Title:A Trust-region Framework for Moment Estimation

Abstract:In this paper, we develop a trust-region framework for understanding the behavior of adaptive moment estimation mechanisms, such as \textsc{Adam}, in stochastic gradient optimization. Specifically, in this framework, the magnitude of the update step for each individual weight is constrained within a trust-region governed by a moment constraint of order $p\in[2,4]$. The resulting derivation then leads to a family of learning-rate mechanisms based on second-moment estimation and a normalized $p$-th moment estimation. When $p=4$, this involves kurtosis-like estimation. The general mechanism, referred to as \textsc{Gmake}, provides a unified interpretation of normalization by moment estimation, learning-rate scheduling, spectral lowpass filtering as momentum, and operator-level spectral normalization within a common trust-region framework. Experiments on GPT2-124M trained on FineWeb-Edu and TinyStories suggest that the fourth-moment realization provides its greatest benefit when trust-region constraints are weak. As progressively stronger trust-region controls are introduced, the second-moment realization becomes increasingly competitive, often achieving slightly lower validation loss than its corresponding fourth-moment realization.

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