Yeshkeyev, A.R., and M.T. Omarova. “Central Types of Convex Fragments of the Perfect Jonsson Theory”. Bulletin of the Karaganda University. Mathematics Series 93, no. 1 (March 30, 2019): 95–101. Accessed December 8, 2025. https://mts.buketov.edu.kz/mathematics-vestnik/article/view/288.