Intersection of Upper Closures is Upper Closure of Join Operands
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Theorem
Let $L = \struct {S, \vee, \preceq}$ be a join semilattice.
Let $x, y \in S$.
Then $\paren {x \vee y}^\succeq = x^\succeq \cap y^\succeq$
Proof
We will prove that:
- $\paren {x \vee y}^\succeq \subseteq x^\succeq \cap y^\succeq$
Let $a \in \paren {x \vee y}^\succeq$
By definition of upper closure of element:
- $x \vee y \preceq a$
- $x \preceq x \vee y$ and $y \preceq x \vee y$
By definition of transitivity:
- $x \preceq a$ and $y \preceq a$
By definition of upper closure of element:
- $a \in x^\succeq$ and $a \in y^\succeq$
Thus by definition of intersection:
- $a \in x^\succeq \cap y^\succeq$
$\Box$
We will prove that:
- $x^\succeq \cap y^\succeq \subseteq \paren {x \vee y}^\succeq$
Let $a \in x^\succeq \cap y^\succeq$
Thus bt definition of intersection:
- $a \in x^\succeq$ and $a \in y^\succeq$
By definition of upper closure of element:
- $x \preceq a$ and $y \preceq a$
By definitions of supremum and Upper bound:
- $x \vee y \preceq a$
Thus by definition of upper closure of element:
- $a \in \paren {x \vee y}^\succeq$
$\Box$
Hence by definition of set equality:
- $\paren {x \vee y}^\succeq = x^\succeq \cap y^\succeq$
$\blacksquare$
Sources
- Mizar article WAYBEL14:4