A Note on Second-Order Expected Maximum-Load Bounds for Binary Linear Hashing

TL;DR

This paper refines bounds on the maximum load in binary linear hashing, showing it nearly matches fully independent hashing at second-order scales and providing sharp tail bounds for individual buckets.

Contribution

It improves the exponential-potential method to show binary linear hashing closely matches fully independent hashing at second-order load scales.

Findings

Expected maximum load matches fully independent hashing up to second-order corrections.

Derived tail bounds for individual bucket loads with asymptotic tightness.

Established that union bounds over all buckets lose only a small factor.

Abstract

Let $S\subseteq F_2^u$ have size $n=2^\ell$, and let $h:F_2^u\to F_2^\ell$ be a uniformly random linear map. For $y\in F_2^\ell$, write $Load_h(y):=|h^{-1}(y)\cap S|$, and let $M(S,h):=\max_{y\in F_2^\ell} Load_h(y)$ be the maximum load. Jaber, Kumar and Zuckerman (STOC 2025) proved that the expected maximum load of $h$ on $S$ is at most $16\log n/\log\log n$, matching the fully independent keys-into-bins scale up to constants. Their proof also gives the tail estimate \[ \Pr\left[ M(S,h)\ge R\frac{\log n}{\log\log n} \right] \le O\left(\frac{1}{R^{2}}\right). \] We record a base optimization in their exponential-potential method showing that binary linear hashing nearly matches fully independent hashing also at the level of the second-order maximum-load scale. For every $R>1$ satisfying $R\ell^{1-1/R}\ge D\ln\ell$, where $D$ is an absolute constant, we prove \[ \Pr\left[ M(S,h)\ge R\frac{\log n}{\log\log n} \right] \le O\left( \frac{(\log\log n)^2}{R^2(\log n)^{2-2/R}} \right). \] Integrating this tail yields \[ E[M(S,h)] \le \left( 1+ (1+o(1)) \frac{\log\log\log n}{\log\log n} \right) \frac{\log n}{\log\log n}. \] Thus binary linear hashing matches fully independent hashing in the leading term and matches the dominant second-order correction up to a $1+o(1)$ factor. We also prove, by an independent self-contained argument, a sharp tail bound for one prescribed bucket: for fixed $y\in F_2^\ell$, \[ \Pr[ Load_h(y)>2^a-2]\le \gamma^{-1}2^{-a^2}, \] where $ \gamma=\prod_{j\ge1}(1-2^{-j}) $. A subspace construction shows that this is asymptotically tight even in the leading constant as $ a\to\infty $. However, this controls only a fixed bucket; a direct union bound over all buckets loses a factor $ 2^\ell $.