A Tight Lower Bound for Smooth Nonconvex Stochastic Optimization with Bounded Gradient Noise

Jikai Jin

In the paper’s small-ε regime, a randomized adaptive method using one independent fresh sample per query needs Ω(ΔL/ε² + ΔLσ²/ε⁴) queries to reach expected gradient norm at most ε for smooth nonconvex objectives with gap Δ, L-Lipschitz gradient and uniformly bounded, unbiased fixed-law gradient noise.

Sources for this record

What was already known

Stochastic gradient methods attain this rate. Earlier matching lower bounds assumed bounded variance but used an oracle without a uniform almost-sure error bound.

AI involvement AI-autonomous

Jikai Jin says GPT-5.6 Sol independently generated the proof after he supplied the prompt. Jin checked and revised the model-generated argument; no human explanation of its sign-vector mechanism is documented.

The paper expressly says the model independently generated the proof after the human supplied the prompt; checking and manuscript polishing came later.

Importance Notable

Closes the minimax-rate gap for the stated one-sample bounded-error oracle model.

Limitations

The oracle need not be a gradient of a sample loss and may be discontinuous in the query point. Conservative, finite-sum, samplewise-smooth and shared-sample variants are outside the theorem.

Technical verification Formal (human)

The named human author reports checking the proof of the stated lower bound, not merely accepting responsibility for it. This is a source-reported complete author check; no independent reviewer or machine formalization is documented and the review was not reproduced here.

Formal checking

Human proof check followed by manuscript revision

Checker
Jikai Jin, author
Scope
The model-generated proof of the K=1 fixed-law uniformly bounded-gradient-noise lower bound in Theorem 4.1
Basis
Reported by the linked checker
Evidence source

Checking a formal statement does not establish that it matches the original problem.

Human understanding Few

Jin reports checking and polishing the proof, but the source does not attribute a human explanation of the sign encoding, KL accounting and escape-control mechanism. Few is the evidence default, not a finding that Jin lacks understanding.

Correction and publication history