# Completely Irreducible implies Meet Irreducible

## Theorem

Let $L = \struct {S, \wedge, \preceq}$ be a meet semilattice.

Let $p \in S$.

Then if $p$ is completely irreducible, then $p$ is meet irreducible.

## Proof

Assume that

$p$ is completely irreducible.
$\exists q \in S: p \prec q \land \left({\forall s \in S: p \prec s \implies q \preceq s}\right) \land p^\succeq = \left\{ {p}\right\} \cup q^\succeq$

where $p^\succeq$ denotes the upper closure of $p$.

Let $a, b \in S$ such that

$p = a \wedge b$

$a \ne p$ and $b \ne p$
$p \preceq b$

By definition of strictly precede:

$p \prec b$

Then

$q \preceq b$

By definition of strictly precede:

$p \preceq q$
$p \preceq a$

By definition of strictly precede:

$p \prec a$

Then

$q \preceq a$
$q \preceq p$

By definition of antisymmetry:

$p = q$

Thus this contradicts $p \prec q$ by definition of strictly precede.

$\blacksquare$