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First page of Lower Bounds for Smooth Nonconvex Zeroth-Order Optimization: The Price of Determinism under Low Curvature Trace

Lower Bounds for Smooth Nonconvex Zeroth-Order Optimization: The Price of Determinism under Low Curvature Trace

Yuxing Peng, Zhiqing Tang, Weijia Jia

cs.LG Oct 20, 2026 · v2
Deterministic lower and upper bounds (Theorem 3.2, Corollary 3.3) and the three-point upper bound are formalized in Lean 4 with Mathlib, provided as an ancillary artifact.
Low curvature trace, an effective rank condition, is a common explanation for why randomized zeroth-order methods scale to high-dimensional tasks such as language model fine-tuning. Can deterministic methods achieve the same savings? We show that they cannot. We study finding $\varepsilon$-stationary points of $L$-smooth nonconvex functions on $\mathbb{R}^d$ from exact function values, assuming that below the starting value the Hessian is dominated by a fixed matrix with trace at most $LE$; this is a fixed-matrix form of the effective rank condition used to explain zeroth-order fine-tuning. When $ET\le d$, the Gaussian two-point method that such fine-tuning uses needs only $O(ET)$ calls if it rejects steps that raise $f$, where $T=LΔ/\varepsilon^2$ is the iteration count of gradient descent from initial gap $Δ$, and we prove that every randomized method needs $Ω(ET)$ calls, so this cost is optimal up to constants. In contrast, every deterministic method needs $\widetildeΩ(d\min\{T,\sqrt{ET}\})$ calls, and adaptive subspace search attains $O(d\min\{T,\sqrt{ET}\})$: a deterministic method pays about $d$ calls per discovered direction, and the trace budget allows about $\sqrt{ET}$ directions. Determinism thus multiplies the cost by a factor of order $d/(E+\sqrt{ET})$ up to logarithmic factors, which grows linearly with $d$ for fixed $E$ and $T$; once $E$ exceeds $T$ by a logarithmic factor, deterministic methods need $Ω(dT)$ calls, as without any trace budget. Our lower bounds hold for $C^\infty$ functions with the matrix bound on all of $\mathbb{R}^d$, even when each call also returns gradient projections priced by rank.

Low curvature trace (an effective rank condition) explains why randomized zeroth-order methods scale to high dimensions. The question is whether deterministic zeroth-order methods can achieve the same savings when finding ε-stationary points of smooth nonconvex functions.

The function class requires the Hessian, below the starting value, to be dominated by a fixed matrix with trace at most LE. Upper bounds come from the Gaussian two-point method with an acceptance test and from an adaptive restarted subspace search. Lower bounds come from resisting-oracle arguments that hide directions behind scalar gates. Key deterministic results and the three-point upper bound are machine-checked in Lean 4 (v4.19.0) with Mathlib, using no extra axioms.

Randomized methods need Θ(ET) calls when ET ≤ d. Deterministic methods need Ω̃(d·min{T, √(ET)}) calls, and subspace search matches this with O(d·min{T, √(ET)}), so determinism costs a factor of about d/(E+√(ET)). Synthetic experiments and GPT-2 curvature measurements illustrate these mechanisms.

SettingAlg.Lower boundUpper bound
ET ≤ dRandomizedΩ(ET)O(ET)
ET ≤ dDeterministicΩ̃(d min{T, √(ET)})O(d min{T, √(ET)})
T log(e+T) ≤ E ≤ d/TDeterministicΩ(dT)O(dT)
Query complexity under low curvature trace (T = LΔ/ε²)