Cardinal number

Uncommon card · No. 142805

In mathematics, a cardinal number, or cardinal for short, is a kind of number that measures the cardinality of a set, i.e., how many elements there are in a set. The cardinal number associated with a set ⁠ A {\displaystyle A} ⁠ is generally denoted by ⁠ | A | {\displaystyle \vert A\vert } ⁠, with a vertical bar on each side, though it may also be denoted by A {\displaystyle A} , card ⁡ ( A ) , {\displaystyle \operatorname {card} (A),} or # A . {\displaystyle \#A.} Cardinality is defined in terms of bijective functions. Two sets have the same cardinality if, and only if, there is a one-to-one correspondence (bijection) between the elements of the two sets. The cardinality of a finite set can be identified...

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