# Axiom of Approximation in Up-Complete Semilattice

## Theorem

Let $\mathscr S = \struct {S, \wedge, \preceq}$ be an up-complete meet semilattice.

Let:

$\forall x \in S: x^\ll$ is directed

Then:

$\mathscr S$ satisfies the axiom of approximation
$\forall x, y \in S: x \npreceq y \implies \exists u \in S: u \ll x \land u \npreceq y$

## Proof

### Sufficient Condition

Let $\mathscr S$ satisfy the axiom of approximation.

Let $x, y \in S$ such that

$x \npreceq y$

By assumption:

$x^\ll$ is directed.

By definition of up-complete:

$x^\ll$ admits a supremum.

By the axiom of approximation:

$x = \map \sup {x^\ll}$

By definition of supremum:

if $y$ is upper bound for $x^\ll$, then $x \preceq y$

By definition of upper bound:

$\exists u \in x^\ll: u \npreceq y$

Thus by definition of way below closure:

$\exists u \in S: u \ll x \land u \npreceq y$

$\Box$

### Necessary Condition

Let:

$\forall x, y \in S: x \npreceq y \implies \exists u \in S: u \ll x \land u \npreceq y$

Let $x \in S$.

By assumption:

$x^\ll$ is directed.

By definition of up-complete:

$x^\ll$ admits a supremum.
$x$ is upper bound for $x^\ll$

We will prove that:

$\forall y \in S: y$ is upper bound for $x^\ll \implies x \preceq y$

Let $y \in S$ such that:

$y$ is upper bound for $x^\ll$

$x \npreceq y$

By assumption:

$\exists u \in S: u \ll x \land u \npreceq y$

By definition of way below closure:

$u \in x^\ll$

By definition of upper bound

$u \preceq y$

This contradicts $u \npreceq y$

$\Box$

Thus by definition of supremum:

$x = \map \sup {x^\ll}$

Thus by definition:

$\mathscr S$ satisfies the axiom of approximation.

$\blacksquare$