Online Beck--Fiala Down to Logarithmic Sparsity

Dylan J. Altschuler, Konstantin Tikhomirov

For every fixed sequence of T=n vectors in [−1,1]^n chosen before the algorithm runs, the model-generated proof gives O(√n) maximum online prefix discrepancy. The authors also prove longer-horizon and sparse-column extensions, including O(√n) for T up to exp(cn/log^(2+η)(en)) with the stated high probability.

Sources for this record

What was already known

Earlier online Komlós bounds were O(√log T), while the online Spencer question was open. The authors combined prior coupling and bounded-walk ingredients in a new compact Metropolis walk.

AI involvement AI-centered

Altschuler and Tikhomirov chose the high-level strategy. ChatGPT 5.6 Pro supplied the initial complete proof and bump density; the authors manually checked, rewrote and extended it.

The human authors chose the key ingredients and problem; the model supplied the complete initial proof and new bump density central to the retained result.

Importance Major

Resolves the online Spencer question and improves online sparse Beck–Fiala discrepancy bounds.

Limitations

The sequence is obliviously fixed in advance; the result does not cover an adaptive adversary. The longer-horizon statement uses its stated probability and parameter conditions.

Technical verification Formal (human)

The named authors explicitly report manual verification of the proofs in the paper, including the assessed original Spencer result. This is source-reported complete human checking, not independently reproduced here.

Formal checking

Manual verification and rewriting of model-generated proofs

Checker
Dylan J. Altschuler and Konstantin Tikhomirov, paper authors
Scope
Original O(√n), T=n online prefix Spencer proof and the expanded manuscript proofs
Basis
Reported by the linked checker
Evidence source

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

Human understanding Full

The human authors explain and rewrite the stationary walk, triplet coupling and bump-law analysis behind the result.

Correction and publication history

2026-10-18 · publication

Resolves the online Spencer question and improves online sparse Beck–Fiala discrepancy bounds.

Read this published revision