@article{GDZPPN001252615, title = {Extension theories for monoids.}, journal = {Semigroup forum}, volume = {16}, year = {1978}, pages = {13-36}, zbl = {0394.18007}, url = {http://dml.mathdoc.fr/item/GDZPPN001252615} }
Wells, C. Extension theories for monoids.. Semigroup forum, Tome 16 (1978) pp. 13-36. http://gdmltest.u-ga.fr/item/GDZPPN001252615/