# Equivalence of Definitions of Convex Set (Order Theory)

## Theorem

The following definitions of the concept of Convex Set in the context of Order Theory are equivalent:

### Definition 1

A subset $A$ of an ordered set $\struct {S, \preceq}$ is convex (in $S$) if and only if:

$\forall x, y \in A: \forall z \in S: x \preceq z \preceq y \implies z \in A$

### Definition 2

A subset $A$ of an ordered set $\struct {S, \preceq}$ is convex (in $S$) if and only if:

$\forall x, y \in A: \forall z \in S: x \prec z \prec y \implies z \in A$

## Proof

### $(1)$ implies $(2)$

Let $A \subseteq S$ be convex in $S$ by definition 1.

Then by definition:

$\forall x, y \in A: \forall z \in S: x \preceq z \preceq y \implies z \in A$

Let $x, y, z \in A$.

Let $x \prec z \prec y$.

Then by definition:

$x \preceq z \preceq y$

and so by hypothesis $z \in A$.

That is:

$\forall x, y \in A: \forall z \in S: x \prec z \prec y \implies z \in A$

Thus $A \subseteq S$ is convex in $S$ by definition 2.

$\Box$

### $(2)$ implies $(1)$

Let $A \subseteq S$ be convex in $S$ by definition 2.

Then by definition:

$\forall x, y \in A: \forall z \in S: x \prec z \prec y \implies z \in A$

Let $x, y, z \in A$.

Let $x \preceq z \preceq y$.

First suppose $z \ne x$ and $z \ne y$.

Then by definition of the relation $\prec$:

$x \prec z \prec y$

and so by hypothesis $z \in A$.

Let $x = z$ or $x = y$

We have that $x \in A$ and $y \in A$.

So $z \in A$.

That is, whether $x \prec z \prec y$ or $z = x$ or $x = y$:

$\forall x, y \in A: \forall z \in S: x \preceq z \preceq y \implies z \in A$

Thus $A \subseteq S$ is convex in $S$ by definition 1.

$\blacksquare$