nLab
cocommutative coalgebra

Contents

Idea

The notion of cocommutative coalgebra is the formal dual of commutative algebra.

Coommutative coalgebras form the category CocommCoalg.

Definition

For (C,,I,σ)(C,\otimes, I, \sigma) a symmetric monoidal category, a comonoid object in CC is an object AA equipped with a morphism

Δ:AAA \Delta : A \to A \otimes A

(“comultiplication?”) and a morphism

ε:AI \varepsilon: A \to I

(“counit”) that satisfy the coassociativity? and couniticity? equations:

A Δ AA Δ IdΔ AA ΔId AAA A Id Δ Id A Idε AA εId A. \array{ A &\stackrel{\Delta}{\to}& A \otimes A \\ ^\mathllap{\Delta} \downarrow && \downarrow^{\mathrlap{Id \otimes \Delta}} \\ A \otimes A &\underset{\Delta \otimes Id}{\to}& A \otimes A \otimes A } \qquad \qquad \qquad \array{ & & A & & \\ & ^\mathllap{Id} \swarrow & \downarrow^\mathrlap{\Delta} & \searrow^\mathrlap{Id} & \\ A & \underset{Id \otimes \varepsilon}{\leftarrow} & A \otimes A & \underset{\varepsilon \otimes Id}{\to} & A } \,.

This is co-commutative if we have a commuting diagram

AA Δ σ A,A A Δ AA, \array{ && A \otimes A \\ & {}^{\mathllap{\Delta}}\nearrow && \searrow^\mathrlap{\sigma_{A,A}} \\ A &&\underset{\Delta}{\to}&& A \otimes A } \,,

where σ A,A:AAAA\sigma_{A,A} : A \otimes A \to A \otimes A is the braiding isomorphism of CC.

In the case of the symmetric monoidal category of modules over a commutative ring RR, such an object is called a cocommutative coalgebra over RR. In the case of the symmetric monoidal category of chain complexes (or differential graded spaces), such an object is called a DG coalgebra.

Sometimes “cocommutative coalgebra” in a symmetric monoidal category is used as a synonym for cocommutative comonoid object. We will follow that convention below. If MM is symmetric monoidal, then CocommCoalg(M)CocommCoalg(M) denotes the category of cocommutative algebras and coalgebra maps in MM.

Relation to cartesian monoidal categories

Cocommutative coalgebras form a bridge between the doctrines of symmetric monoidal categories and cartesian monoidal categories. Notice that every object in a cartesian monoidal category carries a unique cocommutative comonoid structure (the counit is the unique map to the terminal object, and the counicity equations then force the comultiplication map to be the diagonal), and every morphism in a cartesian monoidal category is thereby a coalgebra map.

If CC and DD are two cocommutative coalgebras, then CDC \otimes D becomes a cocommutative coalgebra with comultiplication

CDΔ CΔ DCCDDIdσIdCDCDC \otimes D \stackrel{\Delta_C \otimes \Delta_D}{\to} C \otimes C \otimes D \otimes D \stackrel{Id \otimes \sigma \otimes Id}{\to} C \otimes D \otimes C \otimes D

and counit

CDε Cε DIII.C \otimes D \stackrel{\varepsilon_C \otimes \varepsilon_D}{\to} I \otimes I \cong I.
Proposition

CDC \otimes D is the cartesian product of CC and DD in CocommCoalg(M)CocommCoalg(M).

Theorem

The forgetful 2-functor

U:CartMonCatSymMonCat,U: CartMonCat \to SymMonCat,

from cartesian monoidal categories and product-preserving functors to symmetric monoidal categories and strong symmetric monoidal functors, is coreflective. That is to say, it has a right 2-adjoint which sends MM to CocommCoalg(M)CocommCoalg(M), and the unit of the 2-adjunction is an equivalence.

Corollary

The 2-category of cartesian monoidal categories is comonadic over the 2-category of symmetric monoidal categories.

Revised on August 27, 2011 20:21:00 by Todd Trimble (69.118.58.208)