@article{SF_1994__49_2_135333,
     author = {H. Pei},
     title = {Equivalences, a-semigroups and a-congruences.},
     journal = {Semigroup forum},
     pages = {49--58},
     publisher = {mathdoc},
     volume = {49},
     number = {2},
     year = {1994},
     zbl = {0804.20046},
     url = {http://geodesic.mathdoc.fr/item/SF_1994__49_2_135333/}
}