Sel’kin, V. M. “Minimal Hereditary ω-Local Non-ℌ-Formations”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 3 (March 25, 2002): 373-380. Accessed May 18, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4073.