In mathematics, the cardinality of a set is a measure of the "number of elements of the set". For 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 A is denoted | A |, with a vertical bar on each side; this is the same notation as absolute value and the meaning depends on context.
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