How can the real, integer, and rational number sets be infinite, yet, they aren't all the same size?

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  • Look up "cardinality" and "cardinal numbers" using google. Also "countable set" would be good, and maybe "Cantor's diagnoan proof." BTW integers, rationals same size in cardinality. – coffeemath Dec 16 '15 at 22:16
  • And [this](http://math.stackexchange.com/questions/18969/why-are-the-reals-uncountable?lq=1), and [this](http://math.stackexchange.com/questions/732/proof-that-the-irrational-numbers-are-uncountable)... –  Dec 16 '15 at 22:23

1 Answers1


Because of Cantor's theorem (the diagonal construction).

Given the definitions of what it means for two infinite sets to be of the same size, and of what it means for one set to be at least as large as another, $\Bbb N$ and $\Bbb Q$ are of the same size, because we can prove that there is a bijection between them.

However, Cantor's theorem shows that no 1-1 function $\Bbb N\to \Bbb R$ can exhaust all of $\Bbb R$ — it can't be onto $\Bbb R$, so $\Bbb N$ is strictly smaller than $\Bbb R$.

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