Unbounded strongly irreducible operators and transitive representations   of quivers on infinite-dimensional Hilbert spaces
  We introduce unbounded strongly irreducible operators and transitive operators. These operators are related to a certain class of indecomposable Hilbert representations of quivers on infinite-dimensional Hilbert spaces. We regard the theory of Hilbert representations of quivers is a generalization of the theory of unbounded operators. A non-zero Hilbert representation of a quiver is said to be transitive if the endomorphism algebra is trivial. If a Hilbert representation of a quiver is transitive, then it is indecomposable. But the converse is not true. Let $\Gamma$ be a quiver whose underlying undirected graph is an extended Dynkin diagram. Then there exists an infinite-dimensional transitive Hilbert representation of $\Gamma$ if and only if $\Gamma$ is not an oriented cyclic quiver. 
Masatoshi Enomoto and Yasuo Watatani
math.FA math.OA