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category theory

# Contents

## Idea

Given a pair of adjoint functors

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

there is induced an adjunction of opposite functors between their opposite categories of the form

$\mathcal{C}^{op} \underoverset {\underset{L^{op}}{\longrightarrow}} {\overset{R^{op}}{\longleftarrow}} {\;\;\;\;\bot\;\;\;\;} \mathcal{D}^{op} \,.$

Hence where $L$ was the left adjoint, its opposite becomes the right adjoint, and dually for $R$.

This is immediate from the definition of opposite categories and the characterization of adjoint functors via the corresponding hom-isomorphism.

The adjunction unit of the opposite adjunction has as components the components of the original adjunction counit, regarded in the opposite category, and dually:

$\epsilon^{R^{op} C^{op}}_{d} \;\colon\; R^{op}\circ L^{op}(d) \xrightarrow{\;\; \big( \eta^{R L}_d \big)^{op} \;\;} d \,, {\phantom{AAAAAA}} \eta^{L^{op} R^{op}}_{c} \;\colon\; c \xrightarrow{\;\; \big( \epsilon^{L R}_c \big)^{op} \;\;} L^{op} \circ R^{op}(c) \,.$

Last revised on July 20, 2021 at 09:55:41. See the history of this page for a list of all contributions to it.