@article{SF_1985__31_134757,
     author = {M. Petrich and J.A. Gerhard},
     title = {All varieties of orthogroups.},
     journal = {Semigroup forum},
     pages = {311--352},
     publisher = {mathdoc},
     volume = {31},
     year = {1985},
     zbl = {0557.20033},
     url = {http://geodesic.mathdoc.fr/item/SF_1985__31_134757/}
}