# nLab companion pair

Companion pairs

### Context

#### 2-Category theory

2-category theory

## Structures on 2-categories

#### Higher category theory

higher category theory

# Companion pairs

## Idea

A companion pair in a double category is a way of saying that a horizontal morphism and a vertical morphism are “isomorphic”, even though they do not live in the same 1-category/2-category.

## Definition

Let $f\colon A\to B$ be a vertical arrow (morphism) and $f'\colon A\to B$ a horizontal arrow in a double category. These arrows are said to be a companion pair if they come equipped with 2-cells

$\array{ A & \overset{f'}{\to} & B \\ ^f\downarrow & ^{\phi}\swArrow & \downarrow^{id} \\ B & \underset{id}{\to} & B} \qquad and\qquad \array{ A & \overset{id}{\to} & A \\ ^{id} \downarrow & ^{\psi}\swArrow & \downarrow^f \\ A & \underset{f'}{\to} & B }$

such that $\psi \circ_h \phi = id_f$ and $\phi \circ_v \psi = id_{f'}$, where $\circ_h$ and $\circ_v$ denote horizontal and vertical composition of 2-cells.

Given such a companion pair, we say that $f$ and $f'$ are companions of each other.

## Examples

• In the double category $\mathbf{Sq}(K)$ of squares (quintets) in any 2-category $K$, a companion pair is simply an invertible 2-cell between two parallel 1-morphisms of $K$.

• In the double category $T$-Alg of algebras, lax morphisms, and colax morphisms for a 2-monad, an arrow (of either sort) has a companion precisely when it is a strong (= pseudo) $T$-morphism. This is important in the theory of doctrinal adjunction.

## Properties

• The horizontal (or vertical) dual of a companion pair is a conjunction.

• Companion pairs (and conjunctions) have a mate correspondence generalizing the calculus of mates in 2-categories.

• If every vertical arrow in some double category $D$ has a companion, then the functor $f\mapsto f'$ is a pseudofunctor $V D\to H D$ from the vertical 2-category to the horizontal one, which is the identity on objects, and locally fully faithful by the mate correspondence. A choice of companions that make this a strict 2-functor is called a connection on $D$ (an arbitrary choice of companions may be called a “pseudo-connection”). A double category with a connection is thereby equivalent to an F-category. If every vertical arrow also has a conjoint, then this makes $D$ into a proarrow equipment, or equivalently a framed bicategory.

• Companion pairs and mate-pairs of 2-cells between them in any double category $D$ form a 2-category $Comp(D)$. The functor $Comp\colon DblCat \to 2Cat$ is right adjoint to the functor $Sq\colon 2Cat \to DblCat$ sending a 2-category to its double category of squares.