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arXiv · 2606.23676

Open Problem: Is AdamW Effective Under Heavy-Tailed Noise?

Dingzhi Yu·Hongyi Tao·Yuanyu Wan·Luo Luo·Lijun Zhang
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Abstract

AdamW is the de facto optimizer for training large language models (LLMs), yet the theory behind it still lives mostly in finite-variance regimes. This is increasingly unsatisfying, as empirical evidence indicates that stochastic gradient noise in LLM pretraining is typically heavy-tailed. Recent work shows that sign-based optimizers such as Lion and Muon achieve sharp heavy-tailed rates, and that AdaGrad can also converge under heavy-tailed noise. However, no rigorous convergence theory for AdamW has yet been established in this regime. Can AdamW converge under the same heavy-tailed assumptions, or does its second-moment accumulator create a genuine obstruction? We formulate this as an open problem, prove a positive weighted-metric benchmark, and give a corridor lower-bound mechanism showing how denominator memory can hide large gradients.

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Dingzhi Yu, Hongyi Tao, Yuanyu Wan, Luo Luo, Lijun Zhang. 2026-06-22. Open Problem: Is AdamW Effective Under Heavy-Tailed Noise?. https://arxiv.org/abs/2606.23676

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