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@article{SF_1985__31_134748, author = {I. Levi}, title = {On the set of ranges of a transformation semigroup.}, journal = {Semigroup forum}, pages = {219--226}, publisher = {mathdoc}, volume = {31}, year = {1985}, zbl = {0549.20049}, url = {http://geodesic.mathdoc.fr/item/SF_1985__31_134748/} }
I. Levi. On the set of ranges of a transformation semigroup.. Semigroup forum, Tome 31 (1985), pp. 219-226. http://geodesic.mathdoc.fr/item/SF_1985__31_134748/