References
- A. H. Clifford and G. B. Preston, The Algebraic Theory of Semigroups, Amer. Math. Soc., Providence, Rhode Island, 1(1961).
- B. A. Jensen and D. W. Miller, Commutative Semigroups Which Are Almost Finite, Pacific J. Math., 27(1968), 533-538. https://doi.org/10.2140/pjm.1968.27.533
- I. Kaplansky, Infinite Abelian Groups, The University of Michigan Press, Ann Arbor, Michigan, 1954.