Penalized Spline M-Estimators for Discretely Sampled Functional Data: Existence and Asymptotics

Abstract: Location estimation is a central problem in functional data analysis. In this paper, we investigate penalized spline estimators of location for discretely sampled functional data under a broad class of convex loss functions. Our framework generalizes and extends previously derived results for non-robust estimators to a broad penalized M-estimation framework. The analysis is built on two general-purpose non-asymptotic theoretical tools: (i) a non-asymptotic existence result for penalized spline estimators under minimal design conditions, and (ii) a localization lemma that captures both stochastic variability and approximation error. Under mild assumptions, we establish optimal convergence rates and identify the small- and large-knot regimes, along with the critical breakpoint known from penalized spline theory in nonparametric regression. Our results imply that parametric rates are attainable even with discretely sampled data and numerical experiments demonstrate that our estimators match or exceed the robustness of competing estimators while being considerably more computationally attractive.