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This is my first question and I hope this question is not too brief to be acceptable:

There are well-known bijections $\mathbb{N \times N \to N}$ and well-known associative compositions on countably infinite sets. What if we combine the two properties?

Is there a bijective function $f\colon \mathbb{N \times N \to N}$ which is also associative: $$f(x,f(y,z)) = f(f(x,y),z)\quad\text{ for all } \quad x,y,z \in \mathbb{N}\:?$$

Thanks in advance!

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1 Answer 1

up vote 7 down vote accepted

Suppose $f$ is associative and bijective. For any $x,y, z \in \mathbb{N}$, we have $f(x,f(y,z))=f(f(x,y),z)$ by associativity. Thus by injectivity we have $x=f(x,y), f(y,z)=z$ for all $x, y, z$. But this is impossible, since it implies for example that $f(1,2)=1$ and $f(1,2)=2$.

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Neat! Thank you. Now I feel a bit silly... –  Heinrich Nov 9 '12 at 1:12
This works for every set with $\geq 2$ elements. –  Martin Brandenburg Jun 13 '13 at 23:31

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