GorbachukV. I., and GorbachukM. L. “Some Problems of Spectral Theory of the Second Order Linear Differential Equation With Unlimited Operator Coefficients ”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 23, no. 1, Dec. 1970, pp. 3-14, https://umj.imath.kiev.ua/index.php/umj/article/view/8473.