13.2.3 Corepresentably Fully Faithful Morphisms

Let $\mathcal{C}$ be a bicategory.

A $1$-morphism $f\colon A\to B$ of $\mathcal{C}$ is corepresentably fully faithful1 if the following equivalent conditions are satisfied:

  1. 1.

    The $1$-morphism $f$ is corepresentably full (Definition 13.2.2.1.1) and corepresentably faithful (Definition 13.2.1.1.1).

  2. 2.

    For each $X\in \operatorname {\mathrm{Obj}}\webleft (\mathcal{C}\webright )$, the functor

    \[ f^{*}\colon \mathsf{Hom}_{\mathcal{C}}\webleft (B,X\webright )\to \mathsf{Hom}_{\mathcal{C}}\webleft (A,X\webright ) \]

    given by precomposition by $f$ is fully faithful.


  1. 1Further Terminology: Corepresentably fully faithful morphisms have also been called lax epimorphisms in the literature (e.g. in [AESV, On Functors Which Are Lax Epimorphisms]), though we will always use the name “corepresentably fully faithful morphism” instead in this work.

In detail, $f$ is corepresentably fully faithful if the conditions in Remark 13.2.1.1.2 and Remark 13.2.2.1.2 hold:

  1. 1.

    For all diagrams in $\mathcal{C}$ of the form

    if we have

    \[ \alpha \mathbin {\star }\operatorname {\mathrm{id}}_{f}=\beta \mathbin {\star }\operatorname {\mathrm{id}}_{f}, \]

    then $\alpha =\beta $.

  2. 2.

    For each $X\in \operatorname {\mathrm{Obj}}\webleft (\mathcal{C}\webright )$ and each $2$-morphism

    of $\mathcal{C}$, there exists a $2$-morphism
    of $\mathcal{C}$ such that we have an equality
    of pasting diagrams in $\mathcal{C}$, i.e. such that we have

    \[ \beta =\alpha \mathbin {\star }\operatorname {\mathrm{id}}_{f}. \]

Here are some examples of corepresentably fully faithful morphisms.

  1. 1.

    Corepresentably Fully Faithful Morphisms in $\mathsf{Cats}_{\mathsf{2}}$. The fully faithful epimorphisms in $\mathsf{Cats}_{\mathsf{2}}$ are characterised in Chapter 11: Categories, Item 10 of Proposition 11.6.3.1.2.

  2. 2.

    Corepresentably Fully Faithful Morphisms in $\boldsymbol {\mathsf{Rel}}$. The corepresentably fully faithful morphisms of $\boldsymbol {\mathsf{Rel}}$ coincide (Chapter 8: Relations, Item 3 of Proposition 8.4.10.1.1) with the corepresentably full morphisms in $\boldsymbol {\mathsf{Rel}}$, which are characterised in Chapter 8: Relations, Item 2 of Proposition 8.4.10.1.1.


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