Category of preordered sets


In mathematics, the category Ord has preordered sets as objects and order-preserving functions as morphisms. This is a category because the composition of two order-preserving functions is order preserving and the identity map is order preserving.
The monomorphisms in Ord are the injective order-preserving functions.
The empty set is the initial object of Ord, and the terminal objects are precisely the singleton preordered sets. There are thus no zero objects in Ord.
The categorical product in Ord is given by the product order on the cartesian product.
We have a forgetful functor Ord → Set that assigns to each preordered set the underlying set, and to each order-preserving function the underlying function. This functor is faithful, and therefore Ord is a concrete category. This functor has a left adjoint and a right adjoint.

2-category structure

The set of morphisms between two preorders actually has more structure than that of a set. It can be made into a preordered set itself by the pointwise relation:
This preordered set can in turn be considered as a category, which makes Ord a 2-category.
With this 2-category structure, a pseudofunctor F from a category C to Ord is given by the same data as a 2-functor, but has the relaxed properties:
where x ≃ y means x ≤ y and y ≤ x.