@article{MAN_1922__86_158933,
     author = {O.D. Kellog},
     title = {On the existence and closure of sets of characteristic functions},
     journal = {Mathematische Annalen},
     pages = {14--17},
     year = {1922},
     volume = {86},
     url = {http://geodesic.mathdoc.fr/item/MAN_1922__86_158933/}
}