“$\prod_{1}^{0}$-POSITIVE INDUCTIVE DEFINABILITY ON ABSTRACT STRUCTURES” (1998) Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, 90, pp. 91–108. Available at: https://annual.uni-sofia.bg/index.php/fmi/article/view/288 (Accessed: 15 December 2025).