@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/}
}