Kuznetsova, V. “MODEL REPRESENTATIONS OF THE LIE ALGEBRA $[A_2, A_1] = iA_1$ OF LINEAR NON-SELFADJOINT OPERATORS”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 100, Dec. 2011, pp. 93-112, https://stipendii.uni-sofia.bg/index.php/fmi/article/view/248.