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@article{SF_1993__47_2_135253, author = {H. Pei}, title = {The a-congruences on {S(X)} and the {S-equivalences} on {X.}}, journal = {Semigroup forum}, pages = {48--59}, publisher = {mathdoc}, volume = {47}, number = {2}, year = {1993}, zbl = {0784.20034}, url = {http://geodesic.mathdoc.fr/item/SF_1993__47_2_135253/} }
H. Pei. The a-congruences on S(X) and the S-equivalences on X.. Semigroup forum, Tome 47 (1993) no. 2, pp. 48-59. http://geodesic.mathdoc.fr/item/SF_1993__47_2_135253/