- Cardinal assignment
set theory, the concept of cardinalityis significantly developable without recourse to actually defining cardinal numbersas objects in theory itself (this is in fact a viewpoint taken by Frege; Frege cardinalsare basically equivalence classes on the entire universe of sets which are equinumerous). The concepts are developed by defining equinumerosity in terms of functions and the concepts of one-to-oneand onto(injectivity and surjectivity); this gives us a pseudo-ordering relation
on the whole universe by size. It is not a true ordering because the trichotomy law need not hold: if both and , it is true by the
Cantor–Bernstein–Schroeder theoremthat i.e. "A" and "B" are equinumerous, but they do not have to be literally equal; that at least one case holds turns out to be equivalent to the Axiom of choice.
Nevertheless, most of the "interesting" results on cardinality and its arithmetic can be expressed merely with =c.
The goal of a cardinal assignment is to assign to every set "A" a specific, unique set which is only dependent on the cardinality of "A". This is in accordance with Cantor's original vision of a cardinals: to take a set and abstract its elements into canonical "units" and collect these units into another set, such that the only thing special about this set is its size. These would be totally ordered by the relation and =c would be true equality. As Y. N. Moschovakis says, however, this is mostly an exercise in mathematical elegance, and you don't gain much unless you are "allergic to subscripts." However, there are various valuable applications of "real" cardinal numbers in various models of set theory.
In modern set theory, we usually use the
Von Neumann cardinal assignmentwhich uses the theory of ordinal numbers and the full power of the Axioms of choice and replacement. Cardinal assignments do need the full Axiom of choice, if we want a decent cardinal arithmetic and an assignment for "all" sets. More on this (and much more good set theory in general!) can be found in Moschovakis' excellent introduction to set theory.
Cardinal assignment without the axiom of choice
Formally, assuming the axiom of choice, cardinality of a set "X" is the least ordinal α such that there is a bijection between "X" and α. This definition is known as the
von Neumann cardinal assignment. If the axiom of choice is not assumed we need to do something different. The oldest definition of the cardinality of a set "X" (implicit in Cantor and explicit in Frege and Principia Mathematica) is as the set of all sets which are equinumerous with "X": this does not work in ZFCor other related systems of axiomatic set theorybecause this collection is too large to be a set, but it does work in type theoryand in New Foundationsand related systems. However, if we restrict from this class to those equinumerous with "X" that have the least rank, then it will work (this is a trick due to Dana Scott: it works because the collection of objects with any given rank is a set).
*Moschovakis, Yiannis N. "Notes on Set Theory". New York: Springer-Verlag, 1994.
Wikimedia Foundation. 2010.