Forward operator monoids

TL;DR

This paper introduces and studies forward operator monoids, a class of Hilbert space operator collections with monoid structure, exploring their associated vectors and connections to major problems like the Riemann Hypothesis.

Contribution

It defines forward operator monoids, analyzes their special vectors, and links these concepts to longstanding mathematical problems such as the Riemann Hypothesis.

Findings

Aleph vectors are unique up to a constant when they exist.

Aleph vectors can characterize all cyclic and inner vectors via least-squares methods.

Connections to the Periodic Dilation Completeness Problem and the Riemann Hypothesis are established.

Abstract

This work studies collections of Hilbert space operators which possess a strict monoid structure under composition. These collections can be thought of as discrete unital semigroups for which no subset of the collection is closed under composition (apart from the trivial subset containing only the identity). We call these collections $\textit{forward operator monoids}$. Although not traditionally painted in this light, monoids of this flavor appear in many areas of analysis; as we shall see, they are intimately linked to several well-known problems. The main aim of this work is to study three classes of vectors associated to a given operator monoid: cyclic vectors, generalized inner vectors, and vectors which are both cyclic and inner (we refer to this last class as aleph vectors). We show, when they exist, aleph vectors are unique up to a multiplicative constant. We also show, under the lens of a least-squares problem, how an aleph vector can be used to characterize all cyclic and inner vectors. As an application of this theory, we consider monoids related to the Periodic Dilation Completeness Problem and the Riemann Hypothesis. After recovering some known results, we conclude by giving a further reformulation of the Riemann Hypothesis based on works of B\'{a}ez-Duarte and Noor.