@article{SF_1971__3_133865,
     author = {B.P. jr. Brooks and W.E. Clark},
     title = {On the categori-city of semigroup-theoretical properties.},
     journal = {Semigroup forum},
     pages = {259--266},
     publisher = {mathdoc},
     volume = {3},
     year = {1971},
     zbl = {0239.20079},
     url = {http://geodesic.mathdoc.fr/item/SF_1971__3_133865/}
}