Dimov, Georgi, and Dimiter Vakarelov. “ON THE INVESTIGATIONS OF IVAN PRODANOV IN THE THEORY OF ABSTRACT SPECTRA”. Ann. Sofia Univ. Fac. Math. and Inf. 102 (December 12, 2015): 31–70. Accessed January 15, 2025. https://admin.uni-sofia.bg/index.php/fmi/article/view/63.