• Characterisations. If $\mathcal{C}$ and $\mathcal{D}$ are small1 then the following conditions are equivalent:

    2

    1. The functor $F$ is an equivalence of categories.
    2. The functor $F$ is fully faithful and essentially surjective.
    3. The induced functor
      \[ \left.F\right\vert _{\mathsf{Sk}\webleft (\mathcal{C}\webright )}\colon \mathsf{Sk}\webleft (\mathcal{C}\webright )\to \mathsf{Sk}\webleft (\mathcal{D}\webright ) \]

      is an isomorphism of categories.

    4. For each $X\in \text{Obj}\webleft (\mathsf{Cats}\webright )$, the precomposition functor
      \[ F^{*}\colon \mathsf{Fun}\webleft (\mathcal{D},\mathcal{X}\webright )\to \mathsf{Fun}\webleft (\mathcal{C},\mathcal{X}\webright ) \]

      is an equivalence of categories.

    5. For each $X\in \text{Obj}\webleft (\mathsf{Cats}\webright )$, the postcomposition functor
      \[ F_{*}\colon \mathsf{Fun}\webleft (\mathcal{X},\mathcal{C}\webright )\to \mathsf{Fun}\webleft (\mathcal{X},\mathcal{D}\webright ) \]

      is an equivalence of categories.


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