@article{SF_1991__43_3_135099,
     author = {I. Levi and S. Seif},
     title = {On congruences of  ...x-normal semigroups.},
     journal = {Semigroup forum},
     pages = {93--114},
     publisher = {mathdoc},
     volume = {43},
     number = {3},
     year = {1991},
     zbl = {0727.20043},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135099/}
}