Sel’kin, V. M. “Minimal Hereditary ω-Local Non-ℌ-Formations”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 3 (March 25, 2002): 373–380. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4073.