defining the cardinals


Intuitively, one would like to define cardinal numbers as equivalence classes of sets, but unfortunately, these equivalence classes are too large to be sets. Instead, the cardinal numbers can be defined from the ordinal numbers. This construction requires the Axiom of Choice.


This concept has the prerequisites:


  • Show that the Well-Ordering Theorem is equivalent to the Axiom of Choice
  • Show that the trichotomy of the dominance relation is equivalent to the Well-Ordering Theorem
  • Construct the cardinal numbers in terms of the ordinals
  • Know why one cannot simply define cardinals as equivalence classes of sets under equinumerosity

Core resources (read/watch one of the following)


Supplemental resources (the following are optional, but you may find them useful)


Notes on Set Theory (2013)
Lecture notes for a course on axiomatic set theory.
  • Section 6, "The Axiom of Choice," only the parts dealing with the "Well-Ordering Theorem," pages 25-28
  • Section 7, "Cardinals," pages 30-34
  • Section 8, "Cardinal arithmetic," pages 34-40
Author: Henry Cohn

See also

-No Additional Notes-