arXiv Machine Learning By Katie Everett, Elliot Paquette

Dynamics of Stochastic Momentum with Sparse Updates in High Dimensions

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The paper studies momentum-based optimization when updates are sparse, focusing on two tractable models: a least‑squares problem with sparse inputs and a logistic regression problem with a rare class. It derives exact closed‑form second‑moment dynamics and analyzes their high‑dimensional limits across three scaling exponents for sparsity, batch size, and momentum decay. The resulting phase diagram is governed by the ratio of a momentum retention timescale and a learning timescale, revealing regimes where the dynamics match SGD, become unstable, or recover classical heavy‑ball behavior, and showing that oscillatory behavior depends on token sparsity, leading to a spectral conflict for global momentum.

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