Free Semigroupoid Algebras

TL;DR

This paper develops a comprehensive structure theory for free semigroupoid algebras generated by directed graphs, including their classification, radical, automorphisms, and invariant subspaces, linking algebraic properties to graph structures.

Contribution

It introduces a new framework for analyzing free semigroupoid algebras, characterizing their structure, radicals, automorphisms, and invariants through graph-theoretic methods.

Findings

Graph determines the algebra up to unitary equivalence.

Explicit description of the Jacobson radical for finite graphs.

Classification of partly free algebras via graph properties.

Abstract

Every countable directed graph generates a Fock space Hilbert space and a family of partial isometries. These operators also arise from the left regular representations of free semigroupoids derived from directed graphs. We develop a structure theory for the weak operator topology closed algebras generated by these representations, which we call free semigroupoid algebras. We characterize semisimplicity in terms of the graph and show explicitly in the case of finite graphs how the Jacobson radical is determined. We provide a diverse collection of examples including; algebras with free behaviour, and examples which can be represented as matrix function algebras. We show how these algebras can be presented and decomposed in terms of amalgamated free products. We determine the commutant, consider invariant subspaces, obtain a Beurling theorem for them, conduct an eigenvalue analysis, give an elementary proof of reflexivity, and discuss hyper-reflexivity. Our main theorem shows the graph to be a complete unitary invariant for the algebra. This classification theorem makes use of an analysis of unitarily implemented automorphisms. We give a graph-theoretic description of when these algebras are partly free, in the sense that they contain a copy of a free semigroup algebra.