Tuesday 4 September 2018

combinatorics - Let $X$ denote an infinite set. Is every partitioning of $X^2$ induced by an associative operation $f:X^2 rightarrow X$?





Proposition. Let $X$ denote an infinite set. Then for each partitioning $\Pi$ of $X$, there exists a function $f : X \rightarrow X$ whose coimage is $\Pi$.




I'd like to know whether the analogous statement holds for associative binary operations.




Question. Let $X$ denote an infinite set. Is it true that for each partitioning $\Pi$ of $X^2$, there exists an associative operation $f : X^2 \rightarrow X$ whose coimage is $\Pi$?





This question occurred to me as a result of Behrouz Maleki's answer here.


Answer



Consider the discrete partition of $X^2$, and assume it is the coimage of an associative binary operation. Then for all $a,b,c\in X$ we have $a(bc)=(ab)c$, and thus $a=ab$, and this determines the operation as projection to the left argument. Similarly, $bc=c$, and this determines the operation as projection to the right argument. This can only happen if $X$ is empty or a singleton.


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