A partial order relation is not an object in itself. But, a set can have a partial order. We call such a set a Partially Ordered Set, or POSet for short.
So, if we make a partial order on X and call that partial order â¤, then we can just refer to the POSet as (X,â¤)
We usually write sets with structure as a list*, the first being the set of elements and the rest being the bits of structure on it.
The first thing we should notice is that a POSet fulfills all the conditions to be a directed graph. This is great, since we have a convention about how to show an ordered graph. You represent each element as a circle, and each connection (a,b) by an arrow from point a to point b.
But, we can take advantage of the structure of the partial order to simplify the diagram.
Reflexivity/anti-reflexivity: We choose before-hand if we want every element to be related to itself or not. So, we add no information by including self-loops.
Transitivity: If xâ¤y and yâ¤z, then xâ¤z. This means that we should just show the closer pairs, and not xâ¤z, since it is implied.
Anti-symmetry: Since if two elements are comparable, only one comparison is true, we can take advantage of height, and make earlier things higher.
The representation that takes advantage of this structure is called a Hasse Diagram
Here are some examples:
The power set, ordered by subset:
It can have multiple bottoms:
A family tree ordered by âis a descendantâ:
*I know I mean tuple, but I don't want to get too bogged down with the distinction.