**Set theory** is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly concerned with those that are relevant to mathematics as a whole.

- History
- Basic concepts and notation
- Some ontology
- Formalized set theory
- Applications
- Areas of study
- Combinatorial set theory
- Descriptive set theory
- Fuzzy set theory
- Inner model theory
- Large cardinals
- Determinacy
- Forcing
- Cardinal invariants
- Set-theoretic topology
- Objections to set theory
- Set theory in mathematical education
- See also
- Notes
- References
- Further reading
- External links

The modern study of set theory was initiated by the German mathematicians Richard Dedekind and Georg Cantor in the 1870s. In particular, Georg Cantor is commonly considered the founder of set theory. The non-formalized systems investigated during this early stage go under the name of * naive set theory *. After the discovery of paradoxes within naive set theory (such as Russell's paradox, Cantor's paradox and Burali-Forti paradox) various axiomatic systems were proposed in the early twentieth century, of which Zermelo–Fraenkel set theory (with or without the axiom of choice) is still the best-known and most studied.

Set theory is commonly employed as a foundational system for the whole of mathematics, particularly in the form of Zermelo–Fraenkel set theory with the axiom of choice.^{ [1] } Beside its foundational role, set theory also provides the framework to develop a mathematical theory of infinity, and has various applications in computer science, philosophy and formal semantics ^{[ disambiguation needed ]}. Its foundational appeal, together with its paradoxes, its implications for the concept of infinity and its multiple applications, have made set theory an area of major interest for logicians and philosophers of mathematics. Contemporary research into set theory covers a vast array of topics, ranging from the structure of the real number line to the study of the consistency of large cardinals.

Mathematical topics typically emerge and evolve through interactions among many researchers. Set theory, however, was founded by a single paper in 1874 by Georg Cantor: "On a Property of the Collection of All Real Algebraic Numbers".^{ [2] }^{ [3] }

Since the 5th century BC, beginning with Greek mathematician Zeno of Elea in the West and early Indian mathematicians in the East, mathematicians had struggled with the concept of infinity. Especially notable is the work of Bernard Bolzano in the first half of the 19th century.^{ [4] } Modern understanding of infinity began in 1870–1874, and was motivated by Cantor's work in real analysis.^{ [5] } An 1872 meeting between Cantor and Richard Dedekind influenced Cantor's thinking, and culminated in Cantor's 1874 paper.

Cantor's work initially polarized the mathematicians of his day. While Karl Weierstrass and Dedekind supported Cantor, Leopold Kronecker, now seen as a founder of mathematical constructivism, did not. Cantorian set theory eventually became widespread, due to the utility of Cantorian concepts, such as one-to-one correspondence among sets, his proof that there are more real numbers than integers, and the "infinity of infinities" ("Cantor's paradise") resulting from the power set operation. This utility of set theory led to the article "Mengenlehre", contributed in 1898 by Arthur Schoenflies to Klein's encyclopedia.

The next wave of excitement in set theory came around 1900, when it was discovered that some interpretations of Cantorian set theory gave rise to several contradictions, called antinomies or paradoxes. Bertrand Russell and Ernst Zermelo independently found the simplest and best known paradox, now called Russell's paradox: consider "the set of all sets that are not members of themselves", which leads to a contradiction since it must be a member of itself and not a member of itself. In 1899, Cantor had himself posed the question "What is the cardinal number of the set of all sets?", and obtained a related paradox. Russell used his paradox as a theme in his 1903 review of continental mathematics in his * The Principles of Mathematics *.

In 1906, English readers gained the book *Theory of Sets of Points*^{ [6] } by husband and wife William Henry Young and Grace Chisholm Young, published by Cambridge University Press.

The momentum of set theory was such that debate on the paradoxes did not lead to its abandonment. The work of Zermelo in 1908 and the work of Abraham Fraenkel and Thoralf Skolem in 1922 resulted in the set of axioms ZFC, which became the most commonly used set of axioms for set theory. The work of analysts, such as that of Henri Lebesgue, demonstrated the great mathematical utility of set theory, which has since become woven into the fabric of modern mathematics. Set theory is commonly used as a foundational system, although in some areas—such as algebraic geometry and algebraic topology—category theory is thought to be a preferred foundation.

Set theory begins with a fundamental binary relation between an object *o* and a set *A*. If *o* is a * member * (or *element*) of *A*, the notation *o* ∈ *A* is used.^{ [7] } A set is described by listing elements separated by commas, or by a characterizing property of its elements, within braces { }.^{ [8] } Since sets are objects, the membership relation can relate sets as well.

A derived binary relation between two sets is the subset relation, also called *set inclusion*. If all the members of set *A* are also members of set *B*, then *A* is a * subset * of *B*, denoted *A* ⊆ *B*.^{ [7] } For example, {1, 2} is a subset of {1, 2, 3}, and so is {2} but {1, 4} is not. As implied by this definition, a set is a subset of itself. For cases where this possibility is unsuitable or would make sense to be rejected, the term * proper subset * is defined. *A* is called a *proper subset* of *B* if and only if *A* is a subset of *B*, but *A* is not equal to *B*. Also, 1, 2, and 3 are members (elements) of the set {1, 2, 3}, but are not subsets of it; and in turn, the subsets, such as {1}, are not members of the set {1, 2, 3}.

Just as arithmetic features binary operations on numbers, set theory features binary operations on sets.^{ [9] } The following is a partial list of them:

*Union*of the sets*A*and*B*, denoted*A*∪*B*,^{ [7] }is the set of all objects that are a member of*A*, or*B*, or both.^{ [10] }For example, the union of {1, 2, 3} and {2, 3, 4} is the set {1, 2, 3, 4}.*Intersection*of the sets*A*and*B*, denoted*A*∩*B*,^{ [7] }is the set of all objects that are members of both*A*and*B*. For example, the intersection of {1, 2, 3} and {2, 3, 4} is the set {2, 3}.*Set difference*of*U*and*A*, denoted*U*\*A*, is the set of all members of*U*that are not members of*A*. The set difference {1, 2, 3} \ {2, 3, 4} is {1}, while conversely, the set difference {2, 3, 4} \ {1, 2, 3} is {4}. When*A*is a subset of*U*, the set difference*U*\*A*is also called the*complement*of*A*in*U*. In this case, if the choice of*U*is clear from the context, the notation*A*^{c}is sometimes used instead of*U*\*A*, particularly if*U*is a universal set as in the study of Venn diagrams.*Symmetric difference*of sets*A*and*B*, denoted*A*△*B*or*A*⊖*B*,^{ [7] }is the set of all objects that are a member of exactly one of*A*and*B*(elements which are in one of the sets, but not in both). For instance, for the sets {1, 2, 3} and {2, 3, 4}, the symmetric difference set is {1, 4}. It is the set difference of the union and the intersection, (*A*∪*B*) \ (*A*∩*B*) or (*A*\*B*) ∪ (*B*\*A*).*Cartesian product*of*A*and*B*, denoted*A*×*B*,^{ [7] }is the set whose members are all possible ordered pairs (*a*,*b*), where*a*is a member of*A*and*b*is a member of*B*. For example, the Cartesian product of {1, 2} and {red, white} is {(1, red), (1, white), (2, red), (2, white)}.*Power set*of a set*A*, denoted ,^{ [7] }is the set whose members are all of the possible subsets of*A*. For example, the power set of {1, 2} is { {}, {1}, {2}, {1, 2} }.

Some basic sets of central importance are the set of natural numbers, the set of real numbers and the empty set—the unique set containing no elements. The empty set is also occasionally called the *null set*,^{ [11] } though this name is ambiguous and can lead to several interpretations.

A set is pure if all of its members are sets, all members of its members are sets, and so on. For example, the set {{}} containing only the empty set is a nonempty pure set. In modern set theory, it is common to restrict attention to the * von Neumann universe * of pure sets, and many systems of axiomatic set theory are designed to axiomatize the pure sets only. There are many technical advantages to this restriction, and little generality is lost, because essentially all mathematical concepts can be modeled by pure sets. Sets in the von Neumann universe are organized into a cumulative hierarchy, based on how deeply their members, members of members, etc. are nested. Each set in this hierarchy is assigned (by transfinite recursion) an ordinal number , known as its *rank.* The rank of a pure set is defined to be the least ordinal that is strictly greater than the rank of any of its elements. For example, the empty set is assigned rank 0, while the set {{}} containing only the empty set is assigned rank 1. For each ordinal , the set is defined to consist of all pure sets with rank less than . The entire von Neumann universe is denoted .

Elementary set theory can be studied informally and intuitively, and so can be taught in primary schools using Venn diagrams. The intuitive approach tacitly assumes that a set may be formed from the class of all objects satisfying any particular defining condition. This assumption gives rise to paradoxes, the simplest and best known of which are Russell's paradox and the Burali-Forti paradox. **Axiomatic set theory** was originally devised to rid set theory of such paradoxes.^{ [note 1] }

The most widely studied systems of axiomatic set theory imply that all sets form a cumulative hierarchy. Such systems come in two flavors, those whose ontology consists of:

*Sets alone*. This includes the most common axiomatic set theory,**Z**ermelo–**F**raenkel set theory with the Axiom of**C**hoice (ZFC). Fragments of**ZFC**include:- Zermelo set theory, which replaces the axiom schema of replacement with that of separation;
- General set theory, a small fragment of Zermelo set theory sufficient for the Peano axioms and finite sets;
- Kripke–Platek set theory, which omits the axioms of infinity, powerset, and choice, and weakens the axiom schemata of separation and replacement.

*Sets and proper classes*. These include Von Neumann–Bernays–Gödel set theory, which has the same strength as ZFC for theorems about sets alone, and Morse–Kelley set theory and Tarski–Grothendieck set theory, both of which are stronger than ZFC.

The above systems can be modified to allow * urelements *, objects that can be members of sets but that are not themselves sets and do not have any members.

The * New Foundations * systems of **NFU** (allowing urelements) and **NF** (lacking them) are not based on a cumulative hierarchy. NF and NFU include a "set of everything", relative to which every set has a complement. In these systems urelements matter, because NF, but not NFU, produces sets for which the axiom of choice does not hold.

Systems of constructive set theory, such as CST, CZF, and IZF, embed their set axioms in intuitionistic instead of classical logic. Yet other systems accept classical logic but feature a nonstandard membership relation. These include rough set theory and fuzzy set theory, in which the value of an atomic formula embodying the membership relation is not simply **True** or **False**. The Boolean-valued models of ZFC are a related subject.

An enrichment of ZFC called internal set theory was proposed by Edward Nelson in 1977.^{[ citation needed ]}

Many mathematical concepts can be defined precisely using only set theoretic concepts. For example, mathematical structures as diverse as graphs, manifolds, rings, and vector spaces can all be defined as sets satisfying various (axiomatic) properties. Equivalence and order relations are ubiquitous in mathematics, and the theory of mathematical relations can be described in set theory. ^{[ citation needed ]}

Set theory is also a promising foundational system for much of mathematics. Since the publication of the first volume of * Principia Mathematica *, it has been claimed that most (or even all) mathematical theorems can be derived using an aptly designed set of axioms for set theory, augmented with many definitions, using first or second-order logic. For example, properties of the natural and real numbers can be derived within set theory, as each number system can be identified with a set of equivalence classes under a suitable equivalence relation whose field is some infinite set.^{[ citation needed ]}

Set theory as a foundation for mathematical analysis, topology, abstract algebra, and discrete mathematics is likewise uncontroversial; mathematicians accept (in principle) that theorems in these areas can be derived from the relevant definitions and the axioms of set theory. However, it remains that few full derivations of complex mathematical theorems from set theory have been formally verified, since such formal derivations are often much longer than the natural language proofs mathematicians commonly present. One verification project, Metamath, includes human-written, computer-verified derivations of more than 12,000 theorems starting from ZFC set theory, first-order logic and propositional logic.^{[ citation needed ]}

Set theory is a major area of research in mathematics, with many interrelated subfields.

*Combinatorial set theory* concerns extensions of finite combinatorics to infinite sets. This includes the study of cardinal arithmetic and the study of extensions of Ramsey's theorem such as the Erdős–Rado theorem.

*Descriptive set theory* is the study of subsets of the real line and, more generally, subsets of Polish spaces. It begins with the study of pointclasses in the Borel hierarchy and extends to the study of more complex hierarchies such as the projective hierarchy and the Wadge hierarchy. Many properties of Borel sets can be established in ZFC, but proving these properties hold for more complicated sets requires additional axioms related to determinacy and large cardinals.

The field of effective descriptive set theory is between set theory and recursion theory. It includes the study of lightface pointclasses, and is closely related to hyperarithmetical theory. In many cases, results of classical descriptive set theory have effective versions; in some cases, new results are obtained by proving the effective version first and then extending ("relativizing") it to make it more broadly applicable.

A recent area of research concerns Borel equivalence relations and more complicated definable equivalence relations. This has important applications to the study of invariants in many fields of mathematics.

In set theory as Cantor defined and Zermelo and Fraenkel axiomatized, an object is either a member of a set or not. In * fuzzy set theory * this condition was relaxed by Lotfi A. Zadeh so an object has a *degree of membership* in a set, a number between 0 and 1. For example, the degree of membership of a person in the set of "tall people" is more flexible than a simple yes or no answer and can be a real number such as 0.75.

An *inner model* of Zermelo–Fraenkel set theory (ZF) is a transitive class that includes all the ordinals and satisfies all the axioms of ZF. The canonical example is the constructible universe *L* developed by Gödel. One reason that the study of inner models is of interest is that it can be used to prove consistency results. For example, it can be shown that regardless of whether a model *V* of ZF satisfies the continuum hypothesis or the axiom of choice, the inner model *L* constructed inside the original model will satisfy both the generalized continuum hypothesis and the axiom of choice. Thus the assumption that ZF is consistent (has at least one model) implies that ZF together with these two principles is consistent.

The study of inner models is common in the study of determinacy and large cardinals, especially when considering axioms such as the axiom of determinacy that contradict the axiom of choice. Even if a fixed model of set theory satisfies the axiom of choice, it is possible for an inner model to fail to satisfy the axiom of choice. For example, the existence of sufficiently large cardinals implies that there is an inner model satisfying the axiom of determinacy (and thus not satisfying the axiom of choice).^{ [12] }

A *large cardinal* is a cardinal number with an extra property. Many such properties are studied, including inaccessible cardinals, measurable cardinals, and many more. These properties typically imply the cardinal number must be very large, with the existence of a cardinal with the specified property unprovable in Zermelo–Fraenkel set theory.

*Determinacy* refers to the fact that, under appropriate assumptions, certain two-player games of perfect information are determined from the start in the sense that one player must have a winning strategy. The existence of these strategies has important consequences in descriptive set theory, as the assumption that a broader class of games is determined often implies that a broader class of sets will have a topological property. The axiom of determinacy (AD) is an important object of study; although incompatible with the axiom of choice, AD implies that all subsets of the real line are well behaved (in particular, measurable and with the perfect set property). AD can be used to prove that the Wadge degrees have an elegant structure.

Paul Cohen invented the method of * forcing * while searching for a model of ZFC in which the continuum hypothesis fails, or a model of ZF in which the axiom of choice fails. Forcing adjoins to some given model of set theory additional sets in order to create a larger model with properties determined (i.e. "forced") by the construction and the original model. For example, Cohen's construction adjoins additional subsets of the natural numbers without changing any of the cardinal numbers of the original model. Forcing is also one of two methods for proving relative consistency by finitistic methods, the other method being Boolean-valued models.

A *cardinal invariant* is a property of the real line measured by a cardinal number. For example, a well-studied invariant is the smallest cardinality of a collection of meagre sets of reals whose union is the entire real line. These are invariants in the sense that any two isomorphic models of set theory must give the same cardinal for each invariant. Many cardinal invariants have been studied, and the relationships between them are often complex and related to axioms of set theory.

*Set-theoretic topology* studies questions of general topology that are set-theoretic in nature or that require advanced methods of set theory for their solution. Many of these theorems are independent of ZFC, requiring stronger axioms for their proof. A famous problem is the normal Moore space question, a question in general topology that was the subject of intense research. The answer to the normal Moore space question was eventually proved to be independent of ZFC.

From set theory's inception, some mathematicians have objected to it as a foundation for mathematics. The most common objection to set theory, one Kronecker voiced in set theory's earliest years, starts from the constructivist view that mathematics is loosely related to computation. If this view is granted, then the treatment of infinite sets, both in naive and in axiomatic set theory, introduces into mathematics methods and objects that are not computable even in principle. The feasibility of constructivism as a substitute foundation for mathematics was greatly increased by Errett Bishop's influential book *Foundations of Constructive Analysis*.^{ [13] }

A different objection put forth by Henri Poincaré is that defining sets using the axiom schemas of specification and replacement, as well as the axiom of power set, introduces impredicativity, a type of circularity, into the definitions of mathematical objects. The scope of predicatively founded mathematics, while less than that of the commonly accepted Zermelo–Fraenkel theory, is much greater than that of constructive mathematics, to the point that Solomon Feferman has said that "all of scientifically applicable analysis can be developed [using predicative methods]".^{ [14] }

Ludwig Wittgenstein condemned set theory philosophically for its connotations of Mathematical platonism.^{ [15] } He wrote that "set theory is wrong", since it builds on the "nonsense" of fictitious symbolism, has "pernicious idioms", and that it is nonsensical to talk about "all numbers".^{ [16] } Wittgenstein identified mathematics with algorithmic human deduction;^{ [17] } the need for a secure foundation for mathematics seemed, to him, nonsensical.^{ [18] } Moreover, since human effort is necessarily finite, Wittgenstein's philosophy required an ontological commitment to radical constructivism and finitism. Meta-mathematical statements — which, for Wittgenstein, included any statement quantifying over infinite domains, and thus almost all modern set theory — are not mathematics.^{ [19] } Few modern philosophers have adopted Wittgenstein's views after a spectacular blunder in * Remarks on the Foundations of Mathematics *: Wittgenstein attempted to refute Gödel's incompleteness theorems after having only read the abstract. As reviewers Kreisel, Bernays, Dummett, and Goodstein all pointed out, many of his critiques did not apply to the paper in full. Only recently have philosophers such as Crispin Wright begun to rehabilitate Wittgenstein's arguments.^{ [20] }

Category theorists have proposed topos theory as an alternative to traditional axiomatic set theory. Topos theory can interpret various alternatives to that theory, such as constructivism, finite set theory, and computable set theory.^{ [21] }^{ [22] } Topoi also give a natural setting for forcing and discussions of the independence of choice from ZF, as well as providing the framework for pointless topology and Stone spaces.^{ [23] }

An active area of research is the univalent foundations and related to it homotopy type theory. Within homotopy type theory, a set may be regarded as a homotopy 0-type, with universal properties of sets arising from the inductive and recursive properties of higher inductive types. Principles such as the axiom of choice and the law of the excluded middle can be formulated in a manner corresponding to the classical formulation in set theory or perhaps in a spectrum of distinct ways unique to type theory. Some of these principles may be proven to be a consequence of other principles. The variety of formulations of these axiomatic principles allows for a detailed analysis of the formulations required in order to derive various mathematical results.^{ [24] }^{ [25] }

As set theory gained popularity as a foundation for modern mathematics, there has been support for the idea of introducing the basics of naive set theory early in mathematics education.

In the US in the 1960s, the New Math experiment aimed to teach basic set theory, among other abstract concepts, to primary school students, but was met with much criticism. The math syllabus in European schools followed this trend, and currently includes the subject at different levels in all grades. Venn diagrams are widely employed to explain basic set-theoretic relationships to primary school students (even though John Venn originally devised them as part of a procedure to assess the validity of inferences in term logic).

Set theory is used to introduce students to logical operators (NOT, AND, OR), and semantic or rule description (technically intensional definition ^{ [26] }) of sets (e.g. "months starting with the letter *A*"), which may be useful when learning computer programming, since boolean logic is used in various programming languages. Likewise, sets and other collection-like objects, such as multisets and lists, are common datatypes in computer science and programming.

In addition to that, sets are commonly referred to in mathematical teaching when talking about different types of numbers ( **N** , **Z** , **R** , ...), and when defining a mathematical function as a relation from one set (the domain) to another set (the range).

- Glossary of set theory
- Class (set theory)
- List of set theory topics
- Relational model – borrows from set theory

- ↑ In his 1925 paper "“An Axiomatization of Set Theory”, John von Neumann observed that "set theory in its first, "naive" version, due to Cantor, led to contradictions. These are the well-known antinomies of the set of all sets that do not contain themselves (Russell), of the set of all transfinite ordinal numbers (Burali-Forti), and the set of all finitely definable real numbers (Richard)." He goes on to observe that two "tendencies" were attempting to "rehabilitate" set theory. Of the first effort, exemplified by Bertrand Russell, Julius König, Hermann Weyl and L. E. J. Brouwer, von Neumann called the "overall effect of their activity . . . devastating". With regards to the axiomatic method employed by second group composed of Zermelo, Fraenkel and Schoenflies, von Neumann worried that "We see only that the known modes of inference leading to the antinomies fail, but who knows where there are not others?" and he set to the task, "in the spirit of the second group", to "produce, by means of a finite number of purely formal operations . . . all the sets that we want to see formed" but not allow for the antinomies. (All quotes from von Neumann 1925 reprinted in van Heijenoort, Jean (1967, third printing 1976),
*From Frege to Gödel: A Source Book in Mathematical Logic, 1879–1931*, Harvard University Press, Cambridge MA, ISBN 0-674-32449-8 (pbk). A synopsis of the history, written by van Heijenoort, can be found in the comments that precede von Neumann's 1925 paper.

In mathematics, the **axiom of choice**, or **AC**, is an axiom of set theory equivalent to the statement that *a Cartesian product of a collection of non-empty sets is non-empty*. Informally put, the axiom of choice says that given any collection of bins, each containing at least one object, it is possible to make a selection of exactly one object from each bin, even if the collection is infinite. Formally, it states that for every indexed family of nonempty sets there exists an indexed family of elements such that for every . The axiom of choice was formulated in 1904 by Ernst Zermelo in order to formalize his proof of the well-ordering theorem.

**Naive set theory** is any of several theories of sets used in the discussion of the foundations of mathematics. Unlike axiomatic set theories, which are defined using formal logic, naive set theory is defined informally, in natural language. It describes the aspects of mathematical sets familiar in discrete mathematics, and suffices for the everyday use of set theory concepts in contemporary mathematics.

In mathematics, the **continuum hypothesis** is a hypothesis about the possible sizes of infinite sets. It states:

There is no set whose cardinality is strictly between that of the integers and the real numbers.

In mathematics, a **finite set** is a set that has a finite number of elements. Informally, a finite set is a set which one could in principle count and finish counting. For example,

**Mathematical logic**, also called **formal logic**, is a subfield of mathematics exploring the applications of formal logic to mathematics. It bears close connections to metamathematics, the foundations of mathematics, philosophy, and theoretical computer science. The unifying themes in mathematical logic include the study of the expressive power of formal systems and the deductive power of formal proof systems.

In mathematical logic, **Russell's paradox**, is a set-theoretic paradox discovered by the British philosopher and mathematician Bertrand Russell in 1901. Russell's paradox shows that every set theory that contains an unrestricted comprehension principle leads to contradictions. The paradox had already been discovered independently in 1899 by the German mathematician Ernst Zermelo. However, Zermelo did not publish the idea, which remained known only to David Hilbert, Edmund Husserl, and other academics at the University of Göttingen. At the end of the 1890s, Georg Cantor – considered the founder of modern set theory – had already realized that his theory would lead to a contradiction, which he told Hilbert and Richard Dedekind by letter.

In set theory, the **axiom schema of replacement** is a schema of axioms in Zermelo–Fraenkel set theory (ZF) that asserts that the image of any set under any definable mapping is also a set. It is necessary for the construction of certain infinite sets in ZF.

In set theory, **Zermelo–Fraenkel set theory**, named after mathematicians Ernst Zermelo and Abraham Fraenkel, is an axiomatic system that was proposed in the early twentieth century in order to formulate a theory of sets free of paradoxes such as Russell's paradox. Today, Zermelo–Fraenkel set theory, with the historically controversial axiom of choice (AC) included, is the standard form of axiomatic set theory and as such is the most common foundation of mathematics. Zermelo–Fraenkel set theory with the axiom of choice included is abbreviated **ZFC**, where C stands for "choice", and **ZF** refers to the axioms of Zermelo–Fraenkel set theory with the axiom of choice excluded.

In mathematics and logic, an **axiomatic system** is any set of axioms from which some or all axioms can be used in conjunction to logically derive theorems. A theory is a consistent, relatively-self-contained body of knowledge which usually contains an axiomatic system and all its derived theorems. An axiomatic system that is completely described is a special kind of formal system. A formal theory is an axiomatic system that describes a set of sentences that is closed under logical implication. A formal proof is a complete rendition of a mathematical proof within a formal system.

**Zermelo set theory**, as set out in an important paper in 1908 by Ernst Zermelo, is the ancestor of modern Zermelo-Fraenkel set theory (ZF) and its extensions, such as von Neumann–Bernays–Gödel set theory (NBG). It bears certain differences from its descendants, which are not always understood, and are frequently misquoted. This article sets out the original axioms, with the original text and original numbering.

In mathematics, two sets or classes *A* and *B* are **equinumerous** if there exists a one-to-one correspondence between them, that is, if there exists a function from *A* to *B* such that for every element *y* of *B*, there is exactly one element *x* of *A* with *f*(*x*) = *y*. Equinumerous sets are said to have the same cardinality. The study of cardinality is often called **equinumerosity** (*equalness-of-number*). The terms **equipollence** (*equalness-of-strength*) and **equipotence** (*equalness-of-power*) are sometimes used instead.

In the foundations of mathematics, **von Neumann–Bernays–Gödel set theory** (**NBG**) is an axiomatic set theory that is a conservative extension of Zermelo–Fraenkel-Choice set theory (ZFC). NBG introduces the notion of class, which is a collection of sets defined by a formula whose quantifiers range only over sets. NBG can define classes that are larger than sets, such as the class of all sets and the class of all ordinals. Morse–Kelley set theory (MK) allows classes to be defined by formulas whose quantifiers range over classes. NBG is finitely axiomatizable, while ZFC and MK are not.

In mathematics, a **non-measurable set** is a set which cannot be assigned a meaningful "volume". The mathematical existence of such sets is construed to provide information about the notions of length, area and volume in formal set theory. In Zermelo-Fraenkel set theory, the axiom of choice entails that non-measurable subsets of exist.

In mathematical logic, **New Foundations** (**NF**) is an axiomatic set theory, conceived by Willard Van Orman Quine as a simplification of the theory of types of *Principia Mathematica*. Quine first proposed NF in a 1937 article titled "**New Foundations for Mathematical Logic**"; hence the name. Much of this entry discusses **NFU**, an important variant of NF due to Jensen (1969) and exposited in Holmes (1998). In 1940 and in a revision of 1951, Quine introduced an extension of NF sometimes called "Mathematical Logic" or "ML", that included proper classes as well as sets.

In set theory, the **axiom of limitation of size** was proposed by John von Neumann in his 1925 axiom system for sets and classes. It formalizes the limitation of size principle, which avoids the paradoxes encountered in earlier formulations of set theory by recognizing that some classes are too big to be sets. Von Neumann realized that the paradoxes are caused by permitting these big classes to be members of a class. A class that is a member of a class is a set; a class that is not a set is a proper class. Every class is a subclass of *V*, the class of all sets. The axiom of limitation of size says that a class is a set if and only if it is smaller than *V*—that is, there is no function mapping it onto *V*. Usually, this axiom is stated in the equivalent form: A class is a proper class if and only if there is a function that maps it onto *V*.

An approach to the foundations of mathematics that is of relatively recent origin, **Scott–Potter set theory** is a collection of nested axiomatic set theories set out by the philosopher Michael Potter, building on earlier work by the mathematician Dana Scott and the philosopher George Boolos.

In descriptive set theory, the **Borel determinacy theorem** states that any Gale–Stewart game whose payoff set is a Borel set is determined, meaning that one of the two players will have a winning strategy for the game.

This is a **glossary of set theory**.

- ↑ Kunen 1980, p. xi: "Set theory is the foundation of mathematics. All mathematical concepts are defined in terms of the primitive notions of set and membership. In axiomatic set theory we formulate a few simple axioms about these primitive notions in an attempt to capture the basic "obviously true" set-theoretic principles. From such axioms, all known mathematics may be derived."
- ↑ Cantor, Georg (1874), "Ueber eine Eigenschaft des Inbegriffes aller reellen algebraischen Zahlen",
*Journal für die reine und angewandte Mathematik*(in German),**1874**(77): 258–262, doi:10.1515/crll.1874.77.258, S2CID 199545885 - ↑ Johnson, Philip (1972),
*A History of Set Theory*, Prindle, Weber & Schmidt, ISBN 0-87150-154-6 - ↑ Bolzano, Bernard (1975), Berg, Jan (ed.),
*Einleitung zur Größenlehre und erste Begriffe der allgemeinen Größenlehre*, Bernard-Bolzano-Gesamtausgabe, edited by Eduard Winter et al., Vol. II, A, 7, Stuttgart, Bad Cannstatt: Friedrich Frommann Verlag, p. 152, ISBN 3-7728-0466-7`|volume=`

has extra text (help) - ↑ Dauben, Joseph (1979),
*Georg Cantor: His Mathematics and Philosophy of the Infinite*, Harvard University Press, pp. 30–54, ISBN 0-674-34871-0 . - ↑ Young, William; Young, Grace Chisholm (1906),
*Theory of Sets of Points*, Cambridge University Press - 1 2 3 4 5 6 7 "Comprehensive List of Set Theory Symbols".
*Math Vault*. 2020-04-11. Retrieved 2020-08-20. - ↑ "Introduction to Sets".
*www.mathsisfun.com*. Retrieved 2020-08-20. - ↑ Kolmogorov, A.N.; Fomin, S.V. (1970),
*Introductory Real Analysis*(Rev. English ed.), New York: Dover Publications, pp. 2–3, ISBN 0486612260, OCLC 1527264 - ↑ "set theory | Basics, Examples, & Formulas".
*Encyclopedia Britannica*. Retrieved 2020-08-20. - ↑ Bagaria, Joan (2020), "Set Theory", in Zalta, Edward N. (ed.),
*The Stanford Encyclopedia of Philosophy*(Spring 2020 ed.), Metaphysics Research Lab, Stanford University, retrieved 2020-08-20 - ↑ Jech, Thomas (2003),
*Set Theory*, Springer Monographs in Mathematics (Third Millennium ed.), Berlin, New York: Springer-Verlag, p. 642, ISBN 978-3-540-44085-7, Zbl 1007.03002 - ↑ Bishop, Errett (1967),
*Foundations of Constructive Analysis*, New York: Academic Press, ISBN 4-87187-714-0 - ↑ Feferman, Solomon (1998),
*In the Light of Logic*, New York: Oxford University Press, pp. 280–283, 293–294, ISBN 0195080300 - ↑ Rodych, Victor (Jan 31, 2018). "Wittgenstein's Philosophy of Mathematics". In Zalta, Edward N. (ed.).
*Stanford Encyclopedia of Philosophy*(Spring 2018 ed.).CS1 maint: ref duplicates default (link) - ↑ Wittgenstein, Ludwig (1975),
*Philosophical Remarks, §129, §174*, Oxford: Basil Blackwell, ISBN 0631191305 - ↑ Rodych 2018, §2.1: "When we prove a theorem or decide a proposition, we operate in a purely formal, syntactical manner. In doing mathematics, we do not discover pre-existing truths that were 'already there without one knowing' (PG 481)—we invent mathematics, bit-by-little-bit." Note, however, that Wittgenstein does
*not*identify such deduction with philosophical logic; c.f. Rodych §1, paras. 7-12. - ↑ Rodych 2018, §3.4: "Given that mathematics is a 'motley of techniques of proof' (RFM III, §46), it does not require a foundation (RFM VII, §16) and it cannot be given a self-evident foundation (PR §160; WVC 34 & 62; RFM IV, §3). Since set theory was invented to provide mathematics with a foundation, it is, minimally, unnecessary."
- ↑ Rodych 2018, §2.2: "An expression quantifying over an infinite domain is never a meaningful proposition, not even when we have proved, for instance, that a particular number n has a particular property."
- ↑ Rodych 2018, §3.6.
- ↑ Ferro, Alfredo; Omodeo, Eugenio G.; Schwartz, Jacob T. (September 1980), "Decision Procedures for Elementary Sublanguages of Set Theory. I. Multi-Level Syllogistic and Some Extensions",
*Communications on Pure and Applied Mathematics*,**33**(5): 599–608, doi:10.1002/cpa.3160330503 - ↑ Cantone, Domenico; Ferro, Alfredo; Omodeo, Eugenio G. (1989),
*Computable Set Theory*, International Series of Monographs on Computer Science, Oxford Science Publications, Oxford, UK: Clarendon Press, pp. xii, 347, ISBN 0-19-853807-3 - ↑ Mac Lane, Saunders; Moerdijk, leke (1992),
*Sheaves in Geometry and Logic: A First Introduction to Topos Theory*, Springer-Verlag, ISBN 9780387977102 - ↑ homotopy type theory in
*nLab* - ↑
*Homotopy Type Theory: Univalent Foundations of Mathematics*. The Univalent Foundations Program. Institute for Advanced Study. - ↑ Frank Ruda (6 October 2011).
*Hegel's Rabble: An Investigation into Hegel's Philosophy of Right*. Bloomsbury Publishing. p. 151. ISBN 978-1-4411-7413-0.

- Devlin, Keith (1993),
*The Joy of Sets*(2nd ed.), Springer Verlag, ISBN 0-387-94094-4 - Ferreirós, Jose (2007),
*Labyrinth of Thought: A history of set theory and its role in modern mathematics*, Basel: Birkhäuser, ISBN 978-3-7643-8349-7 - Johnson, Philip (1972),
*A History of Set Theory*, Prindle, Weber & Schmidt, ISBN 0-87150-154-6 - Kunen, Kenneth (1980),
*Set Theory: An Introduction to Independence Proofs*, North-Holland, ISBN 0-444-85401-0 - Potter, Michael (2004),
*Set Theory and Its Philosophy: A Critical Introduction*, Oxford University Press - Tiles, Mary (2004),
*The Philosophy of Set Theory: An Historical Introduction to Cantor's Paradise*, Dover Publications, ISBN 978-0-486-43520-6 - Smullyan, Raymond M.; Fitting, Melvin (2010),
*Set Theory And The Continuum Problem*, Dover Publications, ISBN 978-0-486-47484-7 - Monk, J. Donald (1969),
*Introduction to Set Theory*, McGraw-Hill Book Company, ISBN 978-0898740066

Wikibooks has a book on the topic of: Discrete mathematics/Set theory |

- Daniel Cunningham, Set Theory article in the
*Internet Encyclopedia of Philosophy*. - Jose Ferreiros, The Early Development of Set Theory article in the
*[Stanford Encyclopedia of Philosophy]*. - Foreman, Matthew, Akihiro Kanamori, eds.
*Handbook of Set Theory.*3 vols., 2010. Each chapter surveys some aspect of contemporary research in set theory. Does not cover established elementary set theory, on which see Devlin (1993). - "Axiomatic set theory",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - "Set theory",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - Schoenflies, Arthur (1898). Mengenlehre in Klein's encyclopedia.
- Online books , and library resources in your library and in other libraries about set theory
- Rudin, Walter B. (April 6, 1990). "Set Theory: An Offspring of Analysis".
*Marden Lecture in Mathematics*. University of Wisconsin-Milwaukee – via YouTube.

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