Problem 54660. Generate the Figure-Figure sequence

After discussing Scott Kim’s FIGURE-FIGURE Figure (below) in Gödel, Escher, Bach, Douglas Hofstadter introduced an integer sequence a (say) generated by this rule: it starts with 1, and each later term equals the sum of the previous term in a and the latest term that is not already contained the sequence a.
For example, the second term in the sequence is 3 because the first number not in the sequence is 2, and 1+2 = 3. The third term is 7 because the next term not in a is 4 and 3+4 = 7.
Not only is the complement of the sequence a equal to the differences between the terms, but together the two sequences contain all positive integers.
Write a function that returns the nth term of the sequence.
Solve

Solution Stats

66.67% Correct | 33.33% Incorrect
Last Solution submitted on Dec 18, 2025

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