nLab walking adjoint equivalence

Contents

1. Idea

The walking adjoint equivalence (as in โ€œwalking structureโ€) or free-standing adjoint equivalence is the 2-category (in fact a (2,1)-category) which โ€˜representsโ€™ adjoint equivalences in a 2-category. It is a categorification of the free-standing adjoint isomorphism?.

2. Definition

Definition 2.1. The free-standing adjoint equivalence is the unique (up to isomorphism) 2-category ๐’œโ„ฐ\mathcal{AE} with exactly two objects 00 and 11, 1-morphisms freely generated by an arrow i:0โ†’1i: 0 \rightarrow 1, and an arrow j:1โ†’0j: 1 \rightarrow 0, and 2-morphisms generated by 2-isomorphisms ฮน 0:jโˆ˜iโ‰…id(0)\iota_{0}: j \circ i \cong id(0) and ฮน 1:iโˆ˜jโ‰…id(1)\iota_{1}: i \circ j \cong id(1), subject to the equalities

id(i)=(ฮน 1i)โˆ˜(iฮน 0 โˆ’1),id(j)=(ฮน 0 โˆ’1j)โˆ˜(jฮน 1)id(i) = (\iota_1 i)\circ (i \iota_0^{-1}), id(j) =(\iota_0^{-1} j) \circ (j \iota_1)

Remark 2.2. The arrow i:0โ†’1i: 0 \rightarrow 1 is an adjoint equivalence.

Remark 2.3. The free-standing adjoint equivalence is a (2,1)-category.

3. Representing of adjoint equivalences

๐’œโ„ฐ\mathcal{AE} is the model for all adjoint equivalences in all 2-categories. In other words, any adjoint equivalence in a 2-category ๐’œ\mathcal{A} is just a 2-functor from ๐’œโ„ฐ\mathcal{AE}:

Proposition 3.1. Let ๐’ž\mathcal{C} be a 2-category (weak or strict). Let ff be a 1-arrow of ๐’ž\mathcal{C} which is part of an adjoint equivalence. Then there is a 2-functor F:๐’œโ„ฐโ†’๐’žF: \mathcal{AE} \rightarrow \mathcal{C} such that the arrow i:0โ†’1i:0 \rightarrow 1 of โ„\mathcal{I} maps under FF to ff.

Proof. Immediate from the definitions.ย ย โ–ฎ

Last revised on July 5, 2020 at 10:39:12. See the history of this page for a list of all contributions to it.