Intersection of Upper Closures is Upper Closure of Join Operands

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:
 * $x \vee y \preceq a$

By Join Succeeds Operands:
 * $x \preceq x \vee y$ 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:
 * $a \in x^\succeq \cap y^\succeq$

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:
 * $a \in \paren {x \vee y}^\succeq$

Hence by definition of set equality:
 * $\paren {x \vee y}^\succeq = x^\succeq \cap y^\succeq$