Count of Commutative Binary Operations on Set/Sequence
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Theorem
Let $S$ be a set whose cardinality is $n$.
Let $N$ denote the number of different commutative binary operations that can be applied to $S$:
- $N = n^{\frac {n \paren {n + 1} } 2}$
The sequence of $N$ for each $n$ begins:
$\begin{array} {c|cr} n & \dfrac {n \paren {n + 1} }2 & n^{\frac {n \paren {n + 1} } 2} \\ \hline 1 & 1 & 1 \\ 2 & 3 & 8 \\ 3 & 6 & 729 \\ 4 & 10 & 1 \ 048 \ 576 \\ \end{array}$
and so on.
This sequence is A023813 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).