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by josephcsible 1427 days ago
I noticed this on the page about infinity:

> Are some infinities larger than others?

But didn't Cantor prove the answer to that is "yes" back in the 19th century?

4 comments

https://wikenigma.org.uk/content/philosophy/infinity

Also:

> If a mathematician wants to explore infinity, there are many options - for example by calculating π, or the square root of 2, or dividing any number by 0.

Er, one of these things is not like the others…

This clearly wasn't written by a mathematician.

Yes. Trivially, uncountable infinities have to be larger than countable infinities do they not? The set of reals contains within it the set of integers for example but also contains a bunch of other stuff. (Not a mathematician obviously).
That argument turns out not to be enough! A counterexample is the set of rational numbers, which has the same cardinality as the natural numbers, even though the naturals are a proper subset of the rationals.

Some infinite sets can be put into one-to-one correspondence with some of their proper subsets!

The proof of the lemma in https://en.wikipedia.org/wiki/Cantor%27s_first_set_theory_ar... doesn't look right to me.

They say "Since the endpoints of (a₁, b₁) are x₁ and x₂", but in the example below a₁ = x₅, b₁ = x₁₂.

Am I missing something?

Corrected. 'Fast' wiki in action!
Wait, what's corrected?
I couldn't find the fragment you cited.
It's stil there.
My first thought was that this is an odd wiki to make, as there isn't even consensus on what is known and what is not.