@article{SF_1997__54_2_135532,
     author = {S.A. Annin},
     title = {Hierarchy of efficient generators of the symmetric inverse monoid.},
     journal = {Semigroup forum},
     pages = {327--355},
     publisher = {mathdoc},
     volume = {54},
     number = {2},
     year = {1997},
     zbl = {0873.20043},
     url = {http://geodesic.mathdoc.fr/item/SF_1997__54_2_135532/}
}