@article{MZ_1972__125_171690,
     author = {Kermit Sigmon},
     title = {Idempotent {Topological} {Groupoids} with {Non-Empty} {Center.}},
     journal = {Mathematische Zeitschrift},
     pages = {70--78},
     publisher = {mathdoc},
     volume = {125},
     year = {1972},
     zbl = {0225.55008},
     url = {http://geodesic.mathdoc.fr/item/MZ_1972__125_171690/}
}