Redchuk, I. K., and A. V. Roiter. “Singular Locally Scalar Representations of Quivers in Hilbert Spaces and Separating Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 6, June 2004, pp. 796–809, https://umj.imath.kiev.ua/index.php/umj/article/view/3799.