@article{SF_1974__8_134044,
     author = {Martha O. Bertman},
     title = {Free topological inverse semigroups.},
     journal = {Semigroup forum},
     pages = {266--269},
     publisher = {mathdoc},
     volume = {8},
     year = {1974},
     zbl = {0302.22008},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__8_134044/}
}