Row-finite equivalents exist only for row-countable graphs

Abstract

If $E$ is a not-necessarily row-finite graph, such that each vertex of $E$ emits at most countably many edges, then a {\it desingularization} $F$ of $E$ can be constructed (see e.g. (1) G. Abrams, G. Aranda Pino, Leavitt path algebras of arbitrary graphs, Houston J. Math 34(2) (2008), 423-442, or (2) I. Raeburn, Graph algebras. CBMS Regional Conference Series in Mathematics 103, Conference Board of the Mathematical Sciences, Washington, DC, 2005, ISBN 0-8218-3660-9). The desingularization process has been effectively used to establish various characteristics of the Leavitt path algebras of not-necessarily row-finite graphs. Such a desingularization $F$ of $E$ has the properties that: (1) $F$ is row-finite, and (2) the Leavitt path algebras $L(E)$ and $L(F)$ are Morita equivalent. We show here that for an arbitrary graph $E$, a graph $F$ having properties (1) and (2) exists (we call such a graph $F$ a \emph{row-finite equivalent of} $E$) if and only if $E$ is row-countable; that is, $E$ contains no vertex $v$ for which $v$ emits uncountably many edges.