Abbreviation: Slat
A \emph{semilattice} is a structure $\mathbf{S}=\langle S,\cdot \rangle $, where $\cdot $ is an infix binary operation, called the \emph{semilattice operation}, such that
$\cdot $ is associative: $(xy)z=x(yz)$
$\cdot $ is commutative: $xy=yx$
$\cdot $ is idempotent: $xx=x$
Remark: This definition shows that semilattices form a variety.
Let $\mathbf{S}$ and $\mathbf{T}$ be semilattices. A morphism from $\mathbf{S}$ to $\mathbf{T}$ is a function $h:S\to T$ that is a homomorphism:
$h(xy)=h(x)h(y)$
A \emph{join-semilattice} is a structure $\mathbf{S}=\langle S,\leq,\vee\rangle$, where $\vee $ is an infix binary operation, called the \emph{join}, such that
$\leq $ is a partial order,
$x\leq y\implies x\vee z\leq y\vee z$ and $z\vee x\leq z\vee y$,
$x\le x\vee y$ and $y\leq x\vee y$,
$x\vee x\leq x$.
This definition shows that semilattices form a partially-ordered variety.
A \emph{join-semilattice} is a structure $\mathbf{S}=\langle S,\vee \rangle $, where $\vee $ is an infix binary operation, called the \emph{join}, such that
$\leq $ is a partial order, where $x\leq y\Longleftrightarrow x\vee y=y$
$x\vee y$ is the least upper bound of $\{x,y\}$.
A \emph{meet-semilattice} is a structure $\mathbf{S}=\langle S,\wedge \rangle $, where $\wedge $ is an infix binary operation, called the \emph{meet}, such that
$\leq $ is a partial order, where $x\leq y\Longleftrightarrow x\wedge y=x$
$x\wedge y$ is the greatest lower bound of $\{x,y\}$.
Example 1: $\langle \mathcal{P}_\omega(X)-\{\emptyset\},\cup\rangle $, the set of finite nonempty subsets of a set $X$, with union, is the free join-semilattice with singleton subsets of $X$ as generators.
| Classtype | variety |
|---|---|
| Equational theory | decidable in polynomial time |
| Quasiequational theory | decidable |
| First-order theory | undecidable |
| Locally finite | yes |
| Residual size | 2 |
| Congruence distributive | no |
| Congruence modular | no |
| Congruence meet-semidistributive | yes |
| Congruence n-permutable | no |
| Congruence regular | no |
| Congruence uniform | no |
| Congruence extension property | yes |
| Definable principal congruences | yes |
| Equationally def. pr. cong. | no |
| Amalgamation property | yes |
| Strong amalgamation property | yes |
| Epimorphisms are surjective | yes |
\end{table}
$\begin{array}{lr}
f(1)= &1
f(2)= &1
f(3)= &2
f(4)= &5
f(5)= &15
f(6)= &53
f(7)= &222
f(8)= &1078
f(9)= &5994
f(10)= &37622
f(11)= &262776
f(12)= &2018305
f(13)= &16873364
f(14)= &152233518
f(15)= &1471613387
f(16)= &15150569446
f(17)= &165269824761
\end{array}$
These results follow from the paper below and the observation that semilattices with $n$ elements are in 1-1 correspondence to lattices with $n+1$ elements.
Jobst Heitzig,J\“urgen Reinhold,\emph{Counting finite lattices}, Algebra Universalis, \textbf{48}2002,43–53MRreview