@article{SF_1972__4_133916,
     author = {H.E. Scheiblich},
     title = {Free inverse semigroups.},
     journal = {Semigroup forum},
     pages = {351--359},
     publisher = {mathdoc},
     volume = {4},
     year = {1972},
     zbl = {0259.20054},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__4_133916/}
}