R. Nichols, S. Hofmann, and F. Gesztesy, “On square root domains for non-self-adjoint Sturm-Liouville operators”, MFAT, vol. 19, no. 3, pp. 227–259, Sep. 2013, Accessed: Dec. 16, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/550