A constructive “Closed subgroup theorem” for localic groups and groupoids Peter T. Johnstone (1989) Cahiers de Topologie et Géométrie Différentielle Catégoriques
A few topological problems Mary Ellen Rudin (1988) Commentationes Mathematicae Universitatis Carolinae
A set functor which commutes with all homfunctors is a homfunctor Václav Koubek, Jan Reiterman (1976) Czechoslovak Mathematical Journal