Bedratyuk, L. P. “Poincare Series of the Multigraded Algebras of SL 2-Invariants”. Ukrains’kyi Matematychnyi Zhurnal, vol. 63, no. 6, June 2011, pp. 755-63, https://umj.imath.kiev.ua/index.php/umj/article/view/2760.