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Permutation  ï»¿

A permutation, also called an "arrangement number" or "order," is a rearrangement of the elements of an ordered list S into a one-to-one correspondence with S itself. The number of permutations on a set of nelements is given by n! (n factorial;  For example, there are  2!=2·1=2 permutations of {1,2}, namely {1,2}and {2,1}, and 3!=3·2·1=6 permutations of {1,2,3}, namely {1,2,3}, {1,3,2}, {2,1,3}, {2,3,1}, {3,1,2}, and {3,2,1}.  The number of ways of obtaining an ordered subset of k elements from a set of n elements is given by