@article{SF_1972__4_133887,
     author = {M. Friedberg},
     title = {Some examples of clans.},
     journal = {Semigroup forum},
     pages = {156--164},
     publisher = {mathdoc},
     volume = {4},
     year = {1972},
     zbl = {0259.22005},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__4_133887/}
}