# Kerodon

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Construction 2.2.3.4 (The Conjugate of a $2$-Category). Let $\operatorname{\mathcal{C}}$ be a $2$-category. We define a new $2$-category $\operatorname{\mathcal{C}}^{\operatorname{c}}$ as follows:

• The objects of $\operatorname{\mathcal{C}}^{\operatorname{c}}$ are the objects of $\operatorname{\mathcal{C}}$. To avoid confusion, for each object $X \in \operatorname{\mathcal{C}}$ we will write $X^{\operatorname{c}}$ for the corresponding object of $\operatorname{\mathcal{C}}^{\operatorname{c}}$.

• For every pair of objects $X,Y \in \operatorname{\mathcal{C}}$, we have $\underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}^{\operatorname{c}}}( X^{\operatorname{c}}, Y^{\operatorname{c}} ) = \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}}(X, Y)^{\operatorname{op}}$. In particular, every $1$-morphism $f: X \rightarrow Y$ in the $2$-category $\operatorname{\mathcal{C}}$ can be regarded as a $1$-morphism from $X^{\operatorname{c}}$ to $Y^{\operatorname{c}}$ in the $2$-category $\operatorname{\mathcal{C}}^{\operatorname{c}}$, which we will denote by $f^{\operatorname{c}}: X^{\operatorname{c}} \rightarrow Y^{\operatorname{c}}$. Similarly, if we are given a pair of $1$-morphisms $f,g: X \rightarrow Y$ in the $2$-category $\operatorname{\mathcal{C}}$ having the same source and target, then every $2$-morphism $\gamma : f \Rightarrow g$ in $\operatorname{\mathcal{C}}$ determines a $2$-morphism from $g^{\operatorname{c}}$ to $f^{\operatorname{c}}$ in $\operatorname{\mathcal{C}}^{\operatorname{c}}$, which we will denote by $\gamma ^{\operatorname{c}}: g^{\operatorname{c}} \Rightarrow f^{\operatorname{c}}$.

• For every triple of objects $X,Y,Z \in \operatorname{\mathcal{C}}$, the composition functor

$\circ : \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}^{\operatorname{c}}}( Y^{\operatorname{c}}, Z^{\operatorname{c}}) \times \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}^{\operatorname{c}}}(X^{\operatorname{c}},Y^{\operatorname{c}}) \rightarrow \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}^{\operatorname{op}}}(X^{\operatorname{c}},Z^{\operatorname{c}})$

for the $2$-category $\operatorname{\mathcal{C}}^{\operatorname{c}}$ is induced by the composition functor

$\circ : \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}}(Y,Z) \times \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}}(X,Y) \rightarrow \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}}(X, Z).$

on $\operatorname{\mathcal{C}}$ by passing to opposite categories. In particular, it is given on objects by the formula $g^{\operatorname{c}} \circ f^{\operatorname{c}} = (g \circ f)^{\operatorname{c}}$.

• For every object $X \in \operatorname{\mathcal{C}}$, the identity $1$-morphism $\operatorname{id}_{ X^{\operatorname{c}} } \in \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}^{\operatorname{c}}}( X^{\operatorname{c}}, X^{\operatorname{c}} )$ is given by $\operatorname{id}_{X}^{\operatorname{c}}$, where $\operatorname{id}_{X} \in \underline{\operatorname{Hom}}_{\operatorname{\mathcal{C}}}(X,X)$ is the identity $1$-morphism associated to $X$ in the $2$-category $\operatorname{\mathcal{C}}$, and the unit constraint $\upsilon _{X^{\operatorname{c}}}$ is the isomorphism $(\upsilon _{X}^{\operatorname{c}})^{-1}: \operatorname{id}_{X^{\operatorname{c}} } \circ \operatorname{id}_{X^{\operatorname{c}} } \xRightarrow {\sim } \operatorname{id}_{ X^{\operatorname{c}} }$.

• For every triple of composable $1$-morphisms

$W \xrightarrow {f} X \xrightarrow {g} Y \xrightarrow {h} Z$

in the $2$-category $\operatorname{\mathcal{C}}$, the associativity constraint

$\alpha _{ h^{\operatorname{c}},g^{\operatorname{c}}, f^{\operatorname{c}} }: h^{\operatorname{c}} \circ (g^{\operatorname{c}} \circ f^{\operatorname{c}} ) \xRightarrow {\sim } ( h^{\operatorname{c}} \circ g^{\operatorname{c}} ) \circ f^{\operatorname{c}}$

in the $2$-category $\operatorname{\mathcal{C}}^{\operatorname{c}}$ is given by the inverse $(\alpha ^{\operatorname{c}}_{h,g,f})^{-1}$ of the associativity constraint $\alpha _{h,g,f}: h \circ (g \circ f) \xRightarrow {\sim } (h \circ g) \circ f$ in the $2$-category $\operatorname{\mathcal{C}}$.

We will refer to $\operatorname{\mathcal{C}}^{\operatorname{c}}$ as the conjugate of the $2$-category $\operatorname{\mathcal{C}}$.