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Enriched categories, topoi, abelian categories, monoidal categories, homological algebra.
8
votes
Accepted
Do affine schemes form a Mal'cev category?
(This is a repeat of an above comment.) The category of affine schemes is not Mal'cev. This can be disproven by producing an reflexive, non-symmetric relation on an affine scheme $X$ whose graph is …
10
votes
1
answer
806
views
Subcategories which still give a Yoneda embedding
If $\mathbf{C}$ is a category, then the Yoneda functor which sends $a$ to $Hom_\mathbf{C}(-,a)$ is a fully faithful embedding of categories
$$ \mathbf{C}\rightarrow \mathbf{Func}(\mathbf{C}^{op},\mat …
110
votes
11
answers
13k
views
Why do Groups and Abelian Groups feel so different?
Groups are naturally "the symmetries of an object". To me, the group axioms are just a way of codifying what the symmetries of an object can be so we can study it abstractly.
However, this heuristic …
17
votes
3
answers
1k
views
Freyd-Mitchell for triangulated categories?
Is there a nice analog of the Freyd-Mitchell theorem for triangulated categories (potentially with some requirements)? Freyd-Mitchell is the theorem which says that any small abelian category is a fu …
4
votes
What is an obviously coordinate-independent description of the Chevellay-Eilenberg complex f...
I believe an approach that works is to define the Chevalley-Eilenberg complex as a kind of `Koszul complex over the ring of functions'. The enveloping algebra $U$ is relatively quadratic over the rin …