Beth number

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In mathematics, the infinite cardinal numbers are represented by the Hebrew letter $\aleph$ (aleph) indexed with a subscript that runs over the ordinal numbers (see aleph number). The second Hebrew letter $\beth$ (beth) is also used. To define the beth numbers, start by letting

$\beth_0=\aleph_0$

be the cardinality of any countably infinite set; for concreteness, take the set $\mathbb{N}$ of natural numbers to be a typical case. Denote by P(A) the power set of A, i.e., the set of all subsets of A. Then define

$\beth_{\alpha+1}=2^{\beth_{\alpha}},$

which is the cardinality of the power set of A if $\beth_{\alpha}$ is the cardinality of A.

Then

$\beth_0,\ \beth_1,\ \beth_2,\ \beth_3,\ \dots$

are respectively the cardinalities of

$\mathbb{N},\ P(\mathbb{N}),\ P(P(\mathbb{N})),\ P(P(P(\mathbb{N}))),\ \dots.$

Each set in this sequence has cardinality strictly greater than the one preceding it, because of Cantor's theorem. Note that the first beth number $\beth_1$ is equal to c (or $\mathfrak c$ ), the cardinality of the continuum, and the second beth number $\beth_2$ is the cardinality of the power set of the continuum.

For infinite limit ordinals λ, we define:

$\beth_{\lambda}=\sup\{ \beth_{\alpha}:\alpha<\lambda \}.$

One can also show that the von Neumann universes $V_{\omega+\alpha} \!$ have cardinality $\beth_{\alpha} \!$ .

If we assume the axiom of choice, then infinite cardinalities are linearly ordered; no two cardinalities can fail to be comparable, and so, since by definition no infinite cardinalities are between $\aleph_0$ and $\aleph_1$ , the celebrated continuum hypothesis can be stated in this notation by saying

$\beth_1=\aleph_1.$

The generalized continuum hypothesis says the sequence of beth numbers thus defined is the same as the sequence of aleph numbers.

1 Specific cardinals
- 1.1 Beth one
- 1.2 Beth two
2 Generalization
3 Also

[edit] Specific cardinals

[edit] Beth one

Main article: cardinality of the continuum

Sets with cardinality $\beth_1$ include:

the real numbers R
the irrational numbers
the transcendental numbers
Euclidean space Rⁿ
the complex numbers C
the power set of the natural numbers (the set of all subsets of the natural numbers)
the set of sequences of integers (i.e. all functions N → Z, often denoted Z^N)
the set of sequences of real numbers, R^N
the set of all continuous functions from R to R

[edit] Beth two

$\beth_2$ (pronounced beth two) is also referred to as 2^c (pronounced two to the power of c).

Sets with cardinality $\beth_2$ include:

The power set of the set of real numbers, so it is the number of subsets of the real line, or the number of sets of real numbers
The power set of the power set of the set of natural numbers
The set of all functions from R to R (often denoted R^R)
The power set of the set of all functions from the set of natural numbers to itself, so it is the number of sets of sequences of natural numbers
The set of all real-valued functions of n real variables to the real numbers

[edit] Generalization

The more general symbol $\beth_\alpha(\kappa)$ , for ordinals α and cardinals κ, is occasionally used. It is defined by:

$\beth_0(\kappa)=\kappa,$

$\beth_{\alpha+1}(\kappa)=2^{\beth_{\alpha}(\kappa)},$

$\beth_{\lambda}(\kappa)=\sup\{ \beth_{\alpha}(\kappa):\alpha<\lambda \}$ if λ is a limit ordinal.

So $\beth_{\alpha}=\beth_{\alpha}(\aleph_0).$

In ZF, for any cardinals κ and μ, there is an ordinal α such that:

$\kappa \le \beth_{\alpha}(\mu).$

And in ZF, for any cardinal κ and ordinals α and β:

$\beth_{\beta}(\beth_{\alpha}(\kappa)) = \beth_{\alpha+\beta}(\kappa).$

Consequently, in Zermelo–Fraenkel set theory absent urelements with or without the axiom of choice, for any cardinals κ and μ, there is an ordinal α such that for any ordinal β ≥ α:

$\beth_{\beta}(\kappa) = \beth_{\beta}(\mu).$

This also holds in Zermelo–Fraenkel set theory with urelements with or without the axiom of choice provided the urelements form a set which is equinumerous with a pure set (a set whose transitive closure contains no urelements). If the axiom of choice holds, then any set of urelements is equinumerous with a pure set.

[edit] Also

This article or a past revision is based on the Mandelbrot Set Glossary and Encyclopedia, copyright © 1987-2003 Robert P. Munafo, which is made available under the terms of the GNU Free Documentation License.

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Category: Cardinal numbers