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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,...

9 votes

Merging single-sorted and multi-sorted theories

The answer to your question really depends on just how closely you want the theory of "algebraic theories relative to $\mathcal M$" to mirror the theory of classical ($S$-sorted) algebraic theories. P …
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2 votes

Reflective exponential ideals in presheaf categories

In fact, the converse holds: every cartesian closed locally presentable category is a reflective subcategory of a presheaf category whose reflector preserves finite products. First, a locally presenta …
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14 votes
Accepted

Strict toposes as a finite limit theory

The original reference for the essential algebraicity of elementary toposes is Freyd's Aspects of topoi (in which the notion of essentially algebraic theory, which is equivalent to that of a finite li …
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9 votes
Accepted

Who introduced the notion of 2-categories?

It appears that the definition of 2-category was introduced independently by two authors, both of whom independently introduced the modern notion of enriched category, for which 2-categories appeared …
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7 votes
Accepted

Is $\mathit{Topos}^\text{op} \to \mathit{Pr}^L$ monadic?

Regarding monadicity (rather than comonadicity), the (2-categorical variant of the) question is answered in Bunge–Carboni's The symmetric topos. In their paper, $\mathbf A$ denotes the 2-category of l …
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8 votes

Are there simple conditions on a category C which guaranty that Ind(C) is a Grothendieck to...

In Di Liberti–Ramos González's Gabriel-Ulmer duality for topoi and its relation with site presentations, they state (Theorem 3.17) that $\mathbf{Ind}(\mathscr C)$ is a Grothendieck topos if and only i …
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23 votes

Two interpretations of implication in categorical logic?

There are two concepts here, which are tightly connected. Logically, this corresponds to the distinction between $\vdash$ and $\Rightarrow$. (A) Morphisms $t : \Gamma \to A$ represent (well-formed, ty …
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