Zorn's lemma

/zawrnz/, Math.
a theorem of set theory that if every totally ordered subset of a nonempty partially ordered set has an upper bound, then there is an element in the set such that the set contains no element greater than the specified given element.
[1945-50; after Max August Zorn (born 1906), German mathematician]

* * *

also known as  Kuratowski-Zorn lemma  originally called  maximum principle 

      statement in the language of set theory, equivalent to the axiom of choice, that is often used to prove the existence of a mathematical object when it cannot be explicitly produced.

 In 1935 the German-born American mathematician Max Zorn proposed adding the maximum principle to the standard axioms of set theory (see the table—>). (Informally, a closed collection of sets contains a maximal member—a set that cannot be contained in any other set in the collection.) Although it is now known that Zorn was not the first to suggest the maximum principle (the Polish mathematician Kazimierz Kuratowski discovered it in 1922), he demonstrated how useful this particular formulation could be in applications, particularly in algebra and analysis. He also stated, but did not prove, that the maximum principle, the axiom of choice, and German mathematician Ernst Zermelo's well-ordering principle were equivalent; that is, accepting any one of them enables the other two to be proved. See also set theory: Axioms for infinite and ordered sets (set theory).

      A formal definition of Zorn's lemma requires some preliminary definitions. A collection C of sets is called a chain if, for each pair of members of C (Ci and Cj), one is a subset of the other (Ci ⊆ Cj). A collection S of sets is said to be “closed under unions of chains” if whenever a chain C is included in S (i.e., C ⊆ S), then its union belongs to S (i.e., ∪ Ck ∊ S). A member of S is said to be maximal if it is not a subset of any other member of S. Zorn's lemma is the statement: Any collection of sets closed under unions of chains contains a maximal member.

      As an example of an application of Zorn's lemma in algebra,consider the proof that any vector space V has a basis (a linearly independent subset that spans the vector space; informally, a subset of vectors that can be combined to obtain any other element in the space). Taking S to be the collection of all linearly independent sets of vectors in V, it can be shown that S is closed under unions of chains. Then by Zorn's lemma there exists a maximal linearly independent set of vectors, which by definition must be a basis for V. (It is known that, without the axiom of choice, it is possible for there to be a vector space without a basis.)

      An informal argument for Zorn's lemma can be given as follows: Assume that S is closed under unions of chains. Then the empty set Ø, being the union of the empty chain, is in S. If it is not a maximal member, then some other member that includes it is chosen. This last step is then iterated for a very long time (i.e., transfinitely, by using ordinal numbers to index the stages in the construction). Whenever (at limit ordinal stages) a long chain of larger and larger sets has been formed, the union of that chain is taken and used to continue. Because S is a set (and not a proper class like the class of ordinal numbers), this construction ultimately must stop with a maximal member of S.

Herbert Enderton

* * *

Universalium. 2010.

Look at other dictionaries:

  • Zorn's lemma — Zorn s lemma, also known as the Kuratowski Zorn lemma, is a proposition of set theory that states:Every partially ordered set in which every chain (i.e. totally ordered subset) has an upper bound contains at least one maximal element.It is named… …   Wikipedia

  • zorn's lemma — ˈzȯ(ə)rnz , ˈtsȯ noun Usage: usually capitalized Z Etymology: after Max August Zorn died 1993 American (German born) mathematician : a lemma in set theory: if S is partially ordered and if each subset for which every pair of elements is related …   Useful english dictionary

  • Zorn's lemma — noun Etymology: Max August Zorn died 1993 German mathematician Date: circa 1950 a lemma in set theory: if a set S is partially ordered and if each subset for which every pair of elements is related by exactly one of the relationships “less than,” …   New Collegiate Dictionary

  • Zorn's lemma — A proposition in set theory equivalent to the axiom of choice . Call a set A a chain if for any two members B and C, either B is a subset of C or C is a subset of B. Now consider a set D with the properties that for every chain E that is a subset …   Philosophy dictionary

  • Zorn's lemma — noun A proposition of set theory stating that every partially ordered set, in which every chain (i.e. totally ordered subset) has an upper bound, contains at least one maximal element …   Wiktionary

  • Zorn's Lemma (film) — Infobox Film name = Zorn s Lemma caption = director = Hollis Frampton producer = writer = starring = music = cinematography = editing = distributor = released = runtime = 60 min. country = flagicon|USA USA awards = language = English budget =… …   Wikipedia

  • Zorn's Law — * Zorn s law is a maxim coined by Chicago Tribune columnist Eric Zorn as a Wikipedia prank. * Zorn s lemma is a proposition used in many areas of theoretical mathematics …   Wikipedia

  • Lemma (mathematics) — In mathematics, a lemma (plural lemmata or lemmascite book |last= Higham |first= Nicholas J. |title= Handbook of Writing for the Mathematical Sciences |publisher= Society for Industrial and Applied Mathematics |year= 1998 |isbn= 0898714206 |pages …   Wikipedia

  • Lemma von Zorn — Das Lemma von Zorn, auch bekannt als Lemma von Kuratowski Zorn, ist ein Theorem der Mengenlehre, genauer gesagt, der Zermelo Fraenkel Mengenlehre, die das Auswahlaxiom einbezieht. Es ist benannt nach dem deutsch amerikanischen Mathematiker Max… …   Deutsch Wikipedia

  • Lemma von Teichmüller-Tukey — Das Lemma von Teichmüller Tukey (nach Oswald Teichmüller und John W. Tukey), manchmal auch nur Lemma von Tukey genannt, ist ein Satz aus der Mengenlehre. Es ist im Rahmen der Mengenlehre auf Grundlage der ZF Axiome äquivalent zum Auswahlaxiom und …   Deutsch Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.