1. V. V. Vagner, “Generalized groups,” Dokl. Akad. Nauk SSSR, 84, No. 6, 1119–1122 (1952).
2. O. V. Gutik, “On the Gaui semigroup,” Mat. Met. Fiz.-Mekh. Polya, 42, No. 4, 127–132 (1999).
3. O. Gutik and M. Mykhalevych, “On one generalization of a bicyclic monoid,” Visn. Lviv. Univ., Ser. Mekh.-Mat., Issue 90, 5–19 (2020).
4. O. Andersen, Ein Bericht über die Struktur abstrakter Halbgruppen, PhD Thesis, Hamburg (1952).
5. C. J. Ash, “The J -classes of an inverse semigroup,” J. Austral. Math. Soc., 28, No. 4, 427–432 (1979); https://doi.org/10.1017/S144678870001257X.