Katsura, Takeshi; Muhly, Paul S.; Sims, Aidan; and Tomforde, Mark, 2010, Graph algebras, exel-laca algebras, and ultragraph algebras coincide up to Morita equivalence, Journal fur die Reine und Angewandte Mathematik, 640, 135-165.
We prove that the classes of graph algebras, Exel-Laca algebras, and ultragraph algebras coincide up to Morita equivalence. This result answers the long-standing open question of whether every Exel-Laca algebra is Morita equivalent to a graph algebra. Given an ultragraph G we construct a directed graph E such that C*(G) is isomorphic to a full corner of C*(E). As applications, we characterize real rank zero for ultragraph algebras and describe quotients of ultragraph algebras by gauge-invariant ideals.