M. L. Gorbachuk and Y. G. Mokrousov, “On density of some sets of infinitely differentiable vectors of a closed operator on a Banach space”, MFAT, vol. 8, no. 1, pp. 23–29, Mar. 2002, Accessed: Dec. 18, 2025. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/173