Convergence of Random Products of Countably Infinitely Many Projections

TL;DR

This paper investigates the convergence behavior of random products of infinitely many projections in a Hilbert space, providing new conditions under which such sequences converge strongly or weakly to a common projection.

Contribution

It extends convergence results to countably infinite projections and introduces the concept of pseudo-periodic functions in this context.

Findings

Established conditions for strong and weak convergence of infinite projection products.

Answered an open question by M. Sakai regarding infinite projections.

Integrated pseudo-periodic functions into the analysis of projection sequences.

Abstract

Let $r \in \mathbb{N}\cup\{\infty\}$ be a fixed number and let $P_j\,\, (1 \leq j\leq r )$ be the projection onto the closed subspace $\mathcal{M}_j$ of $\mathscr{H}$. We are interested in studying the sequence $P_{i_1}, P_{i_2}, \ldots \in\{P_1, \ldots, P_r\}$. A significant problem is to demonstrate conditions under which the sequence $\{P_{i_n}\cdots P_{i_2}P_{i_1}x\}_{n=1}^\infty$ converges strongly or weakly to $Px$ for any $x\in\mathscr{H}$, where $P$ is the projection onto the intersection $\mathcal{M}=\mathcal{M}_1\cap \ldots \cap \mathcal{M}_r$. Several mathematicians have presented their insights on this matter since von Neumann established his result in the case of $r=2$. In this paper, we give an affirmative answer to a question posed by M. Sakai. We present a result concerning random products of countably infinitely many projections (the case $r=\infty$) incorporating the notion of pseudo-periodic function.