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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,...

11 votes

Au revoir, law of excluded middle?

In a topos, the question is not whether a sentence is true or false, but where it is true, because toposes--at least the geometric ones you're talking about--are generalized spaces. There can be limit …
Wouter Stekelenburg's user avatar
3 votes
0 answers
187 views

Cloven Kan fibrations

For each groupoid $G$ there is a monad such that the cloven fibred groupoids over $G$ are algebras for this monad. I am looking for an infinite dimensional counterpart: a monad on simplicial sets whos …
5 votes
0 answers
101 views

Ex/reg toposes without generic monomorphisms

A generic monomorphism is a monomorphism of which every monomorphisms in the same category is a pullback; it is like a subobject classifier without uniqueness. I am looking for a locally Cartesian clo …
5 votes

Does this kind of endofunctor ever have an initial algebra?

I don't know any examples with $\Omega$, but $x\mapsto 2^{2^x}$ has an initial algebra in the effective topos. The object $2^{2^x}$ is a quotient of a subobject of the natural number object for any ob …
Wouter Stekelenburg's user avatar
7 votes

On internal functions and arrows in a Topos

Question 1] Morphisms $\lambda:X\times Y\to\Omega$ correspond to subobjects $L\subseteq X\times Y$. The conditions ed says that the projection $\pi_0:L\to X$ is a (regular) epimorphism, and uv says th …
Wouter Stekelenburg's user avatar