@article{SF_1972__4_133914,
     author = {K.K.H. Butler and R. Brandon and D.W. Hardy and G. Markowsky},
     title = {Cardinalities of {D-classes} in {Bn.}},
     journal = {Semigroup forum},
     pages = {341--344},
     publisher = {mathdoc},
     volume = {4},
     year = {1972},
     zbl = {0253.05006},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__4_133914/}
}