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Variant Let $\operatorname{\mathcal{C}}$ be a category and let $\mathscr {F}: \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \operatorname{Set}$ be a functor. We define a category $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$ as follows:

  • The objects of $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$ are pairs $(C, x)$, where $C$ is an object of $\operatorname{\mathcal{C}}$ and $x$ is an element of the set $\mathscr {F}(C)$.

  • If $(C,x)$ and $(C', x')$ are objects of $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$, then a morphism from $(C,x)$ to $(C',x')$ in the category $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$ is a morphism $f: C \rightarrow C'$ in the category $\operatorname{\mathcal{C}}$ for which the induced map $\mathscr {F}(f): \mathscr {F}(C') \rightarrow \mathscr {F}(C)$ carries $x'$ to $x$.

  • Composition of morphisms in $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$ is given by composition of morphisms in $\operatorname{\mathcal{C}}$.

We will refer to $\int ^{\operatorname{\mathcal{C}}} \mathscr {F}$ as the category of elements of the functor $\mathscr {F}$. Note that the construction $(C,x) \mapsto C$ determines a functor $U: \int ^{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$, which we will refer to as the forgetful functor.