If n is a list or set, then numbperm counts the permutations of the elements of n taken r at a time. If n is a non-negative integer, it is interpreted in the same way as a list of the first n integers. If r is not specified, it is taken to be .

The count of permutations takes into account duplicates in n. In the case where there are no duplicates, the count is given by the formula  . Otherwise the generating function is used.

The function permute will compute the number of permutations.  Thus numbperm(n, r) = nops(permute(n, r)).

The command with(combinat,numbperm) allows the use of the abbreviated form of this command.