category theory

duality

# Contents

## Idea

While a (left or right) adjoint to a functor may be understood as the best approximation (from one side or the other) of a possibly non-existent inverse, any pair of adjoint functors restricts to an equivalence of categories on subcategories. These subcategories are sometimes known as the center of the adjunction, their objects are sometimes known as the fixed points of the adjunction.

The equivalences of categories that arise from fixed points of adjunctions this way are often known as dualities. Examples include Pontrjagin duality, Gelfand duality, Stone duality, and the Isbell duality between commutative rings and affine schemes (see Porst-Tholen 91).

## Definition

###### Proposition

(fixed point equivalence of an adjunction)

Let

$\mathcal{D} \underoverset {\underset{ R }{\longrightarrow}} {\overset{ L }{\longleftarrow}} {\phantom{AA}\bot\phantom{AA}} \mathcal{C}$

be a pair of adjoint functors. Say that

1. an object $c \in \mathcal{C}$ is a fixed point of the adjunction if its adjunction unit is an isomorphism

$c \underoverset{\simeq}{\eta_c}{\longrightarrow} R L (c)$

and write

$\mathcal{C}_{fix} \hookrightarrow \mathcal{C}$

for the full subcategory on these fixed objects;

2. an object $d \in \mathcal{D}$ is a fixed point of the adjunction if its adjunction counit is an isomorphism

$L R(d) \underoverset{\simeq}{\epsilon_d}{\longrightarrow} d$

and write

$\mathcal{D}_{fix} \hookrightarrow \mathcal{D}$

for the full subcategory on these fixed objects.

Then the adjunction (co-)restrics to an adjoint equivalence on these full subcategories of fixed points:

$\mathcal{D}_{fix} \underoverset {\underset{ R }{\longrightarrow}} {\overset{ L }{\longleftarrow}} {\phantom{A}\phantom{{}_{\bot}}\simeq_{\bot}\phantom{A}} \mathcal{C}_{fix}$
###### Proof

It is sufficient to see that the functors (co-)restrict as claimed, for then the restricted adjunction unit/counit are isomorphisms by definition, and hence exhibit an adjoint equivalence.

Hence we need to show that

1. for $c \in \mathcal{C}_{fix} \hookrightarrow \mathcal{C}$ we have that $\epsilon_{L(c)}$ is an isomorphism;

2. for $d \in \mathcal{D}_{fix} \hookrightarrow \mathcal{D}$ we have that $\eta_{R(d)}$ is an isomorphism.

For the first case we claim that $L(\eta_{c})$ provides an inverse: by the triangle identity (?) it is a right inverse, but by assumption it is itself an invertible morphism, which implies that $\epsilon_{L(c)}$ is an isomorphism.

The second claim is formally dual.

## Properties

• If the adjunction is idempotent, then the fixed objects in $\mathcal{C}$ are precisely those of the form $G d$, and dually the fixed objects in $\mathcal{D}$ are those of the form $F c$. Indeed, this is essentially the definition of an idempotent adjunction.

## Examples

### Gelfand duality

Gelfand duality is (a further restriction of) the fixed point equivalence of the adjunction between compactly generated Hausdorff spaces and topological algebras over the complex numbers, given by using the complex numbers as dualizing object (Porst-Tholen 91, section 4-c).