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10:01 PM
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Q: Cardinality of the permutations of an infinite set

Roy MacleanIf you have an infinite set X of cardinality k, then what is the cardinality of Sym(X) - the group of permutations of X ?

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Q: An easy proof of the uncountability of bijections on natural numbers?

Abhishek ParabThe proof that I have in mind is as follows - $\text{Gal }(\overline{\mathbb Q}/\mathbb Q)$ is a proper uncountable subgroup of the group of permutations on countably many symbols, hence the latter is uncountable. . But it needs a lot of jargon from topology and algebra. Is there a neat proof ...

It is a nice question and the solution based on Riemann rearrangement theorem is very nice. (I certainly would not get an idea to use something like that.)
However, I was a bit surprised to see the question on MathOverflow. I would consider it a more-or-less standard freshmen exercise, at least the countable case.
There are also a few copies of this question on math.SE:
There are also several posts on MSE about cardinality of the set of all bijections $\mathbb N\to\mathbb N$, for example: math.stackexchange.com/questions/87902/… and math.stackexchange.com/questions/367194/…Martin Sleziak Jun 5 '14 at 6:21
 

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