Example, the set A = {2, 4, 6} contains 3 elements, and therefore A has a cardinality of 3. There are two approaches to cardinality – one which compares sets directly using bijections and injections, and another which uses cardinal numbers.The cardinality of a set is also called its size, when no confusion with other notions of size is possible.

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The cardinality of a set S, denoted |S|, is the number of elements in S. If the set has an infinite number of elements, then its cardinality is ∞. 

Example: |{1, 7, 3}| = 3
Example: |N| =