Mokrousov, Yu. G. “On the Denseness of Subspaces of Analytic Vectors of a Closed Linear Operator in a Banach Space”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 52, no. 7, July 2000, pp. 985-9,
https://umj.imath.kiev.ua/index.php/umj/article/view/4499
.