@article{SF_1993__47_2_135264,
     author = {C . Vachuska and P. Vachuska},
     title = {The bicyclic semigroup has {F4*.}},
     journal = {Semigroup forum},
     pages = {133--134},
     publisher = {mathdoc},
     volume = {47},
     number = {2},
     year = {1993},
     zbl = {0793.20055},
     url = {http://geodesic.mathdoc.fr/item/SF_1993__47_2_135264/}
}