[1] B. Zelinka: Tolerance in algebraic structures II. Czech. Math. J. 25 (1975), 175-178. MR 0457323 | Zbl 0316.08001
[2] B. Zelinka: Tolerance relations on periodic commutative semigroups. Czech. Math. J. 27 (1977), 167-169. MR 0427509 | Zbl 0359.20045
[3] A. H. Clifford, G. B. Preston: The algebraic Theory of Semigroups. Vol. I. Math. Surveys No. 7, Amer. Math. Soc, Providence, R. I. (1961). MR 0132791 | Zbl 0111.03403