Maths - Adjunctions - Examples Preordered Set

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Galois connection

Let (A, ≤) and (B, ≤) be two partially ordered sets. A monotone Galois connection between these posets consists of two monotone functions:

  • F : A -> B
  • G : B -> A

such that for all a in A and b in B, we have

F(a) ≤ b if and only if a ≤ G(b).


This works for composed morphisms like this:

(if we work through each rectangle the above condition holds)

The posets don't have to be isomorphic, morphisms in A can converge to points in B for example: galois
However the morphism F has to be continuous (in some sort of topological sense), that is, we can't have sort of tears like this: galois

Preordered Set

Here we work in the category of preordered sets (I have illustrated this as a directed graph, I find it helpful to use draw directed graphs as described in the box on the right)

preordered set 1

Here we look at a perordered set ≤(red) and a mapping to another perordered set (blue). The morphism between them (green) preserves the ordering. That is:

if a ≤ a' then F(a) ≤ F(a')

  So is there a mapping (G) back from B to A that preserves as much structure as possible in some unique way?
preordered set 2

Here is a first attempt, as we can see we can compose these morphisms and still preserve the order structure
:if a ≤ a' then GF(a) ≤ GF(a'). However, it does not do this in a unique way, there are many mappings for G that could have done this.

preordered set 3

So lets add another condition:

GF(a) ≤ a

This condition is not met for our first choice of G (on the diagram on the left this is interpreted as reversing one of the red arrows).

preordered set 4

However, given the morphism F then there is a way to choose a way to choose the morphism G so that the order is preserved relative to the original sequence.

For instance, if the number the elements of 'A' with ascending values. We then map these values onto 'B' using the morphism 'F'. We can then determine G by mapping the highest value for each element in 'B' to the corresponding number in 'A'.

G is then the adjoint of F.


So, for adjoint morphisms:

F(a) ≤ b if and only if a ≤ G(b)

Now we have a unique mapping G we can go on to show the unit and co-unit:


μ : 1c =>GF

So the unit maps 1c to GF as in this diagram. In this diagram we can't derive mappings from movement of points, we just map the mappings directly.

I added in the purple arrows to make it into a graph, they are not really generated by GF.


ξ : FG => 1d

In this case the co-unit is trivial.

Same Thing Using Definition 2

In this case we take an element 'A' of C and an element 'X' of D. We can then determine GX and FA by following the arrows.

The arrow A->GX is then isomorphic to the arrow FA->X.

This must be so for any 'A' in C and 'X' in D.

preorder adjunction definition 2


An endofunctor on posets models closure. Posets don't have loops, therefore defined by fixpoints.


Define T: P -> P


  • x <= T x
  • T² x <= T x


T² <= T (that is it is idempotent)


Which gives an adjunction: tleft adjointi

This is discussed as a monad on page here.

Implementing Posets in FriCas program is discussed on page here.

type theory quantifiers


(Cartesian maps)

Related Topics

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see also:

Adjunctions from Morphisms

Other Pages on this site

  • Category of graphs described on page here.
  • Implementing Graphs in FriCas program is discussed on page here.
  • Implementing Posets in FriCas program is discussed on page here.
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