[1] A. H. Clifford, G. B. Preston: The Algebraic Theory of Semigroups. Vol. I, American Math. Society, Providence, R. I., 1961. MR 0132791 | Zbl 0111.03403

[2] J. B. Fraleigh: A First Course in Abstract Algebra. Addison Wesley, Reading, Mass., 1967. MR 0225619 | Zbl 0154.26303

[3] M. Petrich: On the structure of a class of commutative semigroups. Czechoslovak Math. J., 14 (1964), 147-153. MR 0166284 | Zbl 0143.03403