@article{SF_1980__20_134412,
     author = {P. Normak},
     title = {Purity in the category of {M-sets.}},
     journal = {Semigroup forum},
     pages = {157--170},
     publisher = {mathdoc},
     volume = {20},
     year = {1980},
     zbl = {0521.20050},
     url = {http://geodesic.mathdoc.fr/item/SF_1980__20_134412/}
}