Axiom of Approximation in Up-Complete Semilattice
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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.
By Operand is Upper Bound of Way Below Closure:
- $x$ 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 $x^\ll$
Aiming for a contradiction, suppose:
- $x \npreceq y$
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.
$\blacksquare$
Sources
- 1980: G. Gierz, K.H. Hofmann, K. Keimel, J.D. Lawson, M.W. Mislove and D.S. Scott: A Compendium of Continuous Lattices
- Mizar article WAYBEL_3:24