@article{SF_1981__23_134517,
     author = {H. Cohen and R.J. Koch},
     title = {Idempotent semigroups generated by threads, {I.}},
     journal = {Semigroup forum},
     pages = {247--254},
     publisher = {mathdoc},
     volume = {23},
     year = {1981},
     zbl = {0533.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1981__23_134517/}
}