User:KBlott/Proofs/Semilattice with Identity is an Idempotent Commutative Monoid
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An algebra $(S, \cdot)$ is a semilattice with identity $e \in S$ iff $(S, \cdot, e)$ is an idempotent commutative monoid.
Proof
$(S, \cdot)$ is a semilattice with identity $e \in S$ iff
- $(S, \cdot)$ is a commutative semigroup, and
- $\forall x \in S$:
- $x \cdot x = x$, and
- $x \cdot e = x = e \cdot x$.
Likewise,$(S, \cdot, e)$ is an idempotent commutative monoid iff
- $(S, \cdot)$ is a commutative semigroup, and
- $\forall x \in S$:
- $x \cdot e = x = e \cdot x$, and
- $x \cdot x = x$.
$\blacksquare$
Examples and some non-examples
- $(\lbrace 0, \N \rbrace, \cup)$ is a semilattice with identity $0 \in \lbrace 0, \N \rbrace$.
- $(\lbrace 0, \N \rbrace, \cap)$ is a semilattice with identity $\N \in \lbrace 0, \N \rbrace$.
- $(\N, \cup)$ is a semilattice with identity $0 \in \N$.
- $(\N, \cap)$ is a semilattice without identity, since $\N \not \in \N$. This can be fixed by taking the successor set $\N + 1 := \N \cup \lbrace \N \rbrace$.
- $(\N + 1, \cup)$ semilattice with identity $0 \in \N + 1$.
- $(\N + 1, \cap)$ semilattice with identity $\N \in \N + 1$. $\N + 1$ is a subset of the power set of $\N$. This allows us to further generalize $\N$ without losing structural integrity.
- $(\mathscr P(\N), \cup)$ semilattice with identity $0 \in \mathscr P(\N)$ where $\mathscr P(\N)$ is the power set of $\N$.
- $(\mathscr P(\N), \cap)$ semilattice with identity $\N \in \mathscr P(\N)$.
- $(\N, +, 0)$ and $(\N, \times, 1)$ are commutative monoids but not semilattices, since they are not idempotent.
- $(1, +, 0)$ is a semilattice with identity $0 \in 1$.
- $(2, \times, 1)$ is a semilattice with identity $1 \in 2$.
Sources
- Proof Theoretical Studies on Semilattice Relevant Logics. 2001. Ryo Kashim.