nLab 2-category of adjunctions

Idea

For any 2-category $K$, there are two 2-categories (each with several variants) that could be called “the 2-category of adjunctions in $K$”.

• The 2-category which here we call $Adj(K)$ has the same objects as $K$, its morphisms are the adjunctions in $K$ (pointing in the direction of, say, the left adjoint), and its 2-morphisms are mate-pairs of 2-morphisms between adjunctions in $K$.

• The 2-category $[Adj,K]$ is the functor 2-category from the walking adjunction $Adj$ to $K$. Thus its objects are the adjunctions in $K$ — or more precisely, triples $(x,y,(f,g,\eta,\epsilon))$ where $x,y$ are objects of $K$ and $(f,g,\eta,\epsilon)$ is an adjunction between $x$ and $y$. Its morphisms are pairs of morphisms $x\to x'$ and $y\to y'$ such that certain squares commute (perhaps up to a transformation or isomorphism), and its 2-cells are similarly composed of cylinders.

Note that the 1-morphisms of $Adj(K)$ are the objects of $[Adj,K]$.

Properties

• The morphisms in $Adj\big(Adj(K)\big)$ are the adjoint triples in $K$.

• The inclusion of $Mnd$, the free monad, in $Adj$ induces a 2-functor from $[Adj,K]$ to $[Mnd,K]$, the 2-category of monads in $K$. The adjoints to this 2-functor are the Kleisli and Eilenberg-Moore constructions on monads in $K$.

Related concepts

• $Cat_{Adj}$

References

• Claude Auderset?, Adjonctions et monades au niveau des $2$-catégories, Cahiers de topologie et géométrie différentielle 15.1 (1974): 3-20.

• Stephen Schanuel and Ross Street, The free adjunction, Cahiers de topologie et géométrie différentielle catégoriques, tome 27, no 1 (1986), p. 81-83