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For questions about coalgebras, comultiplication, cocommutativity, counity, comodules, bicomodules, coactions, corepresentations, cotensor product, subcoalgebras, coideals, coradical, cosemisimplicity, ...

32 votes

What is a coalgebra intuitively?

Coalgebras appear naturally in combinatorics as describing ways one can decompose objects into other objects of the same type. … I learned this point of view from Gian-Carlo Rota's Coalgebras and bialgebras in combinatorics, which I currently cannot find an online copy of... …
3 votes

(Co)Universal Property of Quotients/Subs

1) The answer appears to be no; this was discussed on MO previously. 2) The symmetric algebra functor is left adjoint to the forgetful functor from commutative algebras to vector spaces (over a fiel …
Qiaochu Yuan's user avatar
3 votes
Accepted

Why the preimage rather than image in Stone-type dualities.

You should think of the preimage as taking the pullback of $\mathbb{F}_2$-valued functions. For any topological space $X$, the space of continuous functions $X \to \mathbb{F}_2$ may be identified with …
Qiaochu Yuan's user avatar
7 votes
Accepted

Comultiplication on objects in an (abelian?) category

Sure, we can define such things. Let's work in the Morita 2-category $\text{Mor}(k)$ over a commutative ring $k$, which has objects $k$-algebras $A$, morphisms $k$-bimodules (with composition given …
Qiaochu Yuan's user avatar