@article{SF_1976__12_134151,
     author = {P.G. Trotter},
     title = {Congruence-free regular semigroups with zero.},
     journal = {Semigroup forum},
     pages = {1--5},
     year = {1976},
     volume = {12},
     zbl = {0325.20056},
     url = {http://geodesic.mathdoc.fr/item/SF_1976__12_134151/}
}