TushevA. V. “On Ideals of the Group Algebra of a Free Group of Degree of Freedom Two over the Field of Complex Numbers”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 4, Apr. 1995, pp. 571-2, https://umj.imath.kiev.ua/index.php/umj/article/view/5460.