Vedernikov, V. A., and D. G. Koptyukh. “Hereditary Critical Ω-Compositional Formations”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 53, no. 5, May 2001, pp. 579-88,
https://umj.imath.kiev.ua/index.php/umj/article/view/4281
.