $1/2$, against randomized adaptive algorithms that may retain and repeatedly query each sampled function. The result holds for $\varepsilon\le cL_0$, $Δ\ge Cδ\varepsilon$, and $d\ge C[1+\log(2+ΔL_0^2/(δ\varepsilon^3))]$, on a constructed ball of radius $Θ(Δ/\varepsilon)$. A sequence of localized regions forces repeated direction estimation, and an adaptive Gaussian posterior argument controls sample reuse. The result establishes a joint dimension and accuracy obstruction on solvable bounded-domain instances. The gap is local to the query ball; a matching minimax characterization at a common radius and the corresponding unrestricted global-gap lower bound remain open here.