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Axiomatic set theory

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Set theory is a branch of mathematics and computer science created principally by the German mathematician Georg Cantor at the end of the 19th century. Initially controversial, set theory has come to play the role of a foundational theory in modern mathematics, in the sense of a theory invoked to justify assumptions made in mathematics concerning the existence of mathematical objects (such as numbers or functions) and their properties. Formal versions of set theory also have a foundational role to play as specifying a theoretical ideal of mathematical rigor in proofs. At the same time the basic concepts of set theory are used throughout mathematics, while the subject is pursued in its own right as a speciality by a comparatively small group of mathematicians and logicians. It should be mentioned that there are also mathematicians using and promoting different approaches to the foundations of mathematics.

The basic concepts of set theory are set and membership. A set is thought of as any collection of objects, called the members (or elements) of the set. In mathematics, the members of sets are any mathematical objects, and in particular can themselves be sets. Thus one speaks of the set N of natural numbers {0,1,2,3,4,...}, the set of real numbers, and the set of functions from the natural numbers to the natural numbers; but also, for example, of the set {0,2,N} which has as members the numbers 0 and 2 and the set N.

Initially, what is now known as "naive" or "intuitive" set theory was developed. (See Naive set theory). As it turned out, assuming that one could perform any operations on sets without restriction led to paradoxes such as Russell's paradox. To address these problems, set theory had to be re-constructed, this time using an axiomatic approach.

The origins of rigorous set theory

The important idea of Cantor's, which got set theory going as a new field of study, was to define two sets A and B to have the same number of members (the same cardinality) when there is a way of pairing off members of A exhaustively with members of B. Then the set N of natural numbers has the same cardinality as the set Q of rational numbers (they are both said to be countably infinite), even though N is a proper subset of Q. On the other hand, the set R of real numbers does not have the same cardinality as N or Q, but a larger one (it is said to be uncountable). Cantor gave two proofs that R is not countable, and the second of these, using what is known as the diagonal construction, has been extraordinarily influential and has had manifold applications in logic and mathematics.

Cantor went right ahead and constructed infinite hierarchies of infinite sets, the ordinal and cardinal numbers. This was considered controversial in his day, with the opposition led by the finitist Leopold Kronecker[?], but there is no significant disagreement among mathematicians today that Cantor had the right idea.

Cantor's development of set theory was still "naïve" in the sense that he didn't have a precise axiomatization in mind. In retrospect, we can say that Cantor was tacitly using the axiom of extension, the axiom of infinity, and the axiom schema of (unrestricted) comprehension. However, the last of these leads directly to Russell's paradox, by constructing the set S := {A : A is not in A} of all sets that don't belong to themselves. (If S belongs to itself, then it does not, giving a contradiction, so S must not belong to itself. But then S must belong to itself, giving a final and absolute contradiction.) Therefore, set theorists were forced to abandon either classical logic or unrestricted comprehension, and the latter was far more reasonable to most. (Although intuitionism had a significant following, the paradox still goes through with intuitionistic logic. There is no paradox in Brazilian logic[?], but that was almost completely unknown at the time.)

In order to avoid this and similar paradoxes, Ernst Zermelo put forth a system of axioms for set theory in 1908. He included in this system the axiom of choice, a truly controversial axiom that he needed to prove the well ordering theorem. This system was later refined by Adolf Fraenkel[?] and Thoralf Skolem[?], giving the axioms used today.

Axioms for set theory

The axioms for set theory now most often studied and used, although put in their final form by Skolem, are called the Zermelo-Fraenkel axioms (ZF). Actually, this term usually excludes the axiom of choice, which was once more controversial than it is today. When this axiom is included, the resulting system is called ZFC.

An important feature of ZFC is that every object that it deals with is a set. In particular, every element of a set is itself a set. Other familiar mathematical objects, such as numbers, must be subsequently defined in terms of sets.

The nine axioms of ZFC are listed below. (Strictly speaking, the axioms of ZFC are just strings of logical symbols. What follows should therefore be viewed only as an attempt to express the intended meaning of these axioms in English.) Each axiom has further information in its own article.

  1. Axiom of extension: Two sets are the same if and only if they have the same elements.
  2. Axiom of empty set: There is a set with no elements. We will use {} to denote this empty set.
  3. Axiom of pairing: If x, y are sets, then so is {x,y}, a set containing x and y as its only elements.
  4. Axiom of union: For any set x, there is a set y such that the elements of y are precisely the elements of the elements of x.
  5. Axiom of infinity: There exists a set x such that {} is in x and whenever y is in x, so is the union y U {y}.
  6. Axiom of replacement: Given any set and any mapping, formally defined as a proposition P(x,y) where P(x,y) and P(x,z) implies y = z, there is a set containing precisely the images of the original set's elements.
  7. Axiom of power set: Every set has a power set. That is, for any set x there exists a set y, such that the elements of y are precisely the subsets of x.
  8. Axiom of regularity: Every non-empty set x contains some element y such that x and y are disjoint sets.
  9. Axiom of choice: (Zermelo's version) Given a set x of mutually disjoint nonempty sets, there is a set y (a choice set for x) containing exactly one element from each member of x.

The axioms of choice and regularity are still controversial today among a minority of mathematicians.

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