Contents

### Context

category theory

#### 2-Category theory

2-category theory

Definitions

Transfors between 2-categories

Morphisms in 2-categories

Structures in 2-categories

Limits in 2-categories

Structures on 2-categories

# Contents

## Definition

A monad $(T,\mu,\eta)$ is adjoint to a comonad $(G,\delta,\epsilon)$, if its underlying endofunctor $T$ is left adjoint to the underlying 1-morphism $G$ of the comonad, and $\delta$ and $\epsilon$ are conjugate/adjoint/mate 2-cells to $\mu$ and $\eta$ in the sense explained below.

## Construction

In fact given a monad $\mathbf{T} = (T,\mu^T,\eta^T)$ which has a right adjoint $G$, automatically $G$ is a part of a comonad $\mathbf{G} = (G,\delta^G,\epsilon^G)$ where $\delta^G$ and $\epsilon^G$ are in some sense dual to $\mu^T$ and $\eta^T$.

Thus there is a bijective correspondence between monads having a right adjoint and comonads having a left adjoint (what Alexander Rosenberg calls duality). This is a little more than a consequence of two general facts:

1. If $T\dashv G$ then $T^k \dashv G^k$ for every natural number $k$.

2. Given two adjunctions $S\dashv T$ and $S'\dashv T'$ where $S,S': B\to A$, then there is a bijection between the natural transformations $\phi:S'\Rightarrow S$ and natural transformations $\psi:T\Rightarrow T'$ such that

$\array{ A (S,-) &\to& B(-,T) \\ {}^{\mathllap{A(\phi,-)}}\downarrow &&\downarrow {}^{\mathrlap{B(-,\psi)}} \\ A(S',-)&\to & B(-,T') }$

where the horizontal arrows are the natural bijections given by the adjunctions. Eilenberg & Moore 1965 would write $\phi\dashv\psi$ and talk about “adjointness for morphisms” (of functors), which is of course relative to the given adjunctions among functors. MacLane calls the correspondence conjugation (p 99-102 in Categories for Working Mathematician). It is a special case, of a general construction of mates.

If $\eta,\eta'$ and $\epsilon,\epsilon'$ are their unit and counit of course the upper arrow is $(S M\stackrel{f}\to N)\mapsto T f\circ \eta_M$ and the lower arrow $(S'M\stackrel{g}\to N)\mapsto T'g\circ\eta'_M$. Thus the condition renders as

$T'(f\circ\phi_M)\circ\eta'_M = \psi_N\circ T f\circ\eta_M$

or $T'f\circ T'\phi_M\circ\eta'_M = T'f\circ \psi_{SM}\circ\eta_M$. Given $\phi$, the uniqueness of $B(-,\psi)$ is clear from the above diagram, as the horizontal arrows are invertible. $B(-,\psi)$ determines $\psi$, namely $\psi_N = B(-,\psi)(id_N)$. For the existence of $\psi$ (given $\phi$) satisfying the above equation, one proposes that $\psi$ is the composition $\psi = T'\epsilon \circ T'\phi T \circ \eta'T$, i.e.

$T\stackrel{\eta' T}\longrightarrow T'S' T\stackrel{T'\phi T}\longrightarrow T' S T \stackrel{T'\epsilon}\longrightarrow T'$

and checks that it works. The inverse is similarly given by the composition

$S'\stackrel{S'\eta}\longrightarrow S' T S\stackrel{S'\psi S}\longrightarrow S'T'S\stackrel{\epsilon' S}\longrightarrow S$

This correspondence now enables in our special case to dualize $\mu^T$ to $\delta^G$, and similarly unit to the counit.

## Examples

###### Example

Every adjoint triple (of adjoint functors) $F^*\dashv F_* \dashv F^!$ induces an adjoint pair $F_* F^*\dashv F_* F^!$. The endofunctor $F_* F^*$ is underlying a monad induced by the adjunction $F^*\dashv F_*$ and $F_* F^!$ is underlying a comonad induced by the adjuntion $F_*\dashv F^!$. This pair of a monad and a comonad are adjoint.

## Properties

### General

###### Proposition

$\lozenge \,\dashv\, \Box$

on some category $\mathcal{C}$, then there is an equivalence between their Eilenberg-Moore categories of algebras over $\lozenge$ and coalgebras over $\Box$, compatible with their forgetful functors to $\mathcal{C}$:

This is due to Eilenberg & Moore 1965, where it is implied by the last part of Prop. 3.3. In the more explicit form above the statement may be found in MacLane & Moerdijk 1992, Thm. 2 on p. 249,

### General adjoint (co)algebras $End(A)$

Given a small category $A$, the endofunctor category $End(A)$ (of endofunctors and natural transformations between them, with vertical composition as composition) is monoidal with respect to the composition as the tensor product of objects (endofunctor) and Godement product (horizontal composition) as the tensor product of morphisms (natural transformations). Hence we can consider operads and algebras over operads, as well as, dually, coalgebras over cooperads; or some other framework for general algebras and coalgebras (or even props).

In any case, given an adjunction $T\dashv G$, operations $T^n\to T$ dualize to cooperations $G\to G^n$, and more generally multioperations $T^k\to T^l$ dualize to the multioperations $G^l\to G^k$. We would like to sketch the proof that the identities for operations on $T$, correspond to the identities on cooperations on $G$ (and more generally there is a duality among the identities for multioperations). This is essentially the consequence of

Lemma. (Zoran) Given the adjunction $T\dashv G$ with unit $\eta$ and counit $\epsilon$, and the sequence

$T^k \stackrel{\alpha}\longrightarrow T^l\stackrel\beta\longrightarrow T^s$

the composition $\alpha^*\circ\beta^*$ of the dual (in the above sense) sequence

$G^k \stackrel{\alpha^*}\longleftarrow G^l\stackrel{\beta^*}\longleftarrow G^s$

equal to the dual $(\beta\circ\alpha)^*$ of $\beta\circ\alpha$,

Proof. Mike Shulman notices that this is a special case of known contravariant functoriality of mates, but here is a direct proof.

We need to prove that the composition

$G^s\stackrel{\eta_l G^s}\to G^l T^l G^s\stackrel{G^l\beta G^s}\to G^l T^s G^s\stackrel{G^l \epsilon_s}\to G^l\stackrel{\eta_k G^l}\to G^k T^k G^l\stackrel{G^k\alpha G^l}\to G^k T^l G^l\stackrel{G^k\epsilon_l}\to G^k$

equals the composition

$G^s\stackrel{\eta_k G^l}\to G^k T^k G^s\stackrel{G^k\alpha G^l}\to G^k T^l G^s\stackrel{G^k\beta G^s}\to G^k T^s G^s\stackrel{G^k\epsilon_s}\to G^k.$

Note that in the two compositions there is an opposite order between the expressions involving $\alpha$ and those involving $\beta$. But anyway, their equality reduces to a naturality calculation (which in particular exchanges the order of $\alpha$ and $\beta$ in effect):

$\array{ G^s &\stackrel{\eta_l G^s}\to & G^l T^l G^s &\stackrel{G^l\beta G^s}\to & G^l T^s G^s&\stackrel{G^l \epsilon_s}\longrightarrow& G^l\\ \eta^k G^s\downarrow &&\downarrow \eta_k G^l T^l G^s&&\downarrow \eta_k G^l T^s G^s&& \downarrow \eta_k G^l \\ G^k T^k G^s &\stackrel{G^k T^k\eta_l G^s}\longrightarrow &G^k T^k G^l T^l G^s &\stackrel{G^k T^k G^l \beta G^s}\longrightarrow & G^k T^k G^l T^s G^s &\stackrel{G^k T^k G^l \epsilon_s}\longrightarrow & G^k T^k G^l\\ G^k \alpha G^s \downarrow && \downarrow G^k \alpha G^l T^l G^s&&\downarrow G^k \alpha G^k T^s G^s&&\downarrow G^k \alpha G^l\\ G^k T^l G^s &\stackrel{G^k T^l\eta_l G^s}\longrightarrow & G^k T^l G^l T^l G^s &\stackrel{G^k T^l G^l \beta G^s}\longrightarrow & G^k T^l G^l T^s G^s &\stackrel{G^k T^l G^l\epsilon_s}\longrightarrow & G^k T^l G^l\\ G^k\beta G^s \downarrow &&\downarrow{\rho} &&\downarrow G^k \epsilon_l T^s G^s &&\downarrow G^k\epsilon_l \\ G^k T^s G^s &=&G^k T^s G^l &=&G^k T^s G^s&\stackrel{G^k \epsilon_s}\longrightarrow& G^k }$

where $\rho := G^k \beta G^s \circ G^k \epsilon_l G^s = G^k\epsilon_l T^s G^s\circ G^k T^l G^l \beta G^s$. The commutativity of all small squares in the diagram is evident, except the lower left corner. This one follows by one of the triangle identities for the adjunction $T^l\dashv G^l$. Namely,

$G^k \beta G^s = G^k \beta G^s \circ (G^k \epsilon_l T^l G^s\circ G^k T^l \eta_l G^s) = \rho \circ G^k T^l \eta_l G^s$