How many non-decreasing sequences a1, a2, . . . , a10 are composed entirely of at most three distinct numbers from the set {1, 2, . . . , 9}? For example, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3 and 2, 2, 2, 2, 5, 5, 5, 5, 5, 5 are both allowed.

Guest Mar 20, 2021