@article{SF_1972__5_133949,
     author = {R.J. Warne},
     title = {On the structure of semigroups which are unions of groups.},
     journal = {Semigroup forum},
     pages = {323--330},
     year = {1972},
     volume = {5},
     zbl = {0275.20107},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__5_133949/}
}