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| In the [[foundations of mathematics]], '''von Neumann–Bernays–Gödel set theory''' ('''NBG''') is an [[axiomatic set theory]] that is a [[conservative extension]] of the canonical axiomatic set theory [[ZFC]]. A statement in the language of ZFC is provable in NBG [[if and only if]] it is provable in ZFC. The [[ontology]] of NBG includes [[proper class]]es, objects having members but that cannot be members of other entities. NBG's principle of [[axiom of comprehension|class comprehension]] is [[impredicative|predicative]]; [[quantifier|quantified]] variables in the defining [[first order logic|formula]] can range only over sets. Allowing [[impredicative]] comprehension turns NBG into [[Morse-Kelley set theory]] (MK). NBG, unlike ZFC and MK, can be [[finite set|finite]]ly axiomatized.
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| The defining aspect of NBG is the distinction between [[proper class]] and [[Set (mathematics)|set]]. Let ''a'' and ''s'' be two individuals. Then the [[atomic sentence]] <math>a \in s</math> is defined if ''a'' is a set and ''s'' is a [[class (set theory)|class]]. In other words, <math>a \in s</math> is defined unless ''a'' is a proper class. A proper class is very large; NBG even admits of "the class of all sets", the universal class called ''V''. However, NBG does not admit "the class of all classes" (which fails because proper classes are not "objects" that can be put into classes in NBG) or "the set of all sets" (whose existence cannot be justified with NBG axioms).
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| By NBG's axiom schema of Class Comprehension, all objects satisfying any given formula in the [[first order logic|first order language]] of NBG form a class; if the class would not be a set in [[ZFC]], it is an NBG proper class.
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| The development of classes mirrors the development of [[naive set theory]]. The principle of abstraction is given, and thus classes can be formed out of all individuals satisfying any statement of [[first order logic]] whose [[atomic sentence]]s all involve either the membership relation or predicates definable from membership. Equality, pairing, subclass, and such, are all definable and so need not be axiomatized — their definitions denote a particular [[abstraction]] of a [[formula]].
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| Sets are developed in a manner very similarly to ZF. Let ''Rp''(''A,a''), meaning "the set ''a'' represents the class ''A''," denote a [[binary relation]] defined as follows:
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| :<math>\mathrm{Rp}(A,a) := \forall x(x \in A \leftrightarrow x \in a).</math>
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| That is, ''a'' "represents" ''A'' if every element of ''a'' is an element of ''A'', and conversely. Classes lacking representations, such as the class of all sets that do not contain themselves (the class invoked by the [[Russell paradox]]), are the [[proper class]]es.
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| ==History==
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| In two articles published in 1925 and 1928, [[John von Neumann]] stated his axioms and showed they were adequate to develop set theory.<ref>von Neumann 1925, von Neumann 1928.</ref> Von Neumann took functions and arguments as primitives. His functions correspond to classes, and functions that can be used as arguments correspond to sets. In fact, he defined classes and sets using functions that can take only two values (that is, [[indicator function]]s whose domain is the class of all arguments).
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| Von Neumann's work in set theory was influenced by [[Georg Cantor|Cantor]]'s articles, [[Zermelo set theory|Zermelo's 1908 axioms for set theory]], and the 1922 critiques of Zermelo's set theory that were given independently by [[Abraham Fraenkel|Fraenkel]] and [[Thoralf Skolem|Skolem]]. Both Fraenkel and Skolem pointed out that 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>. They then introduced the [[axiom of replacement]], which would guarantee the existence of such sets.<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, they were reluctant to adopt this axiom: Fraenkel's opinion was "that Replacement was too strong an axiom for 'general set theory' … and … Skolem only wrote that 'we could introduce' Replacement".<ref>Kanamori 2012, p. 62.</ref>
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| Von Neumann worked on the deficiencies in [[Zermelo set theory|Zermelo's set theory]] and introduced several innovations to remedy them, including:
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| * '''A theory of [[ordinal number|ordinal]]s.''' Zermelo's set theory does not contain Cantor's theory of ordinal numbers. Von Neumann recovered this theory by defining the ordinals using sets that are [[well-ordered]] by the ∈-relation.<ref>von Neumann 1923. Von Neumann's definition also used the theory of well-ordered sets. Later, his definition was simplified to the current one: An ordinal α is a set that is well-ordered by ∈ and has the property that every member of α is a subset of α (Kunen 1980, p. 16).</ref> In contrast to Fraenkel and Skolem, von Neumann found the axiom of replacement so essential to his work that he declared: "In fact, I believe that no theory of ordinals is possible at all without this axiom."<ref>von Neumann 1925, p. 223 (footnote); English translation: p. 398 (footnote).</ref>
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| * '''A criterion identifying classes that are too large to be sets.''' Zermelo did not provide such a criterion. His set theory avoids the large classes that lead to the [[Georg Cantor#Paradoxes of set theory|paradoxes]], but it leaves out many sets, such as the one mentioned by Fraenkel and Skolem.<ref>After introducing the [[cumulative hierarchy]], von Neumann could show that Zermelo's axioms do not prove the existence of ordinals α ≥ ω + ω, which include uncountably many [[hereditarily countable set]]s. This follows from Skolem's result that V<sub>ω+ω</sub> satisfies Zermelo's axioms (Kanamori 2012, p. 61) and from α ∈ V<sub>β</sub> implying α < β (Kunen 1980, p. 95-96; Kunen uses the notation R(β) instead of ''V''<sub>β</sub>).</ref> Von Neumann's criterion is: A class is too large to be a set if and only if it can be mapped [[onto]] the universal class. Von Neumann realized that the paradoxes can be avoided by not allowing such large classes to be members of any class. Combining this restriction with his criterion, he obtained his [[axiom of limitation of size]]: A class ''X'' is not a member of any class if and only if ''X'' can be mapped onto the universal class.<ref>Hallett, p. 288-290. Von Neumann stated his axiom in an equivalent functional form (von Neumann 1925, p. 225; English translation: p. 400).</ref> He proved that this axiom implies the axioms of replacement and [[Axiom of separation|separation]], and implies that the universal class can be well-ordered (which is equivalent to the [[axiom of global choice]]).
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| * '''Finite axiomatization.''' Fraenkel and Skolem formalized Zermelo's imprecise concept of "definite [[propositional function]]", which appears in his axiom of separation. Skolem gave the [[axiom schema of separation]] that is currently used in ZFC; Fraenkel gave an equivalent approach. Zermelo rejected both approaches "particularly because they implicitly involve the concept of natural number which, in Zermelo's view, should be based upon set theory."<ref>Fraenkel, ''Historical Introduction'' in Bernays 1991, p. 13.</ref> Von Neumann avoided [[axiom schema]]s by formalizing the concept of "definite propositional function" with his functions, whose construction requires only finitely many axioms. This led to his set theory having finitely many axioms.<ref>von Neumann 1925, p. 224–226; English translation: p. 399–401.</ref> (In 1961, [[Richard Montague|Montague]] proved that ZFC cannot be finitely axiomatized.<ref>Montague 1961.</ref>)
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| * '''The [[axiom of regularity]].''' Zermelo's set theory does not exclude non-[[well-founded]] sets.<ref>Mirimanoff defined well-founded sets in 1917 (Mirimanoff 1917, p. 41).</ref> Fraenkel and von Neumann introduced axioms to exclude these sets. Von Neumann introduced the axiom of regularity, which states that all sets are well-founded.<ref>Von Neumann also analyzed Fraenkel's axiom and stated that it was not "precisely formulated", but it would approximately say: "Besides the sets … whose existence is absolutely required by the axioms, there are no further sets." His analysis led him to reject this axiom and to propose the axiom of regularity instead (von Neumann 1925, p. 230–232 & p. 239; English translation: p. 404–405 & p. 410–411).</ref> Although von Neumann did not adopt regularity as an axiom,<ref>Kanamori 2009, p. 11. In 1930, Zermelo became the first to include regularity in an axiom system (Ferreirós 2007, p. 374).</ref> he proved its relative consistency while studying his axiom of limitation of size. First he weakened his axiom system by replacing the latter axiom with two of its consequences: replacement and a choice axiom equivalent to global choice.<ref>Von Neumann's choice axiom is: "Every relation ''C'' has a subclass which is a function and has the same domain." (Kanamori 2009, p. 7 & p. 10).</ref> Next he proved that if this weaker system is consistent, it remains consistent after adding the axiom of regularity. Finally, he showed that his weaker system augmented with regularity proves the axiom of limitation of size.<ref>von Neumann 1929; Ferreirós 2007, p. 380.</ref> These results establish that the axioms of regularity and limitation of size are relatively consistent with respect to his weaker system, and that (in the presence of regularity and his other axioms) replacement and his choice axiom are equivalent to the axiom of limitation of size.
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| In a series of articles published between 1937 and 1954, [[Paul Bernays]] modified von Neumann's theory by taking sets and classes as primitives. By using sets, Bernays was following the tradition established by Cantor, [[Richard Dedekind|Dedekind]], and Zermelo. His classes followed the tradition of [[Boolean algebra]] since they permit the operation of complement as well as union and intersection.<ref>His classes also used "some of the set-theoretic concepts of the [[Ernst Schröder|Schröder]] logic and of ''[[Principia Mathematica]]''" (quotation from Bernays in Ferreirós 2007, p. 380).</ref> Bernays handled sets and classes in a [[Many-sorted first-order logic|two-sorted logic]]. This required the introduction of two membership primitives: one for membership in sets, and one for membership in classes. With these primitives, Bernays rewrote and simplified von Neumann's axioms. He also adopted the axiom of regularity, and replaced the axiom of limitation of size with the axioms of replacement and von Neumann's choice axiom. (Von Neumann's work shows that the last two changes allow Bernays' axioms to prove the axiom of limitation of size.) <ref>Kanamori 2009, p. 6–12. Bernays' articles are reprinted in Müller 1976, p. 1–117.</ref>
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| [[Kurt Gödel]] simplified Bernays' theory by making every set a class, which allowed him to use just one sort for classes and one membership primitive. He also introduced a [[Predicate (mathematical logic)|predicate]] indicating which classes are sets. Gödel modified some of Bernays' axioms, and introduced the axiom of global choice to replace von Neumann's choice axiom. He used his axioms in his 1940 monograph on the relative consistency of global choice and the [[generalized continuum hypothesis]].<ref>Kanamori 2009, p. 14–16; Gödel 1940.</ref>
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| Several reasons have been given for Gödel choosing NBG for his 1940 monograph.<ref>Gödel used von Neumann's axioms in his 1938 announcement of his theorems, and used ZF in his 1939 outline of his proofs (Ferreirós 2007, p. 382).</ref> Gödel gave a mathematical reason—NBG's global choice produces a stronger consistency theorem: "This stronger form of the axiom [of choice], if consistent with the other axioms, implies, of course, that a weaker form is also consistent."<ref>Gödel 1940, p. 6.</ref> [[Robert Solovay]] conjectured: "My guess is that he wished to avoid a discussion of the technicalities involved in developing the rudiments of [[model theory]] within axiomatic set theory."<ref>Godel 1990, p. 13. Gödel's consistency proof builds the [[constructible universe]]. To build this in ZF requires some model theory; Gödel built it in NBG without model theory. For a discussion of Gödel's technique: see Cohen 1966, p. 99–103.</ref> [[Charles Parsons (philosopher)|Charles Parsons]] stated: "This view [that 'property of set' is a primitive of set theory] may be reflected in Gödel's choice of a theory with class variables as the framework for … [his monograph]."<ref>Godel 1990, p. 108, footnote i.</ref>
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| Gödel's achievement together with the details of his presentation led to the prominence that NBG would enjoy for the next two decades.<ref>Kanamori 2009, p. 15.</ref> Even [[Paul Cohen (mathematician)|Paul Cohen]]'s 1963 [[Independence (mathematical logic)|independence]] proofs for ZF used tools that Gödel developed for his work in NBG.<ref>Cohen 1963.</ref> However, in the 1960s, ZFC became more popular than NBG. This was caused by several factors, including the extra work required to handle [[Forcing (mathematics)|forcing]] in NBG,<ref>Kanamori 2009, p. 23: "Forcing itself went a considerable distance in downgrading any formal theory of classes because of the added encumbrance of having to specify the classes of generic extensions."</ref> Cohen's 1966 presentation of forcing (which uses techniques that naturally belong to ZF),<ref>Cohen 1966, p. 107–147. Cohen also gave a detailed proof of Gödel's relative consistency theorems using ZF (Cohen 1966, p. 85–99).</ref> and the proof that NBG is a conservative extension of ZFC.<ref>Ferreirós 2007, p. 381–382.</ref>
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| == Axiomatizating NBG ==
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| NBG is presented here as a two-sorted theory, with lower case letters denoting variables ranging over sets, and upper case letters denoting variables ranging over classes. Hence "<math>x \in y</math>" should be read "set ''x'' is a member of set ''y''," and "<math>x \in Y</math>" as "set ''x'' is a member of class ''Y''." Statements of equality may take the form
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| "<math>x=y</math>" or "<math>X=Y</math>". "<math>a=A</math>" stands for "<math>\forall x (x \in a \leftrightarrow x \in A)</math>" and is an [[abuse of notation]]. NBG can also be presented as a one-sorted theory of classes, with sets being those classes that are members of at least one other class.
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| We first axiomatize NBG using the {{visible anchor|axiom schema of Class Comprehension}}. This schema is provably equivalent<ref>Mendelson (1997), p. 232, Prop. 4.4, proves Class Comprehension equivalent to the axioms '''B1-B7''' shown on p. 230 and described below.</ref> to 9 of its finite instances, stated in the following section. Hence these 9 finite axioms can replace Class Comprehension. This is the precise sense in which NBG can be finitely axiomatized.
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| === With Class Comprehension schema ===
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| The following five axioms are identical to their [[ZFC]] counterparts:
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| *'''[[axiom of extension|extensionality]]:''' <math>\forall a \forall b [ \forall x (x \in a \leftrightarrow x \in b)\rightarrow a = b ] \,.</math> Sets with the same elements are the same set.
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| *'''[[axiom of pairing|pairing]]:''' <math>\forall x \forall y \exists z \forall w [w \in z \leftrightarrow (w = x \or w = y)].</math> For any sets ''x'' and ''y'', there is a set, <math>\{x,y\}</math>, whose elements are exactly ''x'' and ''y''.
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| :''pairing'' implies that for any set ''x'', the set {''x''} (the [[singleton set]]) exists. Also, given any two sets ''x'' and ''y'' and the usual set-theoretic definition of the [[ordered pair]], the ordered pair (''x,y'') exists and is a set. By Class Comprehension, all relations on sets are classes. Moreover, certain kinds of class relations are one or more of functions, injections, and bijections from one class to another. ''pairing'' is an axiom in [[Zermelo set theory]] and a theorem in [[ZFC]].
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| *'''[[axiom of union|union]]:''' For any set ''x'', there is a set which contains exactly the elements of elements of ''x''.
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| *'''[[axiom of power set|power set]]:''' For any set ''x'', there is a set which contains exactly the subsets of ''x''.
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| *'''[[axiom of infinity|infinity]]:''' There exists an inductive set, namely a set ''x'' whose members are (i) the [[empty set]]; (ii) for every member ''y'' of ''x'', <math>y \cup \{y\}</math> is also a member of ''x''.
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| :''infinity'' can be formulated so as to imply the existence of the empty set.<ref>Mendelson (1997), p. 239, Ex. 4.22(b).</ref>
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| The remaining axioms have capitalized names because they are primarily concerned with classes rather than sets. The next two axioms differ from their [[ZFC]] counterparts only in that their quantified variables range over classes, not sets:
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| *'''Extensionality:''' <math>\forall x (x \in A \leftrightarrow x \in B)\rightarrow A=B.</math>: Classes with the same elements are the same class.
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| *'''Foundation (Regularity):''' Each nonempty class is disjoint from one of its elements.
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| The last two axioms are peculiar to NBG:
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| *'''[[axiom of limitation of size|Limitation of Size]]:''' For any class ''C'', a set ''x'' such that ''x=C'' exists if and only if there is no [[bijection]] between ''C'' and the class ''V'' of all sets.
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| :From this axiom, due to [[Von Neumann]], [[axiom schema of separation|Subsets]], [[axiom schema of replacement|Replacement]], and [[axiom of global choice|Global Choice]] can all be derived. This axiom implies the [[axiom of global choice]] because the class of [[ordinal number|ordinals]] is not a set; hence there exists a [[bijection]] between the ordinals and the [[von Neumann universe|universe]]. If Limitation of Size were weakened to "If the domain of a class function is a set, then the range of that function is likewise a set," then no form of the [[axiom of choice]] is an NBG theorem. In this case, any of the usual local forms of Choice may be taken as an added axiom, if desired.
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| :Limitation of Size cannot be found in Mendelson (1997) NBG. In its place, we find the usual [[axiom of choice]] for sets, and the following form of the [[axiom schema of replacement]]: if the class ''F'' is a [[function (set theory)|function]] whose [[domain (mathematics)|domain]] is a set, the [[range (mathematics)|range]] of ''F'' is also a set .<ref>Mendelson (1997), p. 239, axiom R.</ref>
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| *'''Class Comprehension schema:''' For any formula <math>\phi</math> containing no quantifiers over classes (it may contain class and set parameters), there is a class ''A'' such that <math>\forall x (x \in A \leftrightarrow \phi(x)).</math>
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| :This axiom asserts that invoking the principle of [[unrestricted comprehension]] of [[naive set theory]] yields a class rather than a set, thereby banishing the [[paradoxes of set theory]].
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| :Class Comprehension is the only axiom schema of NBG. In the next section, we show how this schema can be replaced by a number of its own instances. Hence NBG can be finitely axiomatized. If the quantified variables in φ(''x'') range over classes instead of sets, the result is [[Morse–Kelley set theory]], a [[proper extension]] of ZFC which cannot be finitely axiomatized.
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| === Replacing Class Comprehension with finite instances thereof ===
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| An appealing but somewhat mysterious feature of NBG is that its axiom schema of Class Comprehension is equivalent to the conjunction of a finite number of its instances. The axioms of this section may replace the Axiom Schema of Class Comprehension in the preceding section. The finite axiomatization presented below does not necessarily resemble exactly any NBG axiomatization in print.
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| We develop our axiomatization by considering the structure of formulas.
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| *'''Sets:''' For any set ''x'', there is a class ''X'' such that ''x=X''.
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| This axiom, in combination with the set existence axioms from the previous axiomatization, assures the existence of classes from the outset, and enables formulas with class parameters.
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| Let <math>A=\{x \mid \phi\}</math> and <math>B=\{x \mid \psi\}.</math> Then <math>\{x \mid \neg\phi\} = V-A</math> and <math>\{x \mid \phi\wedge \psi\} = A \cap B</math> suffice for handling all [[sentential connective]]s, because ∧ and ¬ are a [[functional completeness|functionally complete]] set of connectives.
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| *'''Complement:''' For any class ''A'', the [[complement (set theory)|complement]] <math>V-A = \{x \mid x \not\in A\}</math> is a class.
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| *'''Intersection:''' For any classes ''A'' and ''B'', the [[Intersection (set theory)|intersection]] <math>A \cap B = \{x \mid x \in A \wedge x \in B\}</math> is a class.
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| We now turn to quantification. In order to handle multiple variables, we need the ability to represent [[Relation (mathematics)|relations]]. Define the [[ordered pair]] <math>(a,b)</math> as <math>\{\{a\},\{a,b\}\},</math> as usual. Note that three applications of '''pairing''' to ''a'' and ''b'' assure that (''a'',''b'') is indeed a set.
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| *'''Products:''' For any classes ''A'' and ''B'', the class <math>A \times B = \{(a,b) \mid a \in A \wedge b \in B\}</math> is a class. (In practice, only <math>V \times A</math> is needed.)
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| *'''Converses:''' For any class ''R'', the classes:
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| : <math>\mathit{Conv}1(R) = \{(b,a)\mid (a,b)\in R\}</math> and
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| : <math>\mathit{Conv}2(R) = \{(b,(a,c)) \mid (a,(b,c)) \in R\}</math> exist.
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| *'''Association:''' For any class ''R'', the classes:
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| : <math>\mathit{Assoc}1(R) = \{((a,b),c) \mid (a,(b,c)) \in R\}</math> and
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| : <math>\mathit{Assoc}2(R) = \{(d,(a,(b,c))) \mid (d,((a,b),c)) \in R\}</math> exist.
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| These axioms license adding dummy arguments, and rearranging the order of arguments, in relations of any [[arity]]. The peculiar form of ''Association'' is designed exactly to make it possible to bring any term in a list of arguments to the front (with the help of ''Converses''). We represent the argument list <math>(x_1,x_2,\ldots,x_n)</math> as <math>(x_1,(x_2,\ldots,x_n))</math> (it is a pair with the first argument as its first projection and the "tail" of the argument list as the second projection). The idea is to apply ''Assoc1'' until the argument to be brought to the front is second, then apply ''Conv1'' or ''Conv2'' as appropriate to bring the second argument to the front, then apply ''Assoc2'' until the effects of the original applications of ''Assoc1'' (which are now behind the moved argument) are corrected.
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| If <math>\{(x,y)\mid \phi(x,y)\}</math> is a class considered as a relation, then its [[range (mathematics)|range]], <math>\{y \mid \exists x[\phi(x,y)]\} ,</math> is a class. This gives us the [[existential quantifier]]. The [[universal quantifier]] can be defined in terms of the existential quantifier and [[negation]]. | |
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| *'''Ranges:''' For any class ''R'', the class <math>\mathit{Rng}(R) = \{y \mid \exists x[(x,y)\in R]\}</math> exists.
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| The above axioms can reorder the arguments of any relation so as to bring any desired argument to the front of the argument list, where it can be quantified.
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| Finally, each [[atomic formula]] implies the existence of a corresponding class relation:
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| *'''Membership:''' The class <math>[\in] = \{(x,y) \mid x\in y\}</math> exists.
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| *'''Diagonal:''' The class <math>[=] = \{(x,y) \mid \,x=y\}</math> exists.
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| ''Diagonal'', together with addition of dummy arguments and rearrangement of arguments, can build a relation asserting the equality of any two of its arguments; thus repeated variables can be handled.
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| ====Mendelson's variant====
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| Mendelson (1997: 230) refers to his axioms '''B1-B7''' of class comprehension as "axioms of class existence." Four of these identical to axioms already stated above: '''B1''' is Membership; '''B2''', Intersection; '''B3''', Complement; '''B5''', Product. '''B4''' is Ranges modified to assert the existence of the domain of ''R'' (by existentially quantifying ''y'' instead of ''x''). The last two axioms are:
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| :'''B6:''' <math> \forall X \exist Y \forall uvw[(u,v,w) \in Y \leftrightarrow (v,w,u) \in X],</math>
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| :'''B7:''' <math> \forall X \exist Y \forall uvw[(u,v,w) \in Y \leftrightarrow (u,w,v) \in X].</math>
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| '''B6''' and '''B7''' enable what Converses and Association enable: given any class ''X'' of ordered triples, there exists another class ''Y'' whose members are the members of ''X'' each reordered in the same way.
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| ==Discussion==
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| For a discussion of some [[ontology|ontological]] and other philosophical issues posed by NBG, especially when contrasted with [[ZFC]] and [[Morse–Kelley set theory|MK]], see Appendix C of Potter (2004).
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| Even though NBG is a [[conservative extension]] of ZFC, a theorem may have a shorter and more elegant proof in NBG than in ZFC (or vice versa). For a survey of known results of this nature, see Pudlak (1998).
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| ===Model theory===
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| ZFC, NBG, and MK have models describable in terms of ''V'', the [[inner model|standard model]] of [[ZFC]] and the [[von Neumann universe]]. Now let the members of ''V'' include the [[inaccessible cardinal]] κ. Also let Def(''X'') denote the Δ<sub>0</sub> definable [[subset]]s of ''X'' (see [[constructible universe]]). Then:
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| *''V''<sub>κ</sub> is an [[intended interpretation|intended model]] of [[ZFC]];
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| * Def(''V''<sub>κ</sub>) is an intended model of NBG;
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| *''V''<sub>κ+1</sub> is an intended model of [[Morse–Kelley set theory|MK]].
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| ===Category theory===
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| The ontology of NBG provides scaffolding for speaking about "large objects" without risking paradox. In some developments of [[category theory]], for instance, a "large category" is defined as one whose [[object (category theory)|object]]s make up a proper class, with the same being true of its [[morphism]]s. A "small category", on the other hand, is one whose objects and morphisms are members of some set. We can thus easily speak of the "[[category of all sets]]" or "[[category of all small categories]]" without risking paradox. Those categories are large, of course. There is no "category of all categories" since it would have to contain large categories which no category can do. Although yet another ontological extension can enable one to talk formally about such a "category" (see for example the "quasicategory of all categories" of Adámek et al. (1990), whose objects and morphisms form a "proper conglomerate").
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| On whether an ontology including classes as well as sets is adequate for [[category theory]], see Muller (2001).
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| == See also ==
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| *[[Impredicativity|Predicativity]]
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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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| * {{cite book |last=Adámek |first=Jiří |coauthors=Herrlich, Horst, and Strecker, George E |title=Abstract and Concrete Categories (The Joy of Cats) |origyear=1990 |url=http://katmat.math.uni-bremen.de/acc/ |format=PDF |year=2004 |publisher= Wiley & Sons |location=New York |isbn=0-471-60922-6 }}
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| * {{cite book | author=Bernays, Paul | title=Axiomatic Set Theory | publisher=Dover Publications | year=1991 | isbn=0-486-66637-9}}
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| * {{Citation | last = Cohen | first = Paul | title = The Independence of the Continuum Hypothesis | url = http://dx.doi.org/10.1073/pnas.50.6.1143 | journal = [[Proceedings of the National Academy of Sciences of the United States of America]] | volume = 51 | pages = 1143–1148| year = 1963}}.
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| * {{Citation | last = Cohen | first = Paul | title = Set Theory and the Continuum Hypothesis | publisher = W. A. Benjamin | year = 1966}}.
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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 = Gödel | first = Kurt | title = The Consistency of the Continuum Hypothesis | publisher = Princeton University Press | year = 1940}}.
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| * {{Citation | last = Gödel | first = Kurt | title = Collected Works, Volume 2 | publisher = Oxford University Press | year = 1990}}.
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| * {{Citation | last = Hallett | first = Michael | title = Cantorian Set Theory and Limitation of Size | place = Oxford | publisher = Clarendon Press | year = 1984}}.
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| * {{Citation | last = Kanamori | first = Akihiro | authorlink = Akihiro Kanamori | title = Bernays and Set Theory | url = http://math.bu.edu/people/aki/17.pdf | journal = [[Bulletin of Symbolic Logic]] | volume = 15 | pages = 43–69 | year = 2009}}. (Page numbering in [[#Notes|Notes]] refers to online article whose numbering starts at 1.)
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| * {{Citation | last = Kanamori | first = Akihiro | title = In Praise of Replacement | url = http://math.bu.edu/people/aki/20.pdf | journal = [[Bulletin of Symbolic Logic]] | volume = 18 | pages = 46–90 | year = 2012}}.
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| * {{Citation | last = Kunen | first = Kenneth | authorlink = Kenneth Kunen | title = [[Set Theory: An Introduction to Independence Proofs]] | publisher = North-Holland | year = 1980 | isbn = 0-444-85401-0}}.
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| * Mendelson, Elliott, (1997), ''An Introduction to Mathematical Logic'', 4th ed. London: Chapman & Hall. ISBN 0-412-80830-7. Pp. 225–86 contain the classic textbook treatment of NBG, showing how it does what we expect of set theory, by grounding [[relation (mathematics)|relations]], [[order theory]], [[ordinal number]]s, [[transfinite number]]s, etc.
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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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| * [[Richard Montague]], (1961), "Semantic Closure and Non-Finite Axiomatizability I," in ''Infinitistic Methods: Proceedings of the Symposium on Foundations of Mathematics'', (Warsaw, 2–9 September 1959). Pergamon: 45-69.
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| * Muller, F. A., (2001), "Sets, classes, and categories," ''British Journal of the Philosophy of Science'' 52: 539-73.
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| * {{Citation | editor-last = Müller | editor-first = Gurt | title = Sets and Classes: On the Work of Paul Bernays | location = Amsterdam | publisher = North Holland | year = 1976}}.
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| * Potter, Michael, (2004), ''Set Theory and Its Philosophy''. Oxford Univ. Press.
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| * Pudlak, P., (1998), "The lengths of proofs" in Buss, S., ed., ''Handbook of Proof Theory''. North-Holland: 547-637.
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| * {{Citation | last = von Neumann | first = John | title = Zur Einführung der transfiniten Zahlen | url = http://bbi-math.narod.ru/newmann/newmann.html | journal = Acta litt. Acad. Sc. Szeged X. | volume = 1 | pages = 199–208| year = 1923}}. 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 = On the introduction of transfinite numbers | pages = 346–354}}.
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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}}.
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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 = von Neumann | first = John | title = Über eine Widerspruchsfreiheitsfrage in der axiomatischen Mengenlehre | url = http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN243919689_0160&DMDID=DMDLOG_0019 | journal = Journal für die Reine und Angewandte Mathematik | volume = 160 | pages = 227–241| year = 1929}}.
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| ==External links==
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| *{{planetmathref|id=4395|title= von Neumann-Bernays-Gödel set theory}}
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| {{Use dmy dates|date=August 2011}}
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| *{{MathWorld |title=von Neumann-Bernays-Gödel Set Theory |id=vonNeumann-Bernays-GoedelSetTheory |author=[[Matthew Szudzik|Szudzik, Matthew]] }}
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| {{Set theory}}
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| {{DEFAULTSORT:Von Neumann-Bernays-Godel set theory}}
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| [[Category:Systems of set theory]]
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