# What should Aleph-Two mean?

Just curious, what should Aleph-Two mean? I know that Aleph-One is distinct from Aleph-Null and Aleph-One is not countable, but does Aleph-Two mean?

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It can be defined as the set of all ordinals that can be injected into $\aleph_1$. (I don't pretend to be able to visualize this.) –  goblin Apr 29 '14 at 18:38

## 2 Answers

$\aleph_0$ is the cardinality of the set of all the finite ordinals, which coincide with our usual interpretation of the natural numbers. $\aleph_1$ is the cardinality of the set of all the countable ordinals.

$\aleph_2$ is the cardinality of the set of all the ordinals whose size is $\leq\aleph_1$ (and in fact equals $\aleph_1$ would suffice).

If we assume the generalized continuum hypothesis, then $\aleph_2=|\mathcal P(\Bbb R)|$.

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To approach this in a slightly different manner than Asaf, in general for every ordinal number $\alpha$ we designate by $\aleph_\alpha$ the unique infinite cardinal number $\kappa$ such that the set of all infinite cardinals $< \kappa$ has order type $\kappa$ (according to the usual order). So one may think of $\aleph_\alpha$ is the "$(\alpha+1)^{\text{st}}$ infinite cardinal".

I'm sure you already know that $\aleph_0$ is the smallest (first) infinite cardinal.

$\aleph_1$ is then the least cardinal number greater than $\aleph_0$, the second infinite cardinal, and the first uncountable cardinal.

$\aleph_2$ is the least cardinal number greater than $\aleph_1$. (The third infinite cardinal, and the second uncountable one.)

Ans we can continue like this ad infinitum.

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