Is there a simple example of a binary operation that is idempotent, but not commutative nor associative?

  • $\begingroup$ The term idempotent refers to objects where we can perform a binary operation, not to the operation itself. $\endgroup$ – Tobias Kildetoft Apr 8 '16 at 11:58
  • $\begingroup$ An idempotent element $a$ is one such that $a\cdot a=a$. Do you want $a\cdot a=a$ for all elements in the set? $\endgroup$ – almagest Apr 8 '16 at 12:04
  • $\begingroup$ yes, I want a*a=a for all a in the set $\endgroup$ – Filip Haglund Apr 8 '16 at 12:08

How about $a\oplus b=pa+(1-p)b$ for $a,b\in\mathbb R$ with $\{0,\frac12,1\}\not\ni p\in\mathbb R$?


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