Concept · Foundations
Cardinality
A measure of the size of a set; two sets have the same cardinality when there exists a bijection between them, and a set is countable when it has the same cardinality as the natural numbers.
Why it matters
Cardinality extends the everyday notion of 'how many' to infinite sets and is the setting in which Cantor's diagonal argument distinguishes the countable from the uncountable.
Where this shows up
Units and lessons using this concept
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