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# Aleph number

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### Aleph number

In set theory, a discipline within mathematics, the aleph numbers are a sequence of numbers used to represent the cardinality (or size) of infinite sets. They are named after the symbol used to denote them, the Hebrew letter aleph (\aleph) (though in older mathematics books the letter aleph is often printed upside down by accident, partly because a Monotype matrix for aleph was mistakenly constructed the wrong way up ).

The cardinality of the natural numbers is \aleph_0 (read aleph-naught or aleph-zero; the German term aleph-null is also sometimes used), the next larger cardinality is aleph-one \aleph_1, then \aleph_2 and so on. Continuing in this manner, it is possible to define a cardinal number \aleph_\alpha for every ordinal number α, as described below.

The concept goes back to Georg Cantor, who defined the notion of cardinality and realized that infinite sets can have different cardinalities.

The aleph numbers differ from the infinity (∞) commonly found in algebra and calculus. Alephs measure the sizes of sets; infinity, on the other hand, is commonly defined as an extreme limit of the real number line (applied to a function or sequence that "diverges to infinity" or "increases without bound"), or an extreme point of the extended real number line.

## Contents

• Aleph-naught 1
• Aleph-one 2
• Continuum hypothesis 3
• Aleph-ω 4
• Aleph-α for general α 5
• Fixed points of omega 6
• Role of axiom of choice 7
• See also 8
• References 9
• External links 10

## Aleph-naught

\aleph_0 (aleph-naught, also aleph zero or the German term Aleph-null) is the cardinality of the set of all natural numbers, and is an infinite cardinal. The set of all finite ordinals, called ω or ω0, has cardinality \aleph_0. A set has cardinality \aleph_0 if and only if it is countably infinite, that is, there is a bijection (one-to-one correspondence) between it and the natural numbers. Examples of such sets are

These infinite ordinals: ω, ω+1, ω·2, ω2, ωω and ε0 are among the countably infinite sets. For example, the sequence (with ordinality ω·2) of all positive odd integers followed by all positive even integers

{1, 3, 5, 7, 9, ..., 2, 4, 6, 8, 10, ...}

is an ordering of the set (with cardinality \aleph_0) of positive integers.

If the axiom of countable choice (a weaker version of the axiom of choice) holds, then \aleph_0 is smaller than any other infinite cardinal.

## Aleph-one

\aleph_1 is the cardinality of the set of all countable ordinal numbers, called ω1 or (sometimes) Ω. This ω1 is itself an ordinal number larger than all countable ones, so it is an uncountable set. Therefore \aleph_1 is distinct from \aleph_0. The definition of \aleph_1 implies (in ZF, Zermelo–Fraenkel set theory without the axiom of choice) that no cardinal number is between \aleph_0 and \aleph_1. If the axiom of choice (AC) is used, it can be further proved that the class of cardinal numbers is totally ordered, and thus \aleph_1 is the second-smallest infinite cardinal number. Using AC we can show one of the most useful properties of the set ω1: any countable subset of ω1 has an upper bound in ω1. (This follows from the fact that a countable union of countable sets is countable, one of the most common applications of AC.) This fact is analogous to the situation in \aleph_0: every finite set of natural numbers has a maximum which is also a natural number, and finite unions of finite sets are finite.

ω1 is actually a useful concept, if somewhat exotic-sounding. An example application is "closing" with respect to countable operations; e.g., trying to explicitly describe the σ-algebra generated by an arbitrary collection of subsets (see e. g. Borel hierarchy). This is harder than most explicit descriptions of "generation" in algebra (vector spaces, groups, etc.) because in those cases we only have to close with respect to finite operations—sums, products, and the like. The process involves defining, for each countable ordinal, via transfinite induction, a set by "throwing in" all possible countable unions and complements, and taking the union of all that over all of ω1.

In popular books \aleph_1 is sometimes incorrectly defined to be 2^{\aleph_0}, but this is wrong if the continuum hypothesis fails.

## Continuum hypothesis

The cardinality of the set of real numbers (cardinality of the continuum) is 2^{\aleph_0}. It cannot be determined from ZFC (Zermelo–Fraenkel set theory with the axiom of choice) where this number fits exactly in the aleph number hierarchy, but it follows from ZFC that the continuum hypothesis, CH, is equivalent to the identity

2^{\aleph_0}=\aleph_1.

The CH states that there is no set whose cardinality is strictly between that of the integers and the real numbers. CH is independent of ZFC: it can be neither proven nor disproven within the context of that axiom system (provided that ZFC is consistent). That CH is consistent with ZFC was demonstrated by Kurt Gödel in 1940 when he showed that its negation is not a theorem of ZFC. That it is independent of ZFC was demonstrated by Paul Cohen in 1963 when he showed, conversely, that the CH itself is not a theorem of ZFC by the (then novel) method of forcing.

## Aleph-ω

Conventionally the smallest infinite ordinal is denoted ω, and the cardinal number \aleph_\omega is the least upper bound of

\left\{\,\aleph_n : n\in\left\{\,0,1,2,\dots\,\right\}\,\right\}

among alephs.

Aleph-ω is the first uncountable cardinal number that can be demonstrated within Zermelo–Fraenkel set theory not to be equal to the cardinality of the set of all real numbers; for any positive integer n we can consistently assume that 2^{\aleph_0} = \aleph_n, and moreover it is possible to assume 2^{\aleph_0} is as large as we like. We are only forced to avoid setting it to certain special cardinals with cofinality \aleph_0, meaning there is an unbounded function from \aleph_0 to it (see Easton's theorem).

## Aleph-α for general α

To define \aleph_\alpha for arbitrary ordinal number \alpha, we must define the successor cardinal operation, which assigns to any cardinal number ρ the next larger well-ordered cardinal ρ+ (if the axiom of choice holds, this is the next larger cardinal).

We can then define the aleph numbers as follows:

\aleph_{0} = \omega
\aleph_{\alpha+1} = \aleph_{\alpha}^+

and for λ, an infinite limit ordinal,

\aleph_{\lambda} = \bigcup_{\beta < \lambda} \aleph_\beta.

The α-th infinite initial ordinal is written \omega_\alpha. Its cardinality is written \aleph_\alpha. In ZFC the \aleph function is a bijection between the ordinals and the infinite cardinals.

## Fixed points of omega

For any ordinal α we have

\alpha\leq\omega_\alpha.

In many cases \omega_{\alpha} is strictly greater than α. For example, for any successor ordinal α this holds. There are, however, some limit ordinals which are fixed points of the omega function, because of the fixed-point lemma for normal functions. The first such is the limit of the sequence

\omega,\ \omega_\omega,\ \omega_{\omega_\omega},\ \ldots.

Any weakly inaccessible cardinal is also a fixed point of the aleph function. This can be shown in ZFC as follows. Suppose \kappa = \aleph_\lambda is a weakly inaccessible cardinal. If \lambda were a successor ordinal, then \aleph_\lambda would be a successor cardinal and hence not weakly inaccessible. If \lambda were a limit ordinal less than \kappa , then its cofinality (and thus the cofinality of \aleph_\lambda) would be less than \kappa and so \kappa would not be regular and thus not weakly inaccessible. Thus \lambda \geq \kappa and consequently \lambda = \kappa which makes it a fixed point.

## Role of axiom of choice

The cardinality of any infinite ordinal number is an aleph number. Every aleph is the cardinality of some ordinal. The least of these is its initial ordinal. Any set whose cardinality is an aleph is equinumerous with an ordinal and is thus well-orderable.

Each finite set is well-orderable, but does not have an aleph as its cardinality.

The assumption that the cardinality of each infinite set is an aleph number is equivalent over ZF to the existence of a well-ordering of every set, which in turn is equivalent to the axiom of choice. ZFC set theory, which includes the axiom of choice, implies that every infinite set has an aleph number as its cardinality (i.e. is equinumerous with its initial ordinal), and thus the initial ordinals of the aleph numbers serve as a class of representatives for all possible infinite cardinal numbers.

When cardinality is studied in ZF without the axiom of choice, it is no longer possible to prove that each infinite set has some aleph number as its cardinality; the sets whose cardinality is an aleph number are exactly the infinite sets that can be well-ordered. The method of Scott's trick is sometimes used as an alternative way to construct representatives for cardinal numbers in the setting of ZF. For example, one can define card(S) to be the set of sets with the same cardinality as S of minimum possible rank. This has the property that card(S) = card(T) if and only if S and T have the same cardinality. (The set card(S) does not have the same cardinality of S in general, but all its elements do.)

## See also

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