Category of Preordered Sets - 2-category Structure

2-category Structure

The set of morphisms (monotonic functions) 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:

(f ≤ g) ⇔ (∀ x, f(x) ≤ g(x))

This preordered set can in turn be considered as a category, which makes Ord a 2-category (the additional axioms of a 2-category trivially hold because any equation of parallel morphisms is true in a posetal 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:

∀ x ∈ F(A), F (id_A) (x) ≃ x

∀ x ∈ F(A), F (g ∘ f) (x) ≃ F(g) (F(f) x)

where x ≃ y means x ≤ y ∧ y ≤ x.