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arXiv · 2610.16030

Limit Theory for Online Expectile Estimation under Short and Long Range Dependence

Younghun Lee·Jake Zummo·Wei Biao Wu
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Abstract

Expectiles are tail-sensitive distributional summaries with important applications in financial risk measurement and forecasting, yet statistical theory for their online estimation from dependent data remains limited. We study a one-pass, stochastic-gradient algorithm with constant learning rate $γ$ for estimating the marginal expectile of a stationary causal process. The recursion admits a unique stationary causal solution and geometrically forgets its initialization. Under short-range dependence, its last iterate is asymptotically Gaussian at the $\sqrtγ$ scale, with variance governed by the population curvature and the long-run variance of the expectile score. For long-memory linear processes the scale changes to $γ^{β-1/2}\ell(1/γ)$, with a Gaussian limit arising from a fractional Brownian motion integral. We further derive the joint long-memory limit for multiple expectile levels and characterize its limiting covariance structure. Various numerical experiments corroborate the predicted Gaussian approximations and regime-specific fluctuation scales.

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Younghun Lee, Jake Zummo, Wei Biao Wu. 2026-10-12. Limit Theory for Online Expectile Estimation under Short and Long Range Dependence. https://arxiv.org/abs/2610.16030

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