ON THE INVESTIGATIONS OF IVAN PRODANOV IN THE THEORY OF ABSTRACT SPECTRA. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 102, p. 31–70, 2015. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/63. Acesso em: 11 jan. 2026.