Topos

In mathematics, a topos (/ˈtoʊpoʊs/, /ˈtoʊpɒs/ or /ˈtɒpɒs/; plural topoi /ˈtoʊpɔɪ/ or /ˈtɒpɔɪ/, or toposes) is a category that behaves like the category of sheaves of sets on a topological space (or more generally: on a site). Topoi behave much like the category of sets and possess a notion of localization; they are in a sense a generalization of point-set topology. The Grothendieck topoi find applications in algebraic geometry; the more general elementary topoi are used in logic.

Grothendieck topoi (topoi in geometry)

Since the introduction of sheaves into mathematics in the 1940s a major theme has been to study a space by studying sheaves on a space. This idea was expounded by Alexander Grothendieck by introducing the notion of a "topos". The main utility of this notion is in the abundance of situations in mathematics where topological heuristics are very effective but an honest topological space is lacking; it is sometimes possible to find a topos formalizing the heuristic. An important example of this programmatic idea to date is the étale topos of a scheme.

∞-topos

In mathematics, an ∞-topos is, roughly, an ∞-category such that its objects are sheaves with some choice of Grothendieck topology; in other words, it gives an intrinsic notion of sheaves without reference to an external space. The prototypical example of an ∞-topos is the ∞-category of sheaves of, say, abelian groups on some topological space. But the notion is more flexible; for example, the ∞-category of étale sheaves on some affine scheme is not the ∞-category of sheaves on any topological space but it is still an ∞-topos.

Precisely, in Lurie's Higher Topos Theory, an ∞-topos is defined as an ∞-category X such that there is an ∞-category C and a left exact localization functor from the ∞-category of presheaves of spaces on C to X. A theorem of Lurie states that an ∞-category is an ∞-topos if and only if it satisfies an ∞-categorical version of Giraud’s axioms in ordinary topos theory. Authors including Wikipedia describes a "topos" as a category behaving like the category of sheaves of sets on a topological space. In analogy, Lurie's definition and characterization theorem of an ∞-topos says that an ∞-topos is an ∞-category behaving like the category of sheaves of spaces.

Topos (disambiguation)

Topos may refer to: