A $2$-functor is the categorification of the notion of a functor to the setting of 2-categories. At the 2-categorical level there are several possible versions of this notion one might want depending on the given setting, some of which collapse to the standard definition of a functor between categories when considered on $2$-categories with discrete hom-categories (viewed as $1$-categories). The least restrictive of these is a lax functor, and the strictest is (appropriately) called a strict 2-functor.
For the various separate definitions that do collapse to standard functors, see:
There is also a notion of ‘lax functor’, however this notion does not necessarily yield a standard functor when considered on discrete hom-categories.
For the generalisation of this to higher categories, see semistrict higher category.
Here we present explicitly the definition for the middling notion of a pseudofunctor, and comment on alterations that yield the stronger and weaker notions.
Let $\mathfrak{C}$ and $\mathfrak{D}$ be strict 2-categories. A pseudofunctor $P:\mathfrak{C}\to\mathfrak{D}$ consists of
A function $P:Ob_\mathfrak{C}\to Ob_\mathfrak{D}$.
For each pair of objects $A,B\in Ob_\mathfrak{C}$ a functor
We will generally write the function and functors as $P$.
whose components are $2$-cell isomorphisms $\gamma_{f,g}:P(f\circ g) \Rightarrow P(f)\circ P(g)$ as below
where $1$ denotes the terminal category and $id_A$ is the identity-selecting functor at $A$. Its component is a $2$-cell isomorphism $\iota_{_*}:P(1_A)\Rightarrow 1_{P(A)}$ as below
These are subject to the following axioms:
where $\circ$ denotes vertical composition and $\star$ denotes horizontal composition, as illustrated by the following commutative $2$-cell diagram in $\mathfrak{D}(P(A),P(D))$:
as illustrated by the commutative $2$-cell diagrams below
To obtain the notion of a lax functor we only require that the coherence morphisms $\gamma_{f,g}$ and $\iota_A$ be $2$-cells, not necessarily $2$-cell isomorphisms. This prevents us from going back and forth between preimages and images of identity $1$-cells and horizontally composed $1$-cells/$2$-cells. Similarly, to obtain an oplax functor we reverse the direction of these 2-cells.
To obtain the notion of a strict $2$-functor we require that $\gamma_{f,g}$ and $\iota_A$ are identity arrows, so horizontal composition and $1$-cell identities literally factor through each functor in the same way vertical composition and $2$-cell identities do.
There is a notion of a ‘weak 2-category’, however it usually doesn't make sense to speak of strict $2$-functors between weak $2$-categories^{1}, but it does make sense to speak of lax (or ‘weak’) $2$-functors between strict $2$-categories. Indeed, the weak $3$-category Bicat of bicategories, pseudofunctors, pseudonatural transformations, and modifications is equivalent to its full sub-3-category spanned by the strict 2-categories. However, it is not equivalent to the $3$-category Str2Cat? of strict $2$-categories, strict $2$-functors, transformations, and modifications. (For discussion of the terminological choice “$2$-functor” and $n$-functor in general, see higher functor.)
2-functor / pseudofunctor / (2,1)-functor
basic properties of…
Although there are certain contexts in which it does. For instance, there is a model structure on the category of bicategories and strict 2-functors between them, which models the homotopy theory of bicategories and weak 2-functors. ↩
Last revised on December 7, 2020 at 14:53:02. See the history of this page for a list of all contributions to it.