@article{SF_1991__43_3_135121,
     author = {W. Williams and I. Levi},
     title = {Normal semigroups of partial one-to-one transformations {II.}},
     journal = {Semigroup forum},
     pages = {344--356},
     publisher = {mathdoc},
     volume = {43},
     number = {3},
     year = {1991},
     zbl = {0734.20031},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135121/}
}