Let $X$ be a finite set of $n$ elements, and consider a binary operation $\odot: X \times X \rightarrow X$. There are $n^{n^2}$ such binary operations, as the $n \times n$ table entries can each be filled with one of $n$ elements of $X$. My question is:

How many of the $n^{n^2}$ binary operations are associative, i.e., $(x \odot y) \odot z = x \odot (y \odot z)$?

Unless I miscomputed this, for $n=2$, exactly half of the $2^4=16$ binary operations are associative. But for $n=3$, only $113$ of the $3^9=19,683$ binary operations are associative, a count I do not trust, because it seems so much smaller than I anticipated. (It is difficult to count among the four billion ($4,294,967,296$) binary operations for $n=4$.)

I would be interested in the asymptotic growth rate. Surely this is all well known...? Thanks for pointers!

**Update**. Following MSE link provided by Darij, I reached (via Gerry Myerson's pointer) the OEIS sequence A023814. The $n=4$ number I couldn't easily compute is $3492$.

Algebra Universalis27(1990), no. 1, 70--79. math.hawaii.edu/~ralph/papers.html $\endgroup$ – Joseph O'Rourke Oct 21 '12 at 4:04