# Ideal is Filter in Dual Ordered Set

## Theorem

Let $P = \left({S, \preceq}\right)$ be an ordered set.

Let $X$ be a subset of $S$.

Then

$X$ is ideal in $P$
$X$ is filter in $P^{-1}$

where $P^{-1} = \left({S, \succeq}\right)$ denotes the dual of $P$.

## Proof

### Sufficient Condition

Let $X$ be ideal in $P$.

By definition of ideal in ordered set:

$X$ is non-empty directed lower.

By definition of directed:

$\forall x, y \in X: \exists z \in X: x \preceq z \land y \preceq z$

Then

$\forall x, y \in X: \exists z \in X: z \succeq x \land z \succeq y$

By definition

$X$ is filtered in $P^{-1}$

By definition of lower set:

$\forall x \in X, y \in S: y \preceq x \implies y \in X$

Then

$\forall x \in X, y \in S: x \succeq y \implies y \in X$

By definition

$X$ is an upper set in $P^{-1}$.

Thus by definition of filter in ordered set

$X$ is a filter in $P^{-1}$.

$\Box$

### Necessary Condition

Let $X$ be a filter in $P^{-1}$.

By definition of filter in ordered set

$X$ is non-empty, filtered in $P^{-1}$, and upper in $P^{-1}$.

By definition of filtered in $P^{-1}$:

$\forall x, y \in X: \exists z \in X: z \succeq x \land z \succeq y$

Then

$\forall x, y \in X: \exists z \in X: x \preceq z \land y \preceq z$

By definition

$X$ is directed.

By definition of upper set in $P^{-1}$:

$\forall x \in X, y \in S: x \succeq y \implies y \in X$

Then

$\forall x \in X, y \in S: y \preceq x \implies y \in X$

By definition

$X$ is a lower set.

Thus by definition of ideal in ordered set:

$X$ is an ideal in $P$.

$\blacksquare$