# Homotopy Type Theory left adjoint (Rev #1)

## Idea

Many naturally arising functors in mathematics have adjoints. This makes them a useful thing to study.

## Definition

A functor $F: A \to B$ is a left adjoint if there exists

• A functor $G : B \to A$
• A natural transformation $\eta:1_A \to G F$ (the unit).
• A natural transformation $\epsilon G \to 1_B$ (the counit).
• Which satisfy the triangle identities (a.k.a zig-zag identities)
• $(\epsilon F)(F \eta) = 1_F$
• $(G \epsilon)(\eta G) = 1_G$

## Properties

### Lemma 9.3.2

If $A$ s a category and $B$ is a precategory then the type “$F$ is a left adjoint” is a mere proposition?.

Proof. Suppose we are given $(G, \eta, \epsilon)$ with the triangle identities and also $(G', \eta', \epsilon')$. Define $\gamma: G \to G'$ to be $(G' \epsilon )(\eta G')$. Then

\begin{aligned} \delta \gamma &= (G \epsilon')(\eta G')(G' \epsilon) (\eta' G)\\ &= (G \epsilon')(G F G' \epsilon_))\eta G' F G)(\eta' G)\\ &= (G \epsilon ')(G \epsilon' F G)(G F \eta' G)(\eta G)\\ &= (G \epsilon)(\eta G)\\ &= 1_G \end{aligned}

using Lemma 9.2.8 (see natural transformation) and the triangle identities. Similarly, we show $\gamma \delta=1_{G'}$, so $\gamma$ is a natural isomorphism $G \cong G'$. By Theorem 9.2.5 (see functor precategory), we have an identity $G=G'$.