The number of multisets of length k on n symbols is sometimes termed "n multichoose k, " denoted ((n k)) by analogy with the binomial coefficient (n k). n multichoose k is given by the simple formula ((n k)) = (n + k - 1 k) = (n - 1, k)!, where (n - 1, k)! is a multinomial coefficient. For example, 3 multichoose 2 is given by 6, since the possible multisets of length 2 on three elements {a, b, c} are {a, a}, {a, b}, {a, c}, {b, b}, {b, c}, and {c, c}.

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