Unique monotonic sequence of nonnegative integers satisfying a(a(n)) = 3n. 0, 2, 3, 6, 7, 8, 9, 12, 15, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 30, 33, 36, 39, 42, 45
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