# nLab quintet construction

The Quintet Construction

### Context

#### 2-Category theory

2-category theory

# The Quintet Construction

## Idea

A quintet construction is an operation that takes a “globular” categorical structure and produces a “cubical” one (see geometric shape for higher structures) in which the squares

$\array{ A & \overset{f}{\to} & B \\ ^g\downarrow & \swArrow & \downarrow^h\\ C & \underset{k}{\to} & D }$

are morphisms between composites $\alpha:h\circ f \to k\circ g$. The name arises because a square in the resulting cubical structure is formally a quintet $(f,g,h,k,\alpha)$ (since just knowing the globular cell $\alpha$ does not determine the decomposition of its domain and codomain as composites).

Hence given a 2-category $\mathcal{C}$, it induces a double category $Sq(\mathcal{C})$ whose

## Applications

A quintet construction is at least morally a left adjoint to a functor that picks out the companion pairs in a cubical structure. Thus, functors out of quintet constructions are nothing new; but some interesting applications involve functors into quintet constructions from other cubical structures. For instance:

## References

The concept is due to

• Charles Ehresmann, Catégorie double des quintettes; applications covariantes, C. R. A. S. Paris 256 (1963), 1891-1894. and in volume III in his collected works.

It appears spelled out also in

• A. Bastiani, Charles Ehresmann, pages 272-273 of Multiple functors. I. Limits relative to double categories, Cah. Top. Géom. Différ. Catég. 15 (1974) 215–292

Last revised on July 13, 2018 at 07:21:46. See the history of this page for a list of all contributions to it.