## 4.29 2-categories

We will give a definition of (strict) $2$-categories as they appear in the setting of stacks. Before you read this take a look at Section 4.28 and Example 4.30.2. Basically, you take this example and you write out all the rules satisfied by the objects, $1$-morphisms and $2$-morphisms in that example.

Definition 4.29.1. A (strict) *$2$-category* $\mathcal{C}$ consists of the following data

A set of objects $\mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$.

For each pair $x, y \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ a category $\mathop{Mor}\nolimits _\mathcal {C}(x, y)$. The objects of $\mathop{Mor}\nolimits _\mathcal {C}(x, y)$ will be called *$1$-morphisms* and denoted $F : x \to y$. The morphisms between these $1$-morphisms will be called *$2$-morphisms* and denoted $t : F' \to F$. The composition of $2$-morphisms in $\mathop{Mor}\nolimits _\mathcal {C}(x, y)$ will be called *vertical* composition and will be denoted $t \circ t'$ for $t : F' \to F$ and $t' : F'' \to F'$.

For each triple $x, y, z\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ a functor

\[ (\circ , \star ) : \mathop{Mor}\nolimits _\mathcal {C}(y, z) \times \mathop{Mor}\nolimits _\mathcal {C}(x, y) \longrightarrow \mathop{Mor}\nolimits _\mathcal {C}(x, z). \]

The image of the pair of $1$-morphisms $(F, G)$ on the left hand side will be called the *composition* of $F$ and $G$, and denoted $F\circ G$. The image of the pair of $2$-morphisms $(t, s)$ will be called the *horizontal* composition and denoted $t \star s$.

These data are to satisfy the following rules:

The set of objects together with the set of $1$-morphisms endowed with composition of $1$-morphisms forms a category.

Horizontal composition of $2$-morphisms is associative.

The identity $2$-morphism $\text{id}_{\text{id}_ x}$ of the identity $1$-morphism $\text{id}_ x$ is a unit for horizontal composition.

This is obviously not a very pleasant type of object to work with. On the other hand, there are lots of examples where it is quite clear how you work with it. The only example we have so far is that of the $2$-category whose objects are a given collection of categories, $1$-morphisms are functors between these categories, and $2$-morphisms are natural transformations of functors, see Section 4.28. As far as this text is concerned all $2$-categories will be sub $2$-categories of this example. Here is what it means to be a sub $2$-category.

Definition 4.29.2. Let $\mathcal{C}$ be a $2$-category. A *sub $2$-category* $\mathcal{C}'$ of $\mathcal{C}$, is given by a subset $\mathop{\mathrm{Ob}}\nolimits (\mathcal{C}')$ of $\mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and sub categories $\mathop{Mor}\nolimits _{\mathcal{C}'}(x, y)$ of the categories $\mathop{Mor}\nolimits _\mathcal {C}(x, y)$ for all $x, y \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}')$ such that these, together with the operations $\circ $ (composition $1$-morphisms), $\circ $ (vertical composition $2$-morphisms), and $\star $ (horizontal composition) form a $2$-category.

The notion of equivalence of categories that we defined in Section 4.2 extends to the more general setting of $2$-categories as follows.

Definition 4.29.4. Two objects $x, y$ of a $2$-category are *equivalent* if there exist $1$-morphisms $F : x \to y$ and $G : y \to x$ such that $F \circ G$ is $2$-isomorphic to $\text{id}_ y$ and $G \circ F$ is $2$-isomorphic to $\text{id}_ x$.

Sometimes we need to say what it means to have a functor from a category into a $2$-category.

Definition 4.29.5. Let $\mathcal{A}$ be a category and let $\mathcal{C}$ be a $2$-category.

A *functor* from an ordinary category into a $2$-category will ignore the $2$-morphisms unless mentioned otherwise. In other words, it will be a “usual” functor into the category formed out of 2-category by forgetting all the 2-morphisms.

A *weak functor*, or a *pseudo functor* $\varphi $ from $\mathcal{A}$ into the 2-category $\mathcal{C}$ is given by the following data

a map $\varphi : \mathop{\mathrm{Ob}}\nolimits (\mathcal{A}) \to \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$,

for every pair $x, y\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{A})$, and every morphism $f : x \to y$ a $1$-morphism $\varphi (f) : \varphi (x) \to \varphi (y)$,

for every $x\in \mathop{\mathrm{Ob}}\nolimits (A)$ a $2$-morphism $\alpha _ x : \text{id}_{\varphi (x)} \to \varphi (\text{id}_ x)$, and

for every pair of composable morphisms $f : x \to y$, $g : y \to z$ of $\mathcal{A}$ a $2$-morphism $\alpha _{g, f} : \varphi (g \circ f) \to \varphi (g) \circ \varphi (f)$.

These data are subject to the following conditions:

the $2$-morphisms $\alpha _ x$ and $\alpha _{g, f}$ are all isomorphisms,

for any morphism $f : x \to y$ in $\mathcal{A}$ we have $\alpha _{\text{id}_ y, f} = \alpha _ y \star \text{id}_{\varphi (f)}$:

\[ \xymatrix{ \varphi (x) \rrtwocell ^{\varphi (f)}_{\varphi (f)}{\ \ \ \ \text{id}_{\varphi (f)}} & & \varphi (y) \rrtwocell ^{\text{id}_{\varphi (y)}}_{\varphi (\text{id}_ y)}{\alpha _ y} & & \varphi (y) } = \xymatrix{ \varphi (x) \rrtwocell ^{\varphi (f)}_{\varphi (\text{id}_ y) \circ \varphi (f)}{\ \ \ \ \alpha _{\text{id}_ y, f}} & & \varphi (y) } \]

for any morphism $f : x \to y$ in $\mathcal{A}$ we have $\alpha _{f, \text{id}_ x} = \text{id}_{\varphi (f)} \star \alpha _ x$,

for any triple of composable morphisms $f : w \to x$, $g : x \to y$, and $h : y \to z$ of $\mathcal{A}$ we have

\[ (\text{id}_{\varphi (h)} \star \alpha _{g, f}) \circ \alpha _{h, g \circ f} = (\alpha _{h, g} \star \text{id}_{\varphi (f)}) \circ \alpha _{h \circ g, f} \]

in other words the following diagram with objects $1$-morphisms and arrows $2$-morphisms commutes

\[ \xymatrix{ \varphi (h \circ g \circ f) \ar[d]_{\alpha _{h, g \circ f}} \ar[rr]_{\alpha _{h \circ g, f}} & & \varphi (h \circ g) \circ \varphi (f) \ar[d]^{\alpha _{h, g} \star \text{id}_{\varphi (f)}} \\ \varphi (h) \circ \varphi (g \circ f) \ar[rr]^{\text{id}_{\varphi (h)} \star \alpha _{g, f}} & & \varphi (h) \circ \varphi (g) \circ \varphi (f) } \]

Again this is not a very workable notion, but it does sometimes come up. There is a theorem that says that any pseudo-functor is isomorphic to a functor. Finally, there are the notions of *functor between $2$-categories*, and *pseudo functor between $2$-categories*. This last notion leads us into $3$-category territory. We would like to avoid having to define this at almost any cost!

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