# Join Semilattice is Ordered Structure

## Theorem

Let $\struct {S, \vee, \preceq}$ be a join semilattice.

Then $\struct {S, \vee, \preceq}$ is an ordered structure.

That is, $\preceq$ is compatible with $\vee$.

## Proof 1

For $\struct {S, \vee, \preceq}$ to be an ordered structure is equivalent to, for all $a, b, c \in S$:

$a \preceq b \implies a \vee c \preceq b \vee c$
$a \preceq b \implies c \vee a \preceq c \vee b$

Since Join is Commutative, it suffices to prove the first of these implications.

By definition of join:

$a \vee c = \sup \set {a, c}$

where $\sup$ denotes supremum.

$b \preceq b \vee c$
$c \preceq b \vee c$

Now also $a \preceq b$, and by transitivity of $\preceq$ we find that:

$a \preceq b \vee c$

Thus $b \vee c$ is an upper bound for $\set {a, c}$.

Hence:

$a \vee c \preceq b \vee c$

by definition of supremum.

$\blacksquare$

## Proof 2

Let $a, b, c \in S$.

Let $a \preceq b$.

By the definition of join semilattice:

$a \vee b = b$

Thus:

$\paren {a \vee b} \vee c = b \vee c$

Since $\vee$ is associative, commutative, and idempotent:

$\paren {a \vee c} \vee \paren {b \vee c} = b \vee c$

Therefore, $a \vee c \preceq b \vee c$.

From Join is Commutative, we conclude that:

$c \vee a \preceq c \vee b$

$\blacksquare$