@article{SF_1993__47_2_135261,
     author = {A. Ehrenfeucht and G. Rozenberg and T. Harju},
     title = {Permutable transformation semigroups.},
     journal = {Semigroup forum},
     pages = {123--125},
     publisher = {mathdoc},
     volume = {47},
     number = {2},
     year = {1993},
     zbl = {0792.20060},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__47_2_135261/}
}