Kungozhin, A. M. “Description of Positively Existentially Closed Models in Any Class of -Structures With Unary Predicates Axiomatizable by Any H-Universal Sentence”. Journal of Mathematics, Mechanics and Computer Science, vol. 69, no. 2, June 2011, pp. 39-43, https://bm.kaznu.kz/index.php/kaznu/article/view/193.