Yeshkeyev, A.R., and M.T. Omarova. “Chains of Existentially Closed Models of Positive (n1, n2)-Jonsson Theories”. Bulletin of the Karaganda University. Mathematics Series, vol. 96, no. 4, Dec. 2019, pp. 69-74, doi:10.31489/2019m4/69-74.