@article{SF_1993__46_1_135237,
     author = {C.M. Campbell and E.F. Robertson and R.M. Thomas},
     title = {On a class of semigroups with symmetric presentations.},
     journal = {Semigroup forum},
     pages = {286--306},
     publisher = {mathdoc},
     volume = {46},
     number = {1},
     year = {1993},
     zbl = {0794.20063},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__46_1_135237/}
}