Gorbachuk, V. I., and M. L. Gorbachuk. “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.