Petravchuk, A. P. “On Strongly Inert Subalgebras of an Infinite-Dimensional Lie Algebra”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 54, no. 7, July 2002, pp. 1025-8,
https://umj.imath.kiev.ua/index.php/umj/article/view/4143
.