The next key idea of Georg Cantor (1845 - 1918) was to use a given set $A$ as a representative for all sets, which are equipotent to it. In order to be able to do so, Cantor realized that any given set can be selected as a representative of a whole class of sets which are equipotent to it. This abstraction process allowed Cantor to ignore which elements a set has and to concentrate on how many elements it has.

# Proposition: Cardinal Number

Let $\mathcal X$ be a universal set. Being equipotent for any two sets $$A,B\subseteq \mathcal X$$ is an equivalence relation $$\sim$$ on $$\mathcal X$$. Each equivalence class $[A]\in \mathcal X/\sim$1 is called the cardinal number or cardinality of $A$.

Instead of $[A]$, the notation $|A|$ is more commonly used for the cardinality of $A.$ Note that all cardinalities are disjoint.

Proofs: 1

Chapters: 1
Definitions: 2 3 4 5
Examples: 6
Explanations: 7
Lemmas: 8
Motivations: 9
Parts: 10
Proofs: 11 12 13 14 15
Propositions: 16 17 18 19 20 21
Theorems: 22 23

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### References

#### Bibliography

1. Reinhardt F., Soeder H.: "dtv-Atlas zur Mathematik", Deutsche Taschenbuch Verlag, 1994, 10th Edition

#### Footnotes

1. "$\mathcal X/\sim$" denotes the quotient set induced by the relation of being equipotent.