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.