@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/