# Semilattices with n elements

How many n-element semilattices there are?

For example, for n-element partially ordered set we can figured out, that there are $2^{n*(n-1)}$ possible sets.

And can I find all possible n-element semilattices? Or maybe there are some useful rule to generate all that kind of sets? I'm trying do something, but have nothing at all with memory problems

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oeis.org/… –  Martin Rubey Feb 16 '11 at 8:59
Note that the oeis sequence above counts semilatices with $n$ generators, not with $n$ elements. Also, the number of $n$-element posets is much less than $2^{n(n-1)}$ (that would be the number of reflexive relations), see oeis.org/A001035 . –  Emil Jeřábek Feb 16 '11 at 11:48

By adding a top element to a (meet) semilattice we get a lattice. Hence the number of $n$-element semilattices is the number of $(n+1)$-element lattices. This number is treated in A006966 of Sloane.
Well, this is a partial solution... You can create semi-lattices as follows: Say $v^1,v^2,\ldots, v^k\in \mathbb{K}^l$ are arbitrary vectors . Here, $\mathbb{K}^l$ denotes the positive part of $\mathbb{R}^l,$ that is, all coordinates of these vectors are positive. You can put a coordinate-wise ordering on $\mathbb{K}^l.$ Then compute the smallest lower semi-lattice generated by these vectors by taking their infimums, for instance, $(v^1 \wedge v^2)_j=v_j^1\wedge v_j^2$ for each coordinate j. According to theory, this lower semi-lattice should be finite. However, I don't know how to generate all semi-lattices containing exactly $n-$elements.
However, if you need an arbitrary semi-lattice $L$ and a morphism $f$ of $L$ onto itself, then there is a method. Choose $n-$ many different sequences, say $c^i:=(c_{ij})_{j=1}^{\infty}$ satisfying $\sum_j c_{ij}=\infty$ for each $i=1\ldots n.$ Then define $T\colon \mathbb{K}^n\rightarrow \mathbb{K}^n$ by $$Tx_{i}:= \Sigma_{\gamma(j,k)=i} c_{jk} \wedge ((x_j-\Sigma_{s=0}^{k-1}c_{js})\vee 0)$$
where $\gamma$ is any function from $\{ 1,2,\ldots n\}\times \mathbb{N}$ into $\{1,2,\ldots,n\}.$ For arbitrary $n,$ the sequences $c_{ij}$ and the rule $\gamma$ should be chosen in such a way that a periodic point of the dynamical system $T\colon \mathbb{K}^n\rightarrow \mathbb{K}^n$ should be computable. I.e, you should choose these variables according to your purpose and $n$. Then a result of Scheutzow say that, the restriction of $T$ onto the semi-lattice generated by the orbit of this periodic point(it is finite) is a morphism.