“$\prod_{1}^{0}$-POSITIVE INDUCTIVE DEFINABILITY ON ABSTRACT STRUCTURES”, Ann. Sofia Univ. Fac. Math. and Inf., vol. 90, pp. 91–108, Dec. 1998, Accessed: Feb. 04, 2026. [Online]. Available: https://annual.uni-sofia.bg/index.php/fmi/article/view/288