11.7.5 Pseudoepic Functors

Let $\mathcal{C}$ and $\mathcal{D}$ be categories.

A functor $F\colon \mathcal{C}\to \mathcal{D}$ is pseudoepic if it satisfies the following conditions:

  1. 1.

    For all diagrams 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 $2$-isomorphism

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

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

Let $F\colon \mathcal{C}\to \mathcal{D}$ be a functor.

  1. 1.

    Characterisations. The following conditions are equivalent:

    1. (a)

      The functor $F$ is pseudoepic.

    2. (b)

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

      \[ F^{*}\colon \mathsf{Fun}\webleft (\mathcal{D},\mathcal{X}\webright )\to \mathsf{Fun}\webleft (\mathcal{C},\mathcal{X}\webright ) \]

      given by precomposition by $F$ is pseudomonic.

    3. (c)

      We have an isococomma square of the form

      in $\mathsf{Cats}_{\mathsf{2}}$ up to equivalence.

  2. 2.

    Dominance. If $F$ is pseudoepic, then $F$ is dominant (Definition 11.7.1.1.1).

Item 1: Characterisations
Omitted.

Item 2: Dominance
If $F$ is pseudoepic, then

\[ F^{*}\colon \mathsf{Fun}\webleft (\mathcal{D},\mathcal{X}\webright )\to \mathsf{Fun}\webleft (\mathcal{C},\mathcal{X}\webright ) \]

is pseudomonic for all $\mathcal{X}\in \operatorname {\mathrm{Obj}}\webleft (\mathsf{Cats}\webright )$, and thus in particular faithful. By Item 5g of Item 5 of Proposition 11.6.1.1.2, this is equivalent to requiring $F$ to be dominant.

Is there a characterisation of functors $F\colon \mathcal{C}\to \mathcal{D}$ such that:

  1. 1.

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

    \[ F^{*}\colon \mathsf{Fun}\webleft (\mathcal{D},\mathcal{X}\webright )\to \mathsf{Fun}\webleft (\mathcal{C},\mathcal{X}\webright ) \]

    is pseudoepic?

  2. 2.

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

    \[ F_{*}\colon \mathsf{Fun}\webleft (\mathcal{X},\mathcal{C}\webright )\to \mathsf{Fun}\webleft (\mathcal{X},\mathcal{D}\webright ) \]

    is pseudoepic?

This question also appears as [Emily, Characterisations of functors $F$ such that $F^*$ or $F_*$ is [property], e.g. faithful, conservative, etc].


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