# Order-Preserving Mapping Not Always Semilattice Homomorphism

## Theorem

Let $\struct {S, \circ}$ and $\struct {T, *}$ be semilattices.

Let $\preceq_1$ be the ordering on $S$ defined by:

- $a \preceq_1 b \iff \paren {a \circ b} = b$

Let $\preceq_2$ be the ordering on $T$ defined by:

- $x \preceq_2 y \iff \paren {x * y} = y$

Let $\phi: \struct {S, \preceq_1} \to \struct {T, \preceq_2}$ be an order-preserving mapping.

Then:

- $\phi: \struct {S, \circ} \to \struct {T, *}$ may not be a semilattice homomorphism

## Proof

Let $S = \set{a, b, c}$.

Let $\circ : S \times S \to S$ be defined by:

- $a \circ b = b \circ a = c$
- $a \circ c = c \circ a = c$
- $b \circ c = c \circ b = c$
- $a \circ a = a$
- $b \circ b = b$
- $c \circ c = c$

Then a manual check shows that $\circ$ is:

So $\struct{S, \circ}$ is a semilattice with ordering

- $\preceq_1 = \set{\tuple{a, a}, \tuple{b, b}, \tuple{c, c}, \tuple{a, c}, \tuple{b,c}}$

Let $T = \set{p, q, r, s}$.

Let $* : S \times S \to S$ be defined by:

- $p * q = q * p = r$
- $p * r = r * p = r$
- $q * r = r * q = r$
- $p * s = s * p = s$
- $q * s = s * q = s$
- $r * s = s * r = s$
- $p * p = p$
- $q * q = q$
- $r * r = r$
- $s * s = s$

Then a manual check shows that $*$ is:

So $\struct{T, *}$ is a semilattice with ordering

- $\preceq_2 = \set{\tuple{p, p}, \tuple{q, q}, \tuple{r, r}, \tuple{s, s}, \tuple{p, r}, \tuple{q,r}, \tuple{p, s}, \tuple{q,s}, \tuple{r, s}}$

Let $\phi : S \to T$ be defined by:

- $\map \phi a = p$
- $\map \phi b = q$
- $\map \phi c = s$

Then a manual check shows that $\phi$ is order-preserving.

Now:

\(\displaystyle \map \phi a * \map \phi b\) | \(=\) | \(\displaystyle p * q\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle r\) | |||||||||||

\(\displaystyle \) | \(\ne\) | \(\displaystyle s\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \map \phi c\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \map \phi {a \circ b}\) |

Hence $\phi$ is not a semilattice homomorphism by definition.

$\blacksquare$

## Sources

- 1982: Peter T. Johnstone:
*Stone spaces*... (previous) ... (next): Chapter $\text I$: Preliminaries, Definition $1.3$