“Linear Cross-Sections and Fredholm Operators in a Class Groupoid $C^*$-Algebras”. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics 108 (December 12, 2021): 17–21. Accessed December 14, 2025. https://annual.uni-sofia.bg/index.php/fmi/article/view/550.