@article{10_4153_CMB_1960_026_3,
     author = {Brainerd, Barron},
     title = {On {Birkhoff's} {Problem} 73 {For} {Monoids}},
     journal = {Canadian mathematical bulletin},
     pages = {217--220},
     year = {1960},
     volume = {3},
     number = {3},
     doi = {10.4153/CMB-1960-026-3},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1960-026-3/}
}