A. Arjomandfar, C.M. Campbell and H. Doostie, Semigroups related to certain group presentations, Semigroup Forum 85 (2012), no. 3, 533--539.
C. M. Campbell, E. F. Robertson, N. Ruskuc and R. M. Thomas, Semigroup and group presentations, Bull. London Math. Soc. 27 (1995), no. 1, 46--50.
A. H. Clifford and G. B. Preston, The algebraic theory of semigroups, I, Amer. Math. Soc. Surveys 7, Providence, 1961.
J. Howie, Fundamentals of Semigroup Theory, London Mathematical Society, New Series, Oxford University Press, New York, 1995.
A. Lucchini and A. Maroti, Rings as the union of proper subrings, Algebr. Represent. Theory 15 (2012), no. 6, 1035--1047.
N. P. Mukherjee, Quasicommutative semigroups I, Czechoslovak Math. J. 22(97) (1972) 449--453.
E. F. Robertson and Y. Ünlü, On semigroup presentations, Proc. Edinburgh Math. Soc. (2) 36 (1993), no. 1, 55--68.
G. Scorza, I gruppi che possono pensarsi come somma di tre loro sottogruppi, Boll. Un. Mat. Ital. 5 (1926) 216--218.