quality:corruption
Cardinality

The cardinality of a set S, denoted | S |, is the number of members of S. For example, if B = {blue, white, red}, then | B | = 3. Repeated members in roster notation are not counted, so | {blue, white, red, blue, white} | = 3, too.

More formally, two sets share the same cardinality if there exists a bijection between them.

The cardinality of the empty set is zero.