- Cardinal assignment
In

set theory , the concept ofcardinality is significantly developable without recourse to actually definingcardinal numbers as objects in theory itself (this is in fact a viewpoint taken by Frege;Frege cardinals are basically equivalence classes on the entire universe of sets which areequinumerous ). The concepts are developed by defining equinumerosity in terms of functions and the concepts ofone-to-one andonto (injectivity and surjectivity); this gives us a pseudo-ordering relation:$A\; leq\_c\; Bquad\; iffquad\; (exists\; f)(f\; :\; A\; o\; B\; mathrm\{is\; injective\})$

on the whole universe by size. It is not a true ordering because the trichotomy law need not hold: if both $A\; leq\_c\; B$ and $B\; leq\_c\; A$, it is true by the

Cantor–Bernstein–Schroeder theorem that $A\; =\_c\; B$ 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 theAxiom 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 $leq\_c$ 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 assignment which 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 andPrincipia Mathematica ) is as the set of all sets which are equinumerous with "X": this does not work inZFC or other related systems ofaxiomatic set theory because this collection is too large to be a set, but it does work intype theory and inNew Foundations and 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 toDana Scott : it works because the collection of objects with any given rank is a set).**References***Moschovakis, Yiannis N. "Notes on Set Theory". New York: Springer-Verlag, 1994.

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