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arXiv Machine Learning

Feature Priming in Online Linear Regression: Sparse-Regret Lower Bounds and a Tight Univariate Rate

The paper investigates feature priming in high‑dimensional online linear regression, showing that estimating feature weights from past data and refitting a minimum‑norm predictor can lead to regret that scales with sparsity rather than ambient dimension. It provides a negative answer to a COLT 2023 open problem by proving that three natural priming rules incur ≥Ω(min{T,√d}) regret against a zero‑loss one‑sparse comparator, due to cheap nuisance interpolation that underweights truly predictive coordinates. The authors also identify conditions under which regret is governed by data rank and present constructions that achieve tight univariate rates, while noting that the multivariate case remains unresolved.

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arXiv Machine Learning
Sep 10

Feature Priming in Online Linear Regression: Sparse-Regret Lower Bounds and Tight Coordinatewise Rates

The paper investigates online linear regression with sparse comparators, focusing on feature priming techniques that reweight features using past data. It establishes sparse‑regret lower bounds that invalidate sparse‑logarithmic guarantees for univariate, Pearson, and multivariate priming rules under a past‑only Moore–Penrose protocol, showing ≥Ω(min{T,√d}) clipped regret for unit‑power rules and linear regret for powered rules in high dimensions. The authors also provide tight rank upper bounds for certain priming schemes and present algebraic constructions yielding Ω(min{T,d^{1/4}}) regret for unit‑power multivariate priming, while noting that the exact multivariate frontier remains open.

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arXiv Machine Learning
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High-dimensional Linear Bandits with Knapsacks

The paper studies high‑dimensional linear contextual bandits with knapsack constraints (CBwK), aiming to exploit sparsity for tighter regret bounds. It introduces an online hard‑thresholding estimator integrated into a primal‑dual framework, achieving sub‑linear regret that grows only logarithmically with the feature dimension. Under either a diverse‑covariate or margin condition, the regret improves to τ‑dependent rates, and when both hold simultaneously, a dual resolving scheme yields an even tighter bound. The approach also recovers optimal rates for high‑dimensional contextual bandits without knapsacks, and experiments demonstrate its practical effectiveness.

By Wanteng Ma, Dong Xia, Jiashuo Jiang
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arXiv Machine Learning
Jun 24

LLMs are Bayesian, In Expectation, Not in Realization

arXiv:2507. 11768v3 Announce Type: replace-cross Abstract: Bayesian accounts of in-context learning face a direct objection: exact posterior predictives for exchangeable data are invariant to task-preserving order, yet transformers change next-token probabilities when the same examples are serialized differently.

By Leon Chlon, Fatima Sheaib, Zein Khamis, Maggie Chlon, Mahdi El Zein, MarcAntonio M. Awada
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