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An unordered selection of r objects from a set of n (≥r) different objects. The number of different combinations is often denoted by nCr. In fact, is the binomial coefficient. Special values are nC0=1, nCn=1, nC1=n.

A frequently used relationship is


which is the defining relationship for Pascal's triangle. For ordered selection, see permutation.

Subjects: Computing — Probability and Statistics.

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