# Equivalence of Definitions of Generalized Ordered Space

## Contents

## Theorem

Let $\struct {S, \preceq}$ be a totally ordered set.

Let $\tau$ be a topology for $S$.

The following definitions of the concept of **Generalized Ordered Space** are equivalent:

### Definition 1

$\left({S, \preceq, \tau}\right)$ is a **generalized ordered space** if and only if:

- $(1): \quad \left({S, \tau}\right)$ is a Hausdorff space

### Definition 2

$\left({S, \preceq, \tau}\right)$ is a **generalized ordered space** if and only if:

- $(1): \quad$ there exists a linearly ordered space $\left({S', \preceq', \tau'}\right)$

- $(2): \quad$ there exists a mapping $\phi: S \to S'$ such that $\phi$ is both an order embedding and a topological embedding.

### Definition 3

$\left({S, \preceq, \tau}\right)$ is a **generalized ordered space** if and only if:

- $(1): \quad \left({S, \tau}\right)$ is a Hausdorff space

- $(2): \quad$ there exists a sub-basis for $\left({S, \tau}\right)$ each of whose elements is an upper set or lower set in $S$.

## Proof

### Definition $(1)$ implies Definition $(3)$

Let $\mathcal B$ be a basis for $\tau$ consisting of convex sets.

Let:

- $\mathcal S = \left\{ {U^\succeq: U \in \mathcal B}\right\} \cup \left\{ {U^\preceq: U \in \mathcal B}\right\}$

where $U^\succeq$ and $U^\preceq$ denote the upper closure and lower closure respectively of $U$.

By Upper Closure is Upper Set and Lower Closure is Lower Set, the elements of $\mathcal S$ are upper and lower sets.

It is to be shown that $\mathcal S$ is a sub-basis for $\tau$.

By Upper and Lower Closures of Open Set in GO-Space are Open:

- $\mathcal S \subseteq \tau$

By Convex Set Characterization (Order Theory), each element of $\mathcal B$ is the intersection of its upper closure with its lower closure.

Thus each element of $\mathcal B$ is generated by $\mathcal S$.

Thus $\mathcal S$ is a sub-basis for $\tau$.

$\blacksquare$

### Definition $(3)$ implies Definition $(1)$

Let $\mathcal S$ be a sub-basis for $\tau$ consisting of upper sets and lower sets.

Let $\mathcal B$ be the set of intersections of finite subsets of $\mathcal S$.

By Upper Set is Convex, Lower Set is Convex and Intersection of Convex Sets is Convex Set (Order Theory) :

But $\mathcal B$ is a basis for $\tau$.

Therefore $\tau$ has a basis consisting of convex sets.

$\blacksquare$

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

Let $x \in U \in \tau$.

Then by the definition of topological embedding:

- $\map \phi U$ is an open neighborhood of $\map \phi x$ in $\map \phi S$ with the subspace topology.

Thus by Basis for Topological Subspace and the definition of the order topology, there is an open interval or open ray $I' \in \tau'$ such that:

- $\map \phi x \in I' \cap \map \phi S \subseteq \map \phi U$

Since $I'$ is an interval or ray, it is convex in $S'$ by Interval of Ordered Set is Convex or Ray is Convex, respectively.

Then:

\(\displaystyle x \in \phi^{-1} \sqbrk {I'}\) | \(=\) | \(\displaystyle \phi^{-1} \sqbrk {I' \cap \phi \sqbrk S}\) | |||||||||||

\(\displaystyle \) | \(\subseteq\) | \(\displaystyle \phi^{-1} \sqbrk {\phi \sqbrk U}\) |

Because $\phi$ is a topological embedding, it is injective by definition.

So:

- $\phi^{-1} \sqbrk {\phi \sqbrk U} = U$

Thus:

- $x \in \phi^{-1} \sqbrk {I'} \subseteq U$

By Inverse Image of Convex Set under Monotone Mapping is Convex:

- $\phi^{-1} \sqbrk {I'}$ is convex.

Thus $\tau$ has a basis consisting of convex sets.

$\blacksquare$

### Definition $(3)$ implies Definition $(2)$

This follows from GO-Space Embeds Densely into Linearly Ordered Space.

$\blacksquare$