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ordinal number

A number used to give the position in an ordered sequence: first, second, third, fourth ... Ordinal numbers are distinct from cardinal numbers (one, two, three, four, ...), which describe the size of a collection. The mathematician Georg Cantor showed in 1897 how to extend the concept of ordinals beyond the natural numbers to the infinite and how to do arithmetic with the resulting transfinite ordinals (see infinity).

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