@article{SF_1994__48_3_135306,
     author = {I. Levi},
     title = {Transformation semigroup diagram and the ...-relation.},
     journal = {Semigroup forum},
     pages = {138--151},
     year = {1994},
     volume = {48},
     number = {3},
     zbl = {0796.20059},
     url = {http://geodesic.mathdoc.fr/item/SF_1994__48_3_135306/}
}