Join Semilattice is Ordered Structure/Proof 1

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

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.

By Join Succeeds Operands:

$b \preceq b \vee c$

$c \preceq b \vee c$

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.

$\blacksquare$