@article{GDZPPN001254235, title = {The Hopf property and K-free products of semigroups.}, journal = {Semigroup forum}, volume = {20}, year = {1980}, pages = {343-368}, zbl = {0448.20055}, url = {http://dml.mathdoc.fr/item/GDZPPN001254235} }
Jones, P.R. The Hopf property and K-free products of semigroups.. Semigroup forum, Tome 20 (1980) pp. 343-368. http://gdmltest.u-ga.fr/item/GDZPPN001254235/