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First page of Pure-DP Statistical Query Release at the Conjectured Square-Root Rate

Pure-DP Statistical Query Release at the Conjectured Square-Root Rate

Jack Fitzsimons

cs.DS Jul 22, 2026 · v1 cs.CR
A Lean 4 development machine-checks the finite mechanism construction, its pure differential privacy after deterministic decoding, and the claimed error upper bound.
Nikolov and Ullman asked whether k statistical queries on a universe of size T can be released under pure differential privacy with expected worst-coordinate error at the square-root rate suggested by known lower bounds. We prove their conjectured upper bound. For every database size n and privacy parameter $\varepsilon>0$, there is an $\varepsilon$-differentially private mechanism with expected error $O(\min\{1,\sqrt{\log(2T)\log(2k)/(\varepsilon n)}\})$. This matches the lower-bound dependence in the standard high-dimensional regimes where those bounds apply; the shifted logarithms and outer minimum make the upper bound valid without additional parameter assumptions. The construction starts from a selection-only private multiplicative weights transcript, then replaces its probability mass function by a distance-penalized likelihood envelope. To prove that the modification preserves accuracy, a likelihood-level Maurey argument upper-bounds each Hamming-ball maximum by a small family of auxiliary PMW laws. Renyi moment bounds control nearby balls, a direct mixture bound controls distant balls, and grouping radii at the privacy scale prevents an additional $1/\varepsilon$ factor in the error. The mechanism is information-theoretic. A companion Lean 4 development machine-checks the finite construction, pure privacy after deterministic decoding, and the displayed all-regimes upper bound.

Nikolov and Ullman asked whether k statistical queries on a universe of size T can be released under pure differential privacy at the square-root expected worst-coordinate error rate suggested by known lower bounds.

An information-theoretic mechanism starts from a selection-only private multiplicative weights (PMW) transcript and replaces its probability mass function with a distance-penalized likelihood envelope. A likelihood-level Maurey argument bounds each Hamming-ball likelihood maximum by a small auxiliary family, with Rényi moment bounds for nearby balls, a direct mixture bound for distant balls, and radius grouping at the privacy scale to avoid an extra 1/ε factor. A companion Lean 4 development machine-checks the finite construction, pure privacy after deterministic decoding, and the displayed all-regimes upper bound.

They prove an ε-differentially private mechanism with expected error O(min{1, sqrt(log(2T)log(2k)/(εn))}), matching the conjectured square-root rate and the lower-bound dependence in standard high-dimensional regimes.