home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: sci.math,sci.answers,news.answers
- Path: senator-bedfellow.mit.edu!bloom-beacon.mit.edu!spool.mu.edu!torn!watserv3.uwaterloo.ca!undergrad.math.uwaterloo.ca!neumann.uwaterloo.ca!alopez-o
- From: alopez-o@neumann.uwaterloo.ca (Alex Lopez-Ortiz)
- Subject: sci.math FAQ: Relevance of AC
- Summary: Part 26 of many, New version,
- Originator: alopez-o@neumann.uwaterloo.ca
- Message-ID: <DI76MH.ACs@undergrad.math.uwaterloo.ca>
- Sender: news@undergrad.math.uwaterloo.ca (news spool owner)
- Approved: news-answers-request@MIT.Edu
- Date: Fri, 17 Nov 1995 17:15:53 GMT
- Expires: Fri, 8 Dec 1995 09:55:55 GMT
- Reply-To: nancym@ii.com
- Nntp-Posting-Host: neumann.uwaterloo.ca
- Organization: University of Waterloo
- Followup-To: sci.math
- Lines: 257
- Xref: senator-bedfellow.mit.edu sci.math:124400 sci.answers:3434 news.answers:57835
-
-
- Archive-Name: sci-math-faq/AC/relevance
- Last-modified: December 8, 1994
- Version: 6.2
-
-
-
-
- THE AXIOM OF CHOICE
-
-
-
- There are several equivalent formulations:
-
- * The Cartesian product of nonempty sets is nonempty, even if the
- product is of an infinite family of sets.
-
- * Given any set S of mutually disjoint nonempty sets, there is a set
- C containing a single member from each element of S . C can thus
- be thought of as the result of ``choosing" a representative from
- each set in S . Hence the name.
-
-
-
-
- Relevance of the Axiom of Choice
-
-
-
- THE AXIOM OF CHOICE
-
- There are many equivalent statements of the Axiom of Choice. The
- following version gave rise to its name:
-
- For any set X there is a function f , with domain X\(0) , so that
- f(x) is a member of x for every nonempty x in X .
-
- Such an f is called a ``choice function" on X . [Note that X\ (0)
- means X with the empty set removed. Also note that in Zermelo-Fraenkel
- set theory all mathematical objects are sets so each member of X is
- itself a set.]
-
- The Axiom of Choice (AC) is one of the most discussed axioms of
- mathematics, perhaps second only to Euclid's parallel postulate. The
- axioms of set theory provide a foundation for modern mathematics in
- the same way that Euclid's five postulates provided a foundation for
- Euclidean geometry, and the questions surrounding AC are the same as
- the questions that surrounded Euclid's Parallel Postulate:
- 1. Can it be derived from the other axioms?
- 2. Is it consistent with the other axioms?
- 3. Should we accept it as an axiom?
-
- For many sets, including any finite set, the first six axioms of set
- theory (abbreviated ZF) are enough to guarantee the existence of a
- choice function but there do exist sets for which AC is required to
- show the existence of a choice function. The existence of such sets
- was proved in 1963 by Paul Cohen. This means that AC cannot be derived
- from the other six axioms; in other words ``AC is independent of ZF."
- This answers question [1] posed above.
-
- The question of whether AC is consistent with the other axioms
- (question [2] above) was answered by Goedel in 1938. Goedel showed
- that if the other axioms are consistent then AC is consistent with
- them. This is a ``relative consistency" proof which is the best we can
- hope for because of Goedel's Second Incompleteness Theorem.
-
- The third question, ``Should we accept it as an axiom?", moves us into
- the realm of philosophy. Today there are three major schools of
- thought concerning the use of AC:
- 1. Accept it as an axiom and use it without hesitation.
- 2. Accept it as an axiom but use it only when you cannot find a proof
- without it.
- 3. AC is unacceptable.
-
- Most mathematicians today belong to school A. Mathematicians who are
- in school B are usually there because of a belief in Occam's Razor
- (use as few assumptions as possible when explaining something) or an
- interest in metamathematics. There are a growing number of people
- moving to school C, especially computer scientists who work on
- automated reasoning using constructive type theories.
-
- Underlying the schools of thought about the use of AC are views about
- truth and the nature of mathematical objects. Three major views are
- platonism, constructivism, and formalism.
-
- Platonism
-
- A platonist believes that mathematical objects exist independent of
- the human mind, and a mathematical statement, such as AC, is
- objectively either true or false. A platonist accepts AC only if it is
- objectively true, and probably falls into school A or C depending on
- her belief. If she isn't sure about AC's truth then she may be in
- school B so that once she finds out the truth about AC she will know
- which theorems are true.
-
-
-
- Constructivism
-
- A constructivist believes that the only acceptable mathematical
- objects are ones that can be constructed by the human mind, and the
- only acceptable proofs are constructive proofs. Since AC gives no
- method for constructing a choice set constructivists belong to school
- C.
-
-
-
- Formalism
-
- A formalist believes that mathematics is strictly symbol manipulation
- and any consistent theory is reasonable to study. For a formalist the
- notion of truth is confined to the context of mathematical models,
- e.g., a formalist would say "The parallel postulate is false in
- Riemannian geometry." but she wouldn't say "The parallel postulate is
- false." A formalist will probably not allign herself with any school.
- She will comfortably switch between A, B, and C depending on her
- current interests.
-
- So: Should you accept the Axiom of Choice? Here are some arguments for
- and against it.
-
-
-
- Against
-
- * It's not as simple, aesthetically pleasing, and intuitive as the
- other axioms.
- * It is equivalent to many statements which are not intuitive such
- as "Every set can be well ordered." How, for example, would you
- well order the reals?
- * With it you can derive non-intuitive results, such as the
- existence of a discontinuous additive function, the existence of a
- non-measurable set of reals, and the Banach-Tarski Paradox (see
- the next section of the sci.math FAQ).
- * It is nonconstructive - it conjures up a set without providing any
- sort of procedure for its construction.
-
-
-
- For
-
- The acceptance of AC is based on the belief that our intuition about
- finite sets can be extended to infinite sets. The main argument for
- accepting it is that it is useful. Many important, intuitively
- plausible theorems are equivalent to it or depend on it. For example
- these statements are equivalent to AC:
- * Every vector space has a basis.
- * Trichotomy of Cardinals: For any cardinals k and l , either k < l
- or k = l or k > l .
- * Tychonoff's Theorem: The product of compact spaces is compact in
- the product topology.
- * Zorn's Lemma: Every nonempty partially ordered set P in which each
- chain has an upper bound in P has a maximal element.
-
- And these statements depend on AC (i.e., they cannot be proved in ZF
- without AC):
- * The union of countably many countable sets is countable.
- * Every infinite set has a denumerable subset.
- * The Loewenheim-Skolem Theorem: Any first-order theory which has a
- model has a denumerable model.
- * The Baire Category Theorem: The reals are not the union of
- countably many nowhere dense sets (i.e., the reals are not
- meager).
- * The Ultrafilter Theorem: Every Boolean algebra has an ultrafilter
- on it.
-
-
-
- Alternatives to AC
-
- * Accept only a weak form of AC such as the Denumerable Axiom of
- Choice (every denumerable set has a choice function) or the Axiom
- of Dependent Choice.
- * Accept an axiom that implies AC such as the Axiom of
- Constructibility ( V = L ) or the Generalized Continuum Hypothesis
- (GCH).
- * Adopt AC as a logical axiom (Hilbert suggested this with his
- epsilon axiom). If set theory is done in such a logical formal
- system the Axiom of Choice will be a theorem.
- * Accept a contradictory axiom such as the Axiom of Determinacy.
- * Use a completely different framework for mathematics such as
- Category Theory. Note that within the framework of Category Theory
- Tychonoff's Theorem can be proved without AC (Johnstone, 1981).
-
-
-
- Test Yourself: When is AC necessary?
-
- If you are working in Zermelo-Fraenkel set theory without the Axiom of
- Choice, can you choose an element from...
- 1. a finite set?
- 2. an infinite set?
- 3. each member of an infinite set of singletons (i.e., one-element
- sets)?
- 4. each member of an infinite set of pairs of shoes?
- 5. each member of inifinite set of pairs of socks?
- 6. each member of a finite set of sets if each of the members is
- infinite?
- 7. each member of an infinite set of sets if each of the members is
- infinite?
- 8. each member of a denumerable set of sets if each of the members is
- infinite?
- 9. each member of an infinite set of sets of rationals?
- 10. each member of a denumerable set of sets if each of the members is
- denumberable?
- 11. each member of an infinite set of sets if each of the members is
- finite?
- 12. each member of an infinite set of finite sets of reals?
- 13. each member of an infinite set of sets of reals?
- 14. each member of an infinite set of two-element sets whose members
- are sets of reals?
-
- The answers to these questions with explanations are accessible
- through http://www.jazzie.com/ii/math/index.html
-
-
-
- References
-
- Benacerraf, Paul and Putnam, Hilary. "Philosophy of Mathematics:
- Selected Readings, 2nd edition." Cambridge University Press, 1983.
-
- Dauben, Joseph Warren. "Georg Cantor: His Mathematics and Philosophy
- of the Infinite." Princeton University Press, 1979.
-
- A. Fraenkel, Y. Bar-Hillel, and A. Levy with van Dalen, Dirk.
- "Foundations of Set Theory, Second Revised Edition." North-Holland,
- 1973.
-
- Johnstone, Peter T. "Tychonoff's Theorem without the Axiom of Choice."
- Fundamenta Mathematica 113: 21-35, 1981.
-
- Leisenring, Albert C. "Mathematical Logic and Hilbert's
- Epsilon-Symbol." Gordon and Breach, 1969.
-
- Maddy, "Believing the Axioms, I", J. Symb. Logic, v. 53, no. 2, June
- 1988, pp. 490-500, and "Believing the Axioms II" in v.53, no. 3.
-
- Moore, Gregory H. "Zermelo's Axiom of Choice: Its Origins,
- Development, and Influence." Springer-Verlag, 1982.
-
- Rubin, Herman and Rubin, Jean E. "Equivalents of the Axiom of Choice
- II." North-Holland, 1985.
-
- This section of the FAQ is Copyright (c) 1994 Nancy McGough. Send
- comments and or corrections relating to this part to nancym@ii.com.
- The most up to date version of this section of the sci.math FAQ is
- accesible through http://www.jazzie.com/ii/math/index.html
-
-
- _________________________________________________________________
-
-
-
-
-
-
-