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| In class theories, the '''axiom of limitation of size''' says that for any class ''C'', ''C'' is a [[proper class]], that is a class which is not a [[Set (mathematics)|set]] (an [[Element (mathematics)|element]] of other classes), if and only if it can be mapped [[onto]] the class ''[[Von Neumann universe|V]]'' of all sets.<ref>This is roughly von Neumann's original formulation, see Fraenkel & al, p. 137.</ref>
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| :<math>\forall C [\lnot \exist W (C \in W) \iff \exist F ( \forall x [\exist W (x \in W) \Rightarrow \exist s (s \in C \and \langle s, x \rangle \in F)] \and </math>
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| ::<math>\forall x \forall y \forall s [(\langle s, x \rangle \in F \and \langle s, y \rangle \in F) \Rightarrow x = y])].</math>
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| This axiom is due to [[John von Neumann]]. It implies the [[axiom schema of specification]], [[axiom schema of replacement]], [[axiom of global choice]], and even, as noticed later by [[Azriel Levy]], [[axiom of union]]<ref>showing directly that a set of ordinals has an upper bound, see A. Levy, " On von Neumann's axiom system for set theory ", Amer. Math. Monthly, 75 (1968), p. 762-763.</ref> at one stroke. The axiom of limitation of size implies the axiom of global choice because the class of ordinals is not a set, so there is a [[surjection]] from the [[ordinal number|ordinals]] to the [[Von Neumann universe|universe]], thus an [[injection (mathematics)|injection]] from the universe to the ordinals, that is, the universe of sets is [[well-order]]ed.
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| Together the [[axiom of replacement]] and the [[axiom of global choice]] (with the other axioms of [[von Neumann–Bernays–Gödel set theory]]) imply this axiom. This axiom can then replace replacement, global choice, specification and union in von Neumann–Bernays–Gödel or [[Morse–Kelley set theory]].
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| However, the axiom of replacement and the usual [[axiom of choice]] (with the other axioms of von Neumann–Bernays–Gödel set theory) do not imply von Neumann's axiom. In 1964, Easton used [[Forcing (set theory)|forcing]] to build a [[Model theory|model]] that satisfies the axioms of von Neumann–Bernays–Gödel set theory with one exception: the axiom of global choice is replaced by the axiom of choice. In Easton's model, the axiom of limitation of size fails dramatically: the universe of sets cannot even be [[linearly ordered]].<ref>Easton 1964.</ref>
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| It can be shown that a class is a proper class if and only if it is [[equinumerous]] to ''V'', but von Neumann's axiom does not capture all of the "[[limitation of size]] doctrine",<ref>Fraenkel & al, p. 137. A guiding principle for ZF to avoid set theoretical paradoxes is to restrict to instances of full (contradictory) comprehension scheme that do not give sets "too much bigger" than the ones they use; it is known as "limitation of size", Fraenkel & al call it "limitation of size doctrine", see p. 32.</ref> because the [[axiom of power set]] is not a consequence of it. Later expositions of class theories ([[Paul Bernays|Bernays]], [[Kurt Gödel|Gödel]], [[John L. Kelley|Kelley]], ...) generally use replacement and a form of the axiom of choice rather than the axiom of limitation of size.
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| ==History==
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| Von Neumann developed the axiom of limitation of size as a new method of identifying sets. [[Zermelo–Fraenkel set theory|ZFC]] identifies sets via its set building axioms. However, as [[Abraham Fraenkel]] pointed out: "The rather arbitrary character of the processes which are chosen in the axioms of '''Z''' [ZFC] as the basis of the theory, is justified by the historical development of set-theory rather than by logical arguments."<ref>''Historical Introduction'' in Bernays 1991, p. 31.</ref>
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| The historical development of the ZFC axioms began in 1908 when [[Ernst Zermelo|Zermelo]] chose axioms to support his proof of the [[well-ordering theorem]] and to avoid contradictory sets.<ref>"... we must, on the one hand, restrict these principles [axioms] sufficiently to exclude all contradictions and, on the other hand, take them sufficiently wide to retain all that is valuable in this theory." (Zermelo 1908, p. 261; English translation, p. 200). Gregory Moore analyzed Zermelo's reasons behind his axiomatization and concluded that "his axiomatization was primarily motivated by a desire to secure his demonstration of the Well-Ordering Theorem …" and "For Zermelo, … the paradoxes were an inessential obstacle to be circumvented with as little fuss as possible." (Moore 1982, p. 159–160).</ref> In 1922, [[Abraham Fraenkel|Fraenkel]] and [[Thoralf Skolem|Skolem]] pointed out that [[Zermelo set theory|Zermelo's axioms]] cannot prove the existence of the set {''Z''<sub>0</sub>, ''Z''<sub>1</sub>, ''Z''<sub>2</sub>, … } where ''Z''<sub>0</sub> is the set of [[natural number]]s, and ''Z''<sub>''n''+1</sub> is the [[power set]] of ''Z''<sub>''n''</sub>.<ref>Frankel 1922, p. 230–231; Skolem 1922 (English translation, p. 296–297).</ref> They also introduced the [[axiom of replacement]], which guarantees the existence of this set.<ref>Ferreirós 2007, p. 369. In 1917, [[Dmitry Mirimanoff|Mirimanoff]] published a form of replacement based on cardinal equivalence (Mirimanoff 1917, p. 49).</ref> However, adding axioms as they are needed neither guarantees the existence of all reasonable sets nor clarifies the difference between sets that are safe to use and collections that lead to contradictions.
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| In a 1923 letter to Zermelo, von Neumann outlined an approach to set theory that identifies the sets that are "too big" (now called proper classes) and that can lead to contradictions.<ref>He gave a detailed exposition of his set theory in two articles: von Neumann 1925 and von Neumann 1928.</ref> Von Neumann identified these sets using the criterion: "A set is 'too big' if and only if it is [[equinumerous|equivalent]] to the set of all things."<ref>Hallett 1984, p. 288.</ref> He then restricted how these sets may be used: "… in order to avoid the paradoxes those [sets] which are 'too big' are declared to be impermissible as ''elements''."<ref>Hallett 1984, p. 290.</ref> By combining this restriction with his criterion, von Neumann obtained the axiom of limitation of size (which in the language of classes states): A class X is not an element of any class if and only if X is equivalent to the class of all sets.<ref>Hallett 1984, p. 290. Von Neumann later changed "equivalent to the class of all sets" to "can be mapped onto the class of all sets."</ref> So von Neumann identified sets as classes that are not equivalent to the class of all sets. Von Neumann realized that, even with his new axiom, his set theory does not fully characterize sets.<ref>To be precise, von Neumann investigated whether his set theory is [[categorical (model theory)|categorical]]; that is, whether it uniquely determines sets in the sense that any two of its models are [[isomorphic]]. He showed that it is not categorical because of a weakness in the [[axiom of regularity]]: this axiom only excludes descending ∈-sequences from existing in the model; descending sequences may still exist outside the model. A model having "external" descending sequences is not isomorphic to a model having no such sequences since this latter model lacks isomorphic images for the sets belonging to external descending sequences. This led von Neumann to conclude "that no categorical axiomatization of set theory seems to exist at all" (von Neumann 1925, p. 239; English translation: p. 412).</ref>
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| Gödel found von Neumann's axiom to be "of great interest":
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| :"In particular I believe that his [von Neumann's] necessary and sufficient condition which a property must satisfy, in order to define a set, is of great interest, because it clarifies the relationship of axiomatic set theory to the paradoxes. That this condition really gets at the essence of things is seen from the fact that it implies the axiom of choice, which formerly stood quite apart from other existential principles. The inferences, bordering on the paradoxes, which are made possible by this way of looking at things, seem to me, not only very elegant, but also very interesting from the logical point of view.<ref>For example, von Neumann's proof that his axiom implies the well-ordering theorem uses the [[Burali-Forte paradox]] (von Neumann 1925, p. 223; English translation: p. 398).</ref> Moreover I believe that only by going farther in this direction, i.e., in the direction opposite to [[constructivism (mathematics)|constructivism]], will the basic problems of abstract set theory be solved."<ref>From a Nov. 8, 1957 letter Gödel wrote to [[Stanislaw Ulam]] (Kanamori 2003, p. 295).</ref>
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| ==Zermelo's models and the axiom of limitation of size==
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| In 1930, Zermelo published an article on models of set theory, in which he proved that some of his models satisfy the axiom of limitation of size. These models are built in [[ZFC]] by using the [[cumulative hierarchy]] ''V''<sub>α</sub>, which is defined by [[transfinite recursion]]:
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| # ''V''<sub>0</sub> = [[empty set|∅]].<ref>This is the standard definition of ''V''<sub>0</sub>. Zermelo let ''V''<sub>0</sub> be a set of [[urelements]] and proved that if this set contains a single element, the resulting model satisfies the axiom of limitation of size (his proof also works for ''V''<sub>0</sub> = ∅). Zermelo stated that the axiom is not true for all models built from a set of urelements. (Zermelo 1930, p. 38; English translation: p. 1227.)</ref>
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| # ''V''<sub>α+1</sub> = ''V''<sub>α</sub> ∪ ''P''(''V''<sub>α</sub>). That is, the [[Union (set theory)|union]] of ''V''<sub>α</sub> and its [[power set]].<ref>This is Zermelo's definition (Zermelo 1930, p. 36; English translation: p. 1225 & p. 1209), which is equivalent to ''V''<sub>α+1</sub> = ''P''(''V''<sub>α</sub>) since ''V''<sub>α</sub> ⊆ ''P''(''V''<sub>α</sub>) (Kunen 1980, p. 95; Kunen uses the notation R(α) instead of ''V''<sub>α</sub>).</ref>
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| # For limit β: ''V''<sub>β</sub> = ∪<sub>α < β</sub> ''V''<sub>α</sub>. That is, ''V''<sub>β</sub> is the union of the preceding ''V''<sub>α</sub>.
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| Zermelo worked with models of the form ''V''<sub>κ</sub> where κ is a [[von Neumann cardinal|cardinal]]. The classes of the model are the [[subsets]] of ''V''<sub>κ</sub>, and the model's ∈-relation is the standard ∈-relation. The sets of the model ''V''<sub>κ</sub> are the classes ''X'' such that ''X'' ∈ ''V''<sub>κ</sub>.<ref>In [[von Neumann–Bernays–Gödel set theory|NBG]], ''X'' is a set if there is a class ''Y'' such that ''X'' ∈ ''Y''. Since ''Y'' ⊆ ''V''<sub>κ</sub>, we have ''X'' ∈ ''V''<sub>κ</sub>. Conversely, if ''X'' ∈ ''V''<sub>κ</sub>, then ''X'' belongs to a class, so ''X'' is a set.</ref> Zermelo identified cardinals κ such that ''V''<sub>κ</sub> satisfies:<ref>These theorems are part of Zermelo's Second Development Theorem. (Zermelo 1930, p. 37; English translation: p. 1226.)</ref>
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| : Theorem 1. A class ''X'' is a set if and only if | ''X'' | < κ.
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| : Theorem 2. | ''V''<sub>κ</sub> | = κ.
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| Since every class is a subset of ''V''<sub>κ</sub>, Theorem 2 implies that every class ''X'' has [[cardinality]] ≤ κ. Combining this with Theorem 1 proves: Every proper class has cardinality κ. Hence, every proper class can be put into one-to-one correspondence with ''V''<sub>κ</sub>, so the axiom of limitation of size holds for the model ''V''<sub>κ</sub>.
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| The proof of the axiom of global choice in ''V''<sub>κ</sub> is more direct than von Neumann's proof. First note that κ (being a [[von Neumann cardinal]]) is a [[well-ordered]] class of cardinality κ. Since Theorem 2 states that ''V''<sub>κ</sub> has cardinality κ, there is a [[one-to-one correspondence]] between κ and ''V''<sub>κ</sub>. This correspondence produces a well-ordering of ''V''<sub>κ</sub>, which implies the axiom of global choice.<ref>The domain of the global [[choice function]] consists of the non-empty sets of ''V''<sub>κ</sub>; this function uses the well-ordering of ''V''<sub>κ</sub> to choose the least element of each set.</ref> Von Neumann uses the [[Burali-Forti paradox]] to [[prove by contradiction]] that the class of all ordinals is a proper class, and then he applies the axiom of limitation of size to well-order the universal class.<ref>Von Neumann 1925, p. 223. English translation: p. 398. Von Neumann's proof, which only uses axioms, has the advantage of applying to all models rather than just to ''V''<sub>κ</sub>.</ref>
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| ===The model ''V''<sub>ω</sub>===
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| To demonstrate that Theorems 1 and 2 hold for some ''V''<sub>κ</sub>, we need to prove that if a set belongs to ''V''<sub>α</sub> then it belongs to all subsequent ''V''<sub>β</sub>, or equivalently: ''V''<sub>α</sub> ⊆ ''V''<sub>β</sub> for α ≤ β. This is proved by [[transfinite induction]] on β:
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| # β = 0: ''V''<sub>0</sub> ⊆ ''V''<sub>0</sub>.
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| # For β+1: By inductive hypothesis, ''V''<sub>α</sub> ⊆ ''V''<sub>β</sub>. Hence, ''V''<sub>α</sub> ⊆ ''V''<sub>β</sub> ⊆ ''V''<sub>β</sub> ∪ ''P''(''V''<sub>β</sub>) = ''V''<sub>β+1</sub>.
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| # For limit β: If α < β, then ''V''<sub>α</sub> ⊆ ∪<sub>ξ < β</sub> ''V''<sub>ξ</sub> = ''V''<sub>β</sub>. If α = β, then ''V''<sub>α</sub> ⊆ ''V''<sub>β</sub>.
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| Note that sets enter the hierarchy only through the power set ''P''(''V''<sub>β</sub>) at step β+1. We will need the following definitions:
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| :If ''x'' is a set, '''rank'''(''x'') is the least ordinal β such that ''x'' ∈ ''V''<sub>β+1</sub>.<ref>Kunen 1980, p. 95.</ref>
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| :The '''supremum''' of a set of ordinals A, denoted by sup A, is the least ordinal β such that α ≤ β for all α ∈ A.
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| Zermelo's smallest model is ''V''<sub>ω</sub>. [[Mathematical induction|Induction]] proves that ''V''<sub>''n''</sub> is [[finite set|finite]] for all ''n'' < ω:
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| # | ''V''<sub>0</sub> | = 0.
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| # | ''V''<sub>''n''+1</sub> | = | ''V''<sub>''n''</sub> ∪ ''P''(''V''<sub>''n''</sub>) | ≤ | ''V''<sub>''n''</sub> | + 2 <sup>| ''V''<sub>''n''</sub> |</sup>, which is finite since ''V''<sub>''n''</sub> is finite by inductive hypothesis.
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| To prove Theorem 1: since a set ''X'' enters ''V''<sub>ω</sub> only through ''P''(''V''<sub>''n''</sub>) for some ''n'' < ω, we have ''X'' ⊆ ''V''<sub>''n''</sub>. Since ''V''<sub>''n''</sub> is finite, ''X'' is finite. [[Converse (logic)|Conversely]]: if a class ''X'' is finite, let ''N'' = sup {rank(''x''): ''x'' ∈ ''X''}. Since rank(''x'') ≤ ''N'' for all ''x'' ∈ ''X'', we have ''X'' ⊆ ''V''<sub>''N''+1</sub>, so ''X'' ∈ ''V''<sub>''N''+2</sub> ⊆ ''V''<sub>ω</sub>. Therefore, ''X'' ∈ ''V''<sub>ω</sub>.
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| To prove Theorem 2, note that ''V''<sub>ω</sub> is the union of [[countably many]] finite sets. Hence, ''V''<sub>ω</sub> is countably infinite and has cardinality <math>\aleph_0</math> (which equals ω by [[von Neumann cardinal assignment]]).
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| It can be shown that the sets and classes of ''V''<sub>ω</sub> satisfy all the axioms of NBG (von Neumann–Bernays–Gödel set theory) except the [[axiom of infinity]].
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| ===The models ''V''<sub>κ</sub> where κ is a strongly inaccessible cardinal===
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| To find models satisfying the axiom of infinity, observe that two properties of finiteness were used to prove Theorems 1 and 2 for ''V''<sub>ω</sub>:
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| # If λ is a finite cardinal, then 2<sup>λ</sup> is finite.
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| # If ''A'' is a set of ordinals such that | ''A'' | is finite, and α is finite for all α ∈ ''A'', then sup ''A'' is finite.
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| Replacing "finite" by "< κ" produces the properties that define [[strongly inaccessible cardinal]]s. A cardinal κ is strongly inaccessible if κ > ω and:
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| # If λ is a cardinal such that λ < κ, then 2<sup>λ</sup> < κ.
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| # If ''A'' is a set of ordinals such that | ''A'' | < κ, and α < κ for all α ∈ ''A'', then sup ''A'' < κ.
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| These properties assert that κ cannot be reached from below. The first property says κ cannot be reached by power sets; the second says κ cannot be reached by the axiom of replacement.<ref>Zermelo introduced strongly inaccessible cardinals κ so that ''V''<sub>κ</sub> would satisfy ZFC. The axioms of power set and replacement led him to the properties of strongly inaccessible cardinals. (Zermelo 1930, p. 31–35; English translation: p. 1221–1224.) Independently, [[Wacław Sierpiński|Sierpiński]] and [[Alfred Tarski|Tarski]] also introduced these cardinals in 1930.</ref> Just as the axiom of infinity is required to obtain ω, an axiom is needed to obtain strongly inaccessible cardinals. Zermelo postulated the existence of an unbounded sequence of strongly inaccessible cardinals.<ref>Zermelo used this sequence of cardinals to obtain a sequence of models that explains the paradoxes of set theory — such as, the Burali-Forti paradox and [[Russell's paradox]]. He stated that the paradoxes "depend solely on confusing ''set theory itself'' … with individual ''models'' representing it. What appears as an 'ultrafinite non- or super-set' in one model is, in the succeeding model, a perfectly good, valid set with both a cardinal number and an ordinal type, and is itself a foundation stone for the construction of a new domain [model]." (Zermelo 1930, p. 46–47; English translation: p. 1233.)</ref>
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| If κ is a strongly inaccessible cardinal, then transfinite induction proves | ''V''<sub>α</sub> | < κ for all α < κ:
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| # α = 0: | ''V''<sub>0</sub> | = 0.
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| # For α+1: | ''V''<sub>α+1</sub> | = | ''V''<sub>α</sub> ∪ ''P''(''V''<sub>α</sub>) | ≤ | ''V''<sub>α</sub> | + 2 <sup>| ''V''<sub>α</sub> |</sup> = 2 <sup>| ''V''<sub>α</sub> |</sup> < κ. Last inequality uses inductive hypothesis and κ being strongly inaccessible.
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| # For limit α: | ''V''<sub>α</sub> | = | ∪<sub>ξ < α</sub> ''V''<sub>ξ</sub> | ≤ sup {| ''V''<sub>ξ</sub> | : ξ < α} < κ. Last inequality uses inductive hypothesis and κ being strongly inaccessible.
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| To prove Theorem 1: since a set ''X'' enters ''V''<sub>κ</sub> only through ''P''(''V''<sub>α</sub>) for some α < κ, we have ''X'' ⊆ ''V''<sub>α</sub>. Since | ''V''<sub>α</sub> | < κ, we have | ''X'' | < κ. Conversely: if a class ''X'' has | ''X'' | < κ, let β = sup {rank(''x''): ''x'' ∈ ''X''}. Since κ is strongly inaccessible, | ''X'' | < κ, and rank(''x'') < κ for all ''x'' ∈ ''X'', we have β < κ. Also, rank(''x'') ≤ β for all ''x'' ∈ ''X'' implies ''X'' ⊆ ''V''<sub>β+1</sub>, so ''X'' ∈ ''V''<sub>β+2</sub> ⊆ ''V''<sub>κ</sub>. Therefore, ''X'' ∈ ''V''<sub>κ</sub>.
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| To prove Theorem 2, we compute: | ''V''<sub>κ</sub> | = | ∪<sub>α < κ</sub> ''V''<sub>α</sub> | ≤ sup {| ''V''<sub>α</sub> | : α < κ}. Let β be this supremum. Since each ordinal in the supremum is less than κ, we have β ≤ κ. Now β cannot be less than κ. If it were, there would be a cardinal λ such that β < λ < κ; for example, take λ = 2<sup> | β |</sup>. Since λ ⊆ ''V''<sub>λ</sub> and | ''V''<sub>λ</sub> | is in the supremum, we have λ ≤ | ''V''<sub>λ</sub> | ≤ β. This contradicts β < λ. Therefore, | ''V''<sub>κ</sub> | = β = κ.
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| It can be shown that the sets and classes of ''V''<sub>κ</sub> satisfy all the axioms of NBG.<ref>Zermelo proved that ZFC without the axiom of infinity is satisfied by ''V''<sub>κ</sub> for κ = ω and κ strongly inaccessible. To prove the class existence axioms of NBG (Gödel 1940, p. 5), note that ''V''<sub>κ</sub> is a set when viewed from the set theory that constructs it. Therefore, the [[axiom of specification]] produces subsets of ''V''<sub>κ</sub> that satisfy the class existence axioms.</ref>
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| ==See also==
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| *[[Axiom of global choice]]
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| *[[Limitation of size]]
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| *[[Von Neumann–Bernays–Gödel set theory]]
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| *[[Morse–Kelley set theory]]
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| == Notes ==
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| <references/> | |
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| == References ==
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| * {{Citation | last = Bernays | first = Paul | authorlink = Paul Bernays | title=Axiomatic Set Theory | publisher=Dover Publications | year=1991 | isbn=0-486-66637-9}}.
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| * William B. Easton (1964), ''Powers of Regular Cardinals'', Ph.D. thesis, Princeton University.
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| * {{Citation | last = Ferreirós | first = José | title = Labyrinth of Thought: A History of Set Theory and Its Role in Mathematical Thought | place = Basel, Switzerland | publisher = Birkhäuser | year = 2007 | edition = 2nd revised | isbn = 3-7643-8349-6}}.
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| * {{Citation | last = Fraenkel | first = Abraham | title = Zu den Grundlagen der Cantor-Zermeloschen Mengenlehre | url = http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN235181684_0086&DMDID=DMDLOG_0019 | journal = [[Mathematische Annalen]] | volume = 86 | | pages = 230–237 | year = 1922}}.
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| * {{Citation | last1 = Fraenkel | first1 = Abraham | last2 = Bar-Hillel | first2 = Yehoshua | last3 = Levy | first3 = Azriel | title = Foundations of Set Theory | place = Basel, Switzerland | publisher = Elsevier | year = 1973 | edition = 2nd revised | isbn = 0-7204-2270-1}}.
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| * {{Citation | last = Gödel | first = Kurt | authorlink = Kurt Gödel | title = The Consistency of the Continuum Hypothesis | publisher = Princeton University Press | year = 1940}}.
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| * {{Citation | last = Kanamori | first = Akihiro | authorlink = Akihiro Kanamori | chapter = Stanislaw Ulam | url = http://math.bu.edu/people/aki/9.pdf}} in: {{Citation | author = Solomon Fefermann and John W. Dawson, Jr. (editors-in-chief) | title = Kurt Gödel Collected Works, Volume V, Correspondence H-Z | publisher = Clarendon Press | pages = 280–300 | year = 2003}}.
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| *{{Citation | last = Kunen | first = Kenneth | title = [[Set Theory: An Introduction to Independence Proofs]] | publisher = North-Holland | year = 1980 | isbn = 0-444-85401-0}}.* {{Citation | last = Hallett | first = Michael | title = Cantorian Set Theory and Limitation of Size | place = Oxford | publisher = Clarendon Press | year = 1984 | isbn = 0-444-86839-9}}.
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| * {{Citation | last1=Mirimanoff | first1=Dmitry | title=Les antinomies de Russell et de Burali-Forti et le probleme fondamental de la theorie des ensembles | url=http://retro.seals.ch/digbib/view?rid=ensmat-001:1917:19::9&id=hitlist | year = 1917 | journal=L'Enseignement Mathématique | volume=19 | pages=37–52}}.
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| * {{Citation | last = Moore| first = Gregory H. | title = Zermelo's Axiom of Choice: Its Origins, Development, and Influence | publisher = Springer | year = 1982 | isbn = 0-387-90670-3}}.
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| *{{Citation | last1=Sierpiński | first1=Wacław | last2=Tarski | first2=Alfred | title=Sur une propriété caractéristique des nombres inaccessibles | url=http://matwbn.icm.edu.pl/ksiazki/fm/fm15/fm15129.pdf | year=1930 | journal=[[Fundamenta Mathematicae]] | issn=0016-2736 | volume=15 | pages=292–300}}.
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| * {{Citation | last = Skolem | first = Thoralf | chapter = Einige Bemerkungen zur axiomatischen Begründung der Mengenlehre | title = Matematikerkongressen i Helsingfors den 4-7 Juli, 1922 | pages = 217–232 | year = 1922}}. English translation: {{Citation | last=van Heijenoort | first =Jean | authorlink = Jean van Heijenoort | year =1967 | publisher = Harvard University Press | title = From Frege to Godel: A Source Book in Mathematical Logic, 1879-1931 | chapter = Some remarks on axiomatized set theory | pages = 290–301 | isbn = 978-0-674-32449-7}}.
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| * {{Citation | last = von Neumann | first = John | title = Eine Axiomatisierung der Mengenlehre | url = http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN243919689_0154&DMDID=DMDLOG_0025 | journal = [[Journal für die Reine und Angewandte Mathematik]] | volume = 154 | pages = 219–240 | year = 1925}}. English translation: {{citation|last=van Heijenoort | first =Jean | year =1967 | publisher = Harvard University Press | title = From Frege to Godel: A Source Book in Mathematical Logic, 1879-1931 | chapter = An axiomatization of set theory | pages = 393–413 | isbn = 978-0-674-32449-7}}.
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| * {{Citation | last = von Neumann | first = John | title = Die Axiomatisierung der Mengenlehre | url = http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN266833020_0027&DMDID=DMDLOG_0042 | journal = [[Mathematische Zeitschrift]] | volume = 27 | pages = 669–752 | year = 1928}}.
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| * {{Citation | last = Zermelo | first = Ernst | authorlink = Ernst Zermelo |title = Über Grenzzahlen und Mengenbereiche: neue Untersuchungen über die Grundlagen der Mengenlehre | url = http://matwbn.icm.edu.pl/ksiazki/fm/fm16/fm1615.pdf | journal = [[Fundamenta Mathematicae]] | volume = 16 | pages = 29–47| year = 1930}}. English translation: {{Citation | last = Ewald | first = William B. (ed.) | title = From Immanuel Kant to David Hilbert: A Source Book in the Foundations of Mathematics | chapter = On boundary numbers and domains of sets: new investigations in the foundations of set theory | pages = 1208–1233 | publisher = Oxford University Press | year = 1996 | isbn = 978-0-19-853271-2}}.
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| * {{Citation|first=Ernst|last= Zermelo|year=1908|title=Untersuchungen über die Grundlagen der Mengenlehre I|journal=Mathematische Annalen |volume=65|issue=2|pages= 261–281|url = http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN235181684_0065&DMDID=DMDLOG_0018}}. English translation: {{citation|last=van Heijenoort | first =Jean | year =1967 | publisher = Harvard University Press | title = From Frege to Godel: A Source Book in Mathematical Logic, 1879-1931 | chapter = Investigations in the foundations of set theory | pages = 199–215| isbn = 978-0-674-32449-7}}.
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| [[Category:Axioms of set theory]]
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| [[Category:Wellfoundedness]]
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