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On a certain relation for closure operations on a semigroup. Czechoslovak Math. J. 20 (95), 1970, 220-231.
MR 0260905 |
Zbl 0198.34304
[4] PONDĚLÍČEK B.:
A certain equivalence on a semigroup. Czechoslovak Math. J. 21 (96), 1971. 109-117.
MR 0283110 |
Zbl 0217.36404