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A topos is a category that behaves very much like the category of sets and possesses a good notion of localization. Related to topos are: sheaves, presheaves, descent, stacks, localization,...

2 votes

What information is lost in $X \to \mathrm{Sh}(X)$?

Benjamin Steinberg's answer covers the case of topological spaces, so I'll answer this question for sites. The functor $$\mathrm{Sh}:\mathbf{Site}\rightarrow\mathbf{Topos}^{\mathrm{op}}$$ (here $\mat …
Oscar Cunningham's user avatar
16 votes
2 answers
1k views

Is there a universal way to force the Axiom of Choice to be true?

Given a model of set theory $V$ there are various ways to construct a model in which the Axiom of Choice holds, such as Gödel's constructible universe $L^V$ or by using forcing*. I'm wondering if any …
Oscar Cunningham's user avatar
19 votes
1 answer
876 views

Has this "backwards" perspective on toposes been studied?

Topos theory can be seen as a categorification of topology via the following analogies. \begin{array}{|c|c|} \hline \text{locales}&\text{Grothendieck toposes}\\\hline \text{open sets}&\text{sheaves}\ …
Oscar Cunningham's user avatar