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What would be an example of bijection between $\mathbb{Q}$ and $\mathbb{Q}\times \mathbb{Q}$.

I can think of one: $x \mapsto (x,x+1)$ Does this work? I am not sure.<

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Is it surjective? –  Lord_Farin Apr 17 '13 at 10:30
    
@Lord_Farin: Oh sorry. I think it isn't. Because the point $(3,5)$ doesn't have a preimage at all. Thanks. Foolish on my behalf –  limit Apr 17 '13 at 10:32

1 Answer 1

Using the fact that $\mathbb Q$ is countable and a finite Cartesian product of countable sets is itself countable, there exist 2 bijections:

$$ f: \mathbb Q \rightarrow \mathbb N$$ $$ g: \mathbb N \rightarrow \mathbb Q \times \mathbb Q$$

And the composition $g \circ f$ will yield a bijection from $\mathbb Q$ to $\mathbb Q \times \mathbb Q$

I will leave finding an explicit formula for f and g to you.

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