Almost everywhere Convergence of Spline Sequences

TL;DR

This paper extends the Martingale Convergence Theorem to polynomial spline sequences in Banach spaces with the Radon-Nikodým property, proving almost everywhere convergence of spline projections in a new setting.

Contribution

It establishes the convergence of spline sequences in Banach spaces, generalizing classical scalar results to vector-valued functions with the Radon-Nikodým property.

Findings

Proves convergence of spline sequences in Banach spaces.

Extends scalar convergence results to vector-valued functions.

Demonstrates almost everywhere convergence in a new functional setting.

Abstract

We prove the analogue of the Martingale Convergence Theorem for polynomial spline sequences. Given a natural number $k $ and a sequence $(t_i)$ of knots in $[0,1]$ with multiplicity $\le k-1$, we let $P_n $ be the orthogonal projection onto the space of spline polynomials in $[0,1] $ of degree $k-1$ corresponding to the grid $(t_i)_{i=1}^n$. Let $X$ be a Banach space with the Radon-Nikod\'{y}m property. Let $(g_n)$ be a bounded sequence in the Bochner-Lebesgue space $L^1_X [0,1]$ satisfying $$ g_n = P_n ( g_{n+1} ),\qquad n \in \mathbb N . $$ We prove the existence of $\lim_{n\to \infty} g_n(t) $ in $X$ for almost every $t \in [0,1]. $ Already in the scalar valued case $X = \mathbb R $ the result is new.