A friend and I discussed whether there exist operations that are never associative in the sense that $$x(yz)\neq (xy)z$$ for all x, y, z and after pondering I found a simple example on a set with 2 elements. An example of an operation on a set with 4 elements was also readily found. But 3 elements eludes me. I have found these three restrictions that may all serve to disprove this existence.
- At least two pairs of elements do not commute
- No element can be idempotent
- The following identity can never hold $$xy=yz=y$$ for a triple of x,y,z.
An example or a proof of non-existence would be much appreciated.
Here's a simple example of an anti-associative structure on any set $A$ with more than one element. Let $f:A\to A$ be a mapping with no fixed points and define $xy=f(y)$. Then for any $x,y,z\in A$, $$(xy)z=f(z)\ne f(f(z))=x(yz).$$