NazarovaL. A., and RoiterA. V. “The Norm of a Relation, Separating Functions, and Representations of Marked Quivers”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 6, June 2002, pp. 808-40, https://umj.imath.kiev.ua/index.php/umj/article/view/4119.