Canonical traces and directly finite Leavitt path algebras

TL;DR

This paper studies traces on Leavitt path algebras, characterizing positivity and faithfulness, and establishes a correspondence with traces on graph $C^*$-algebras, while also characterizing directly finite Leavitt path algebras.

Contribution

It provides a characterization of positive and faithful traces on Leavitt path algebras and links these to traces on graph $C^*$-algebras, also characterizing directly finite Leavitt path algebras.

Findings

Characterization of positive traces on Leavitt path algebras.

Bijective correspondence between algebraic and operator algebra traces.

Classification of directly finite Leavitt path algebras based on graph cycles.

Abstract

Motivated by the study of traces on graph $C^*$-algebras, we consider traces (additive, central maps) on Leavitt path algebras, the algebraic counterparts of graph $C^*$-algebras. In particular, we consider traces which vanish on nonzero graded components of a Leavitt path algebra and refer to them as {\em canonical} since they are uniquely determined by their values on the vertices. A desirable property of a $\mathbb{C}$-valued trace on a $C^*$-algebra is that the trace of an element of the positive cone is nonnegative. We adapt this property to traces on a Leavitt path algebra $L_K(E)$ with values in any involutive ring. We refer to traces with this property as positive. If a positive trace is injective on positive elements, we say that it is faithful. We characterize when a canonical, $K$-linear trace is positive and when it is faithful in terms of its values on the vertices. As a consequence, we obtain a bijective correspondence between the set of faithful, gauge invariant, $\mathbb{C}$-valued (algebra) traces on $L_{\mathbb{C}}(E)$ of a countable graph $E$ and the set of faithful, semifinite, lower semicontinuous, gauge invariant (operator theory) traces on the corresponding graph $C^*$-algebra $C^*(E)$. With the direct finite condition (i.e $xy=1$ implies $yx=1$) for unital rings adapted to rings with local units, we characterize directly finite Leavitt path algebras as exactly those having the underlying graphs in which no cycle has an exit. Our proof involves consideration of "local" Cohn-Leavitt subalgebras of finite subgraphs. Lastly, we show that, while related, the class of locally noetherian, the class of directly finite, and the class of Leavitt path algebras which admit a faithful trace are different in general.