definition. Ob, Arr [leinster2016basic, 4.1.6] [tt-003U]

✍️source

Given a small category \({\cal C}\), there is a functor \(\operatorname {Ob} : \mathbf {Cat} \to \mathbf {Set}\) that sends \({\cal C}\) to its set of objects where \(\mathbf {Cat}\) is the category of small categories. Thus, \[ \mathscr {H}^{\mathrm {1}}({\cal C}) \cong \operatorname {Ob}({\cal C}) \] where \(\mathscr {H}\) is a Yoneda embedding functor.

This isomorphism is natural in \({\cal C}\); hence \(\operatorname {Ob} \cong \mathbf {Cat}(\mathrm {1}, -)\) where \(\mathbf {Cat}(\mathrm {1}, -)\) is a covariant hom-functor.

Functor \(\operatorname {Ob}\) is representable. Similarly, the functor \(\operatorname {Arr} : \mathbf {Cat} \to \mathbf {Set}\) sending a small category to its set of arrows is representable.