Permutations and Combinations

A permutation is a particular sequence of a given set of objects. A combination is the set itself without reference to order.

1. The number of different permutations of n distinct objects taken r at a time is



The number of different combinations of n distinct objects taken r at a time is



The number of different permutations of n objects taken n at a time, given that n_i are of type i, where i = 1, 2,..., k and Σn_i = n, is

See also: Probability.