# Zermelo set theory

﻿
Zermelo set theory

Zermelo set theory, as set out in an important paper in 1908 by Ernst Zermelo, is the ancestor of modern set theory. 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 (translated into English) and original numbering.

The axioms of Zermelo set theory

:AXIOM I. Axiom of extensionality ("Axiom der Bestimmtheit") "If every element of a set "M" is also an element of "N" and vice versa ... then "M" $equiv$ "N". Briefly, every set is determined by its elements".: AXIOM II. Axiom of elementary sets ("Axiom der Elementarmengen") "There exists a (fictitious) set, the null set, ∅, that contains no element at all. If "a" is any object of the domain, there exists a set {"a"} containing "a" and only "a" as element. If "a" and "b" are any two objects of the domain, there always exists a set {"a", "b"} containing as elements "a" and "b" but no object "x" distinct from them both." See Axiom of pairs.: AXIOM III. Axiom of separation ("Axiom der Aussonderung") "Whenever the propositional function &ndash;("x") is definite for all elements of a set "M", "M" possesses a subset "M' " containing as elements precisely those elements "x" of "M" for which &ndash;("x") is true".: AXIOM IV. Axiom of the power set ("Axiom der Potenzmenge") "To every set "T" there corresponds a set "T' ", the power set of "T", that contains as elements precisely all subsets of "T".: AXIOM V. Axiom of the union ("Axiom der Vereinigung") "To every set "T" there corresponds a set "∪T", the union of "T", that contains as elements precisely all elements of the elements of "T".: AXIOM VI. Axiom of choice ("Axiom der Auswahl"): "If "T" is a set whose elements all are sets that are different from ∅ and mutually disjoint, its union "∪T" includes at least one subset "S"1 having one and only one element in common with each element of "T".: AXIOM VII. Axiom of infinity ("Axiom des Unendlichen") "There exists in the domain at least one set "Z" that contains the null set as an element and is so constituted that to each of its elements "a" there corresponds a further element of the form {"a"}, in other words, that with each of its elements "a" it also contains the corresponding set {"a"} as element".

Connection with standard set theory

The accepted standard for set theory is Zermelo-Fraenkel set theory. The links show where the axioms of Zermelo's theory correspond. There is no exact match for "elementary sets". (It was later shown that the singleton set could be derived from what is now called "Axiom of pairs". If "a" exists, "a" and "a" exist, thus {"a","a"} exists. By extensionality {"a","a"} = {"a"}.) The empty set axiom is already assumed by axiom of infinity, and is now included as part of it.

The axioms do not include the Axiom of regularity and Axiom of replacement. These were added as the result of work by Thoralf Skolem in 1922, based on earlier work by Abraham Fraenkel in the same year.

In the modern ZFC system, the "propositional function" referred to in the axiom of separation is interpreted as "any property definable by a first order formula with parameters", so the separation axiom is replaced by an axiom scheme. The notion of "first order formula" was not known in 1904 when Zermelo published his axiom system, and he later rejected this interpretation as being too restrictive. Zermelo set theory is usually taken to be a first-order theory with the separation axiom replaced by an axiom scheme with an axiom for each first-order formula. It can also be considered as a theory in second-order logic, where now the separation axiom is just a single axiom. The second-order interpretation of Zermelo set theory is probably closer to Zermelo's own conception of it, and is stronger than the first-order interpretation.

In the usual cumulative hierarchy "V"α of ZFC set theory (for ordinals α), any one of the sets"V"α for α a limit ordinal larger than the first infinite ordinal ω (such as "V"&omega;&middot;2) forms a model of Zermelo set theory. So the consistency of Zermelo set theory is a theorem of ZFC set theory. Zermelo's axioms do not imply the existence of &alefsym;&omega; or larger infinite cardinals, as the model "V"&omega;&middot;2 does not contain such cardinals. (Cardinals have to be defined differently in Zermelo set theory, as the usual definition of cardinals and ordinals does not work very well: with the usual definition it is not even possible to prove the existence of the ordinal &omega;2.)

The axiom of infinity is usually now modified to assert the existence of the first infinitevon Neumann ordinal $omega$; the original Zermeloaxioms cannot prove the existence of this set, nor can the modified Zermelo axioms prove Zermelo'saxiom of infinity. Zermelo's axioms (original or modified) cannot prove the existence of $V_\left\{omega\right\}$ as a set nor of any rank of the cumulative hierarchy of sets with infinite index.

Zermelo set theory is similar in strength to topos theory with a natural number object, or to the system in Principia mathematica. It is strong enough to carry out almost all ordinary mathematics not directly connected with set theory or logic.

The aim of Zermelo's paper

The introduction states that the very existence of the discipline of set theory "seems to be threatened by certain contradictions or "antinomies", that can be derived from its principles &ndash; principles necessarily governing our thinking, it seems &ndash; and to which no entirely satisfactory solution has yet been found". Zermelo is of course referring to the "Russell antinomy".

He says he wants to show how the original theory of Cantor and Dedekind can be reduced to a few definitions and seven principles or axioms. He says he has "not" been able to prove that the axioms are consistent. A Platonistic argument for their consistency goes as follows. Define "V"α for α one of the symbols ("ordinals") 0, 1, 2, ...,&omega;, &omega;+1, &omega;+2,..., &omega;2 as follows. For α a successor of the form &beta;+1, "V"α is defined to be the collection of all subsets of "V"&beta;. If α is a limit (0, &omega;, or ω·2) then "V"α is defined to be the union of "V"&beta; for &beta;<α. Then the axioms of Zermelo set theory are (Platonically) consistent because they are true in the (Platonic) model "V"ω·2. While a Platonist might regard this as a valid argument, a constructivist would probably not: while there are no problems with the construction of the sets up to "V"&omega;, the construction of "V"&omega;+1 is less clear because one cannot constructively define every subset of "V"&omega;. (This argument can be turned into a valid proof in Zermelo-Frenkel set theory, but this does not really help because the consistency of Zermelo-Frenkel set theory is less clear than the consistency of Zermelo set theory.)

The axiom of separation

Zermelo comments that Axiom III of his system is the one responsible for eliminating the antinomies. It differs from the original definition by Cantor, as follows.

Sets cannot be independently defined by any arbitrary logically definable notion. They must be constructed in some way from previously constructed sets. For example they can be constructed by taking powersets, or they can be "separated" as subsets of sets already "given". This, he says, eliminates contradictory ideas like "the set of all sets" or "the set of all ordinal numbers".

He disposes of the Russell paradox by means of a Theorem. "Every set $M$ possesses at least one subset $M_0$ that is not an element of "M". Let $M_0$ be the subset of $M$ for which, by AXIOM III, is separated out by the notion "$x otin x$". Then $M_0$ cannot be in $M$. For

# If $M_0$ is in $M_0$, then $M_0$ contains an element "x" for which "x" is in "x" (i.e. $M_0$ itself), which would contradict the definition of $M_0$.
# If $M_0$ is not in $M_0$, $M_0$ is an element of "M" that satisfies the definition "$x otin x$", and so is in $M_0$.

So $M_0$ cannot be in $M$, hence not all objects of the universal domain "B" can be elements of one and the same set. "This disposes of the Russell antinomy as far as we are concerned".

This left the problem of "the domain "B" which seems to refer to something. This led to the idea of a proper class.

Cantor's theorem

Zermelo's paper is notable for what may be the first mention of Cantor's theorem explicitly and by name. This appeals strictly to set theoretical notions, and is thus not exactly the same as Cantor's diagonal argument.

Cantor's Theorem: "If "M" is an arbitrary set, then always "M" < P("M") [the power set of "M"] . Every set is of lower cardinality than the set of its subsets".

Zermelo proves this by considering a function φ: "M" → P("M"). By AXIOM III this defines the following set "M' ":

:"M' " = {"m": "m" ∉ φ("m")}

But no element "m' " of "M " could correspond to "M' ", i.e. such that φ("m' ") = "M' ". Otherwise

:1) If "m' " is in "M' " then by definition "m' " ∉ φ("m' ") = "M' ", which is a contradiction

:2) If "m' " is in not in "M' " but in "M " then by definition "m' " ∉ "M' " = φ("m' ") which by definition implies that "m' " is in "M' ", which is a contradiction.

so by contradiction "m' " does not exist. Note the close resemblance of this proof to the way Zermelo disposes of Russell's paradox.

References

*citation|authorlink=Ernst Zermelo|first=Ernst|last= Zermelo|year=1908|title=Untersuchungen über die Grundlagen der Mengenlehre I|journal=Mathematische Annalen |volume=65|pages= 261-281|doi= 10.1007/BF01449999 English translation in citation|authorlink=Jean van Heijenoort|first=Jean van|last= Heijenoort |year=1967 |title= From Frege to Gödel: A Source Book in Mathematical Logic, 1879-1931 |series=Source Books in the History of the Sciences |chapter=Investigations in the foundations of set theory|publisher=Harvard Univ. Press|pages=199-215|ISBN= 978-0674324497

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Zermelo–Fraenkel set theory — Zermelo–Fraenkel set theory, with the axiom of choice, commonly abbreviated ZFC, is the standard form of axiomatic set theory and as such is the most common foundation of mathematics.ZFC consists of a single primitive ontological notion, that of… …   Wikipedia

• Set theory — This article is about the branch of mathematics. For musical set theory, see Set theory (music). A Venn diagram illustrating the intersection of two sets. Set theory is the branch of mathematics that studies sets, which are collections of objects …   Wikipedia

• set theory — the branch of mathematics that deals with relations between sets. [1940 45] * * * Branch of mathematics that deals with the properties of sets. It is most valuable as applied to other areas of mathematics, which borrow from and adapt its… …   Universalium

• set theory — The modern theory of sets was largely inspired by Cantor, whose proof that the set of real numbers could not be put into a one to one correspondence with the set of natural numbers opened the door to the set theoretic hierarchy, and to the study… …   Philosophy dictionary

• Zermelo–Fraenkel set theory — The first rigorous axiomatization of set theory was presented by Ernst Zermelo (1871–1953) in 1908, and its development by A. A. Fraenkel (1891–1965), adding the axiom of replacement, is known as ZF. If the axiom of choice is added it is known as …   Philosophy dictionary

• Ackermann set theory — is a version of axiomatic set theory proposed by Wilhelm Ackermann in 1956. The languageAckermann set theory is formulated in first order logic. The language L A consists of one binary relation in and one constant V (Ackermann used a predicate M… …   Wikipedia

• Constructive set theory — is an approach to mathematical constructivism following the program of axiomatic set theory. That is, it uses the usual first order language of classical set theory, and although of course the logic is constructive, there is no explicit use of… …   Wikipedia

• Naive set theory — This article is about the mathematical topic. For the book of the same name, see Naive Set Theory (book). Naive set theory is one of several theories of sets used in the discussion of the foundations of mathematics.[1] The informal content of… …   Wikipedia

• Class (set theory) — In set theory and its applications throughout mathematics, a class is a collection of sets (or sometimes other mathematical objects) which can be unambiguously defined by a property that all its members share. The precise definition of class… …   Wikipedia

• Quasi-set theory — is a formal mathematical theory of collections of indistinguishable objects, mainly motivated by the assumption that certain objects treated in quantum physics are indistinguishable. Quasi set theory is closely related to, yet distinct from,… …   Wikipedia

### Share the article and excerpts

Do a right-click on the link above