Cardinal characteristic of the continuum

In the mathematical discipline of set theory, a cardinal characteristic of the continuum is an infinite cardinal number that may consistently lie strictly between (the cardinality of the set of natural numbers), and the cardinality of the continuum, that is, the cardinality of the set of all real numbers. The latter cardinal is denoted or . A variety of such cardinal characteristics arise naturally, and much work has been done in determining what relations between them are provable, and constructing models of set theory for various consistent configurations of them.

Background

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Cantor's diagonal argument shows that   is strictly greater than  , but it does not specify whether it is the least cardinal greater than   (that is,  ). Indeed the assumption that   is the well-known Continuum Hypothesis, which was shown to be consistent with the standard ZFC axioms for set theory by Kurt Gödel and to be independent of it by Paul Cohen. If the Continuum Hypothesis fails and so   is at least  , natural questions arise about the cardinals strictly between   and  , for example regarding Lebesgue measurability. By considering the least cardinal with some property, one may get a definition for an uncountable cardinal that is consistently less than  . Generally one only considers definitions for cardinals that are provably greater than   and at most   as cardinal characteristics of the continuum, so if the Continuum Hypothesis holds they are all equal to  .

Examples

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As is standard in set theory, we denote by   the least infinite ordinal, which has cardinality  ; it may be identified with the set of natural numbers.

A number of cardinal characteristics naturally arise as cardinal invariants for ideals which are closely connected with the structure of the reals, such as the ideal of Lebesgue null sets and the ideal of meagre sets.

non(N)

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The cardinal characteristic   is the least cardinality of a non-measurable set; equivalently, it is the least cardinality of a set that is not a Lebesgue null set.

Bounding number and dominating number

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We denote by   the set of functions from   to  . For any two functions   and   we denote by   the statement that for all but finitely many  . The bounding number   is the least cardinality of an unbounded set in this relation, that is,  

The dominating number   is the least cardinality of a set of functions from   to   such that every such function is dominated by (that is,  ) a member of that set, that is,  

Clearly any such dominating set   is unbounded, so   is at most  , and a diagonalisation argument shows that  . Of course if   this implies that  , but Hechler[1] has shown that it is also consistent to have   strictly less than  

Splitting number and reaping number

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We denote by   the set of all infinite subsets of  . For any  , we say that   splits   if both   and   are infinite. The splitting number   is the least cardinality of a subset   of   such that for all  , there is some   such that   splits  . That is,  

The reaping number   is the least cardinality of a subset   of   such that no element   of   splits every element of  . That is,  

Ultrafilter number

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The ultrafilter number   is defined to be the least cardinality of a filter base of a non-principal ultrafilter on  . Kunen[2] gave a model of set theory in which   but   and using a countable support iteration of Sacks forcings, Baumgartner and Laver[3] constructed a model in which   and  .

Almost disjointness number

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Two subsets   and   of   are said to be almost disjoint if   is finite, and a family of subsets of   is said to be almost disjoint if its members are pairwise almost disjoint. A maximal almost disjoint ("mad") family of subsets of   is thus an almost disjoint family   such that for every subset   of   not in  , there is a set   such that   and   are not almost disjoint (that is, their intersection is infinite). The almost disjointness number   is the least cardinality of an infinite maximal almost disjoint family. A basic result[4] is that  ; Shelah[5] showed that it is consistent to have the strict inequality  .

Cichoń's diagram

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A well-known diagram of cardinal characteristics is Cichoń's diagram, showing all pairwise relations provable in ZFC between 10 cardinal characteristics.

References

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  1. ^ Stephen Hechler. On the existence of certain cofinal subsets of  . In T. Jech (ed), Axiomatic Set Theory, Part II. Volume 13(2) of Proc. Symp. Pure Math., pp 155–173. American Mathematical Society, 1974
  2. ^ Kenneth Kunen. Set Theory An Introduction to Independence Proofs. Studies in Logic and the Foundations of Mathematics vol. 102, Elsevier, 1980
  3. ^ James Earl Baumgartner and Richard Laver. Iterated perfect-set forcing. Annals of Mathematical Logic 17 (1979) pp 271–288.
  4. ^ Eric van Douwen. The Integers and Topology. In K. Kunen and J.E. Vaughan (eds) Handbook of Set-Theoretic Topology. North-Holland, Amsterdam, 1984.
  5. ^ Saharon Shelah. On cardinal invariants of the continuum. In J. Baumgartner, D. Martin and S. Shelah (eds) Axiomatic Set Theory, Contemporary Mathematics 31, American Mathematical Society, 1984, pp 183-207.

Further reading

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  • Tomek Bartoszyński and Haim Judah. Set Theory On the Structure of the Real Line. A K Peters, 1995.
  • Vaughan, Jerry E. (1990). "Chapter 11: Small uncountable cardinals and topology" (PDF). In van Mill, Jan; Reed, George M. (eds.). Open Problems in Topology. Amsterdam: North-Holland Publishing Company. pp. 196–218. ISBN 0-444-88768-7. Retrieved December 5, 2011.
  • Blass, Andreas (January 12, 2010). "Chapter 6 : Combinatorial Cardinal Characteristics of the Continuum". In Foreman, Matthew; Kanamori, Akihiro (eds.). Handbook of Set Theory (PDF). Vol. 1. Springer. pp. 395–490. ISBN 978-1-4020-4843-2. Retrieved December 5, 2011.
  • Bartoszyński, Tomek (January 12, 2010). "Chapter 7 : Invariants of Measure and Category". In Foreman, Matthew; Kanamori, Akihiro (eds.). Handbook of Set Theory. Vol. 1. Springer. pp. 491–556. arXiv:math.LO/9910015. ISBN 978-1-4020-4843-2.
  • Jech, Thomas (2003). Set Theory. Springer Monographs in Mathematics (Third Millennium ed.). Berlin, New York: Springer-Verlag. ISBN 978-3-540-44085-7. Zbl 1007.03002.
  • Halbeisen, Lorenz J. (2012). Combinatorial Set Theory: With a Gentle Introduction to Forcing. Springer Monographs in Mathematics. Springer Monographs in Mathematics. London: Springer-Verlag. doi:10.1007/978-1-4471-2173-2. ISBN 978-1-4471-2172-5.