In point set topology, every subspace of a space has a unique largest subspace in which is dense, namely simply the closure . Using maps, this amounts to say that factors as followed by .
The (dense,closed)-factorization generalizes this idea from topology to topos theory. It can be viewed as a way to associate to every subtopos a closure .
A geometric embedding of elementary toposes
factors as
where (the “exterior” of ) denotes the -closure of and
the closed topology that corresponds to the subterminal object .
Here the first inclusion exhibits a dense subtopos and the second a closed subtopos.
can be viewed as the ‘best approximation’ of by a closed subtopos and therefore might be called the closure of .1
Its complement, the open subtopos corresponding to the subterminal object deserves in turn to be called the exterior of .
The (dense,closed)-factorization is a special case for inclusions of a slightly more general factorization which attaches to a general geometric morphism the closure of its image.
Recall that an inclusion is dense precisely if it is a dominant geometric morphism, hence the following is pertinent for the (dense,closed)-factorization as well.
Let be dominant and a closed inclusion at the same time. Then is an isomorphism.
Proof: Recall that are in the closed subtopos precisely when they satisfy with the subterminal object associated to . But is dominant, or what comes to the same for inclusions: dense, hence is in and therefore . From this follows , which in turn implies that all are in .
Let be a geometric morphism. Then factors as a dominant geometric morphism followed by a closed inclusion .
Proof: Let be the surjection-inclusion factorization of . Since is surjective, it is dominant (cf. this proposition). Then we use the (dense,closed)-factorization to factor into . Since both are dominant, so is and yields the demanded factorization of .
M. Artin, A. Grothendieck, J. L. Verdier, Théorie des Topos et Cohomologie Etale des Schémas (SGA4), LNM 269 Springer Heidelberg 1972. (Exposé IV 9.3.4-9.4., pp.456ff)
Peter Johnstone, Sketches of an Elephant vol.I , Oxford UP 2002. (around Lemma A 4.5.19, p. 219)
Olivia Caramello, Lattices of theories , arXiv:0905.0299 (2009). (section 8)
Last revised on May 31, 2022 at 16:02:27. See the history of this page for a list of all contributions to it.