Examples of HNN-extensions with nontrivial quasi-kernels. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [S. l.], v. 107, p. 107–129, 2020. DOI: 10.60063/gsu.fmi.107.107-129. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/107. Acesso em: 4 feb. 2026.