In informal usage, a cardinal number is a number used in counting (a Counting Number), such as 1, 2, 3, ....

Formally, a cardinal number is a type of number defined in such a way that any method of counting Sets using it gives the same result. (This is not true for the Ordinal Numbers.) In fact, the cardinal numbers are obtained by collecting all Ordinal Numbers which are obtainable by counting a given set. A set has (Aleph-0) members if it can be put into a One-to-One correspondence with the finite Ordinal Numbers.

Two sets are said to have the same cardinal number if all the elements in the sets can be paired off One-to-One. An Inaccessible Cardinal cannot be expressed in terms of a smaller number of smaller cardinals.

**References**

Cantor, G. *Über unendliche, lineare Punktmannigfaltigkeiten, Arbeiten zur Mengenlehre aus dem Jahren 1872-1884.*
Leipzig, Germany: Teubner, 1884.

Conway, J. H. and Guy, R. K. ``Cardinal Numbers.'' In *The Book of Numbers.* New York: Springer-Verlag, pp. 277-282, 1996.

Courant, R. and Robbins, H. ``Cantor's `Cardinal Numbers.''' §2.4.3 in
*What is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed.*
Oxford, England: Oxford University Press, pp. 83-86, 1996.

© 1996-9

1999-05-26