Contents

# Contents

## Idea

A vertical transformation is an analogue of a natural transformation which goes between double functors of double categories, and whose components are vertical arrows and squares. There is a dual notion of horizontal transformation.

## Definition

If $C$ and $D$ are strict double categories regarded as internal categories in $Cat$ and $F,G\colon C\to D$ are double functors regarded as internal functors in $Cat$, then a transformation between them is simply an internal natural transformation in $Cat$. Whether this is a vertical or horizontal transformation depends on how we identify double categories with internal categories in $Cat$ (there being two ways).

More explicitly, a vertical transformation $\alpha\colon F\to G$ consists of

• For every object $c\in C$, a vertical arrow

$\array{ F c \\ \downarrow ^{\alpha_c} \\ G c}$

in $D$, which are natural with respect to vertical composition of vertical arrows in $C$.

• For every horizontal arrow $p\colon c_1 \to c_2$ in $C$, a square

$\array{F c_1 & \overset{F p}{\to} & F c_2\\ ^{\alpha_{c_1}}\downarrow & \Downarrow^{\alpha_p}& \downarrow^{\alpha_{c_2}}\\ G c_1& \underset{G p}{\to} & G c_2}$

in $D$, which are natural with respect to vertical composition of squares in $C$.

• For each $c\in C$, if $1_c\colon c\to c$ is its horizontal identity, then the square $\alpha_{1_c}$ is equal to $1_{\alpha_c}$, the identity square on the arrow $\alpha_c$.

• For $p\colon c_1\to c_2$ and $q\colon c_2\to c_3$, the horizontal composite of $\alpha_p$ and $\alpha_q$ is equal to $\alpha_{q p}$.

The notion of horizontal transformation is dual.

## The double category of double functors

Another characterization of transformations between double categories comes from observing that the 1-category $DblCat$ is cartesian closed, and so any two double categories have an exponential $D^C$. The objects of $D^C$ are double functors, its vertical arrows are vertical transformations, and its horizontal arrows are horizontal transformations. Its squares are a sort of “square modification” relating a pair of vertical and a pair of horizontal transformations.

## Generalizations

It is easy to modify the explicit definition to handle the cases when $C$ and $D$ are weak in one direction or the other, and/or when $F$ and $G$ are pseudo functors in one direction or the other, by composing with appropriate coherence constraints. In this way, we obtain many 2-categories of double categories.

It is also easy to define vertical transformations between double functors which are horizontally lax or colax, and dually. In fact, given double categories $C,D,C',D'$, lax functors $F\colon C\to D$ and $F'\colon C'\to D'$, and colax functors $G\colon C\to C'$ and $G'\colon D\to D'$, we can define a vertical transformation having the shape

$\array{C & \overset{F}{\to} & D \\ ^G\downarrow & \Downarrow& \downarrow^{G'}\\ C'& \underset{F'}{\to} & D'}$

despite the fact that the composites $G' \circ F$ and $F'\circ G$ do not exist as double functors of any sort. Such transformations are the squares of a large double category $Dbl$ whose objects are double categories, whose horizontal arrows are lax functors, and whose vertical arrows are colax functors. This, in turn, is a special case of a construction which works for algebras over any 2-monad.

Finally, we can also define vertical transformations between functors of (horizontally) virtual double categories.