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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.

 Mar 20, 2021
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There are 582 such sequences.

 Mar 21, 2021

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