@article{SF_1982__25_134584,
     author = {R.D. Kopperman},
     title = {Lengths on semigroups and groups.},
     journal = {Semigroup forum},
     pages = {345--360},
     publisher = {mathdoc},
     volume = {25},
     year = {1982},
     zbl = {0502.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1982__25_134584/}
}