Count of 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 possible different binary operations that can be applied to $S$:
- $N = n^{\paren {n^2} }$
The sequence of $N$ for each $n$ begins:
$\begin{array} {c|rr} n & n^2 & N = n^{\paren {n^2} } \\ \hline 1 & 1 & 1 \\ 2 & 4 & 16 \\ 3 & 9 & 19 \ 683 \\ 4 & 16 & 4 \ 294 \ 967 \ 296 \\ \end{array}$
There are still only $4$ elements in a set, and already there are over $4$ thousand million different possible algebraic structures.
This sequence is A002489 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).