Cloitre's density conjecture for the self-generating sequence

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Let (an)n≥1(a_n)_{n \geq 1} be the unique sequence over the alphabet 1,2\\{1,2 \\}, beginning with a1=1a_1=1, whose runs have sums equal to twice the corresponding sequence terms:

(an)n≥1=1,1,⏟2 2,⏟2 1,1,1,1,⏟4 2,⏟2 1,1,⏟2 2,⏟2 1,1,⏟2 2,2,⏟4 1,1,⏟2 2,⏟2 1,1,1,1,⏟4…(a_n)_{n \geq 1}=\underbrace{1,1,}_2 \, \underbrace{2,}_2 \, \underbrace{1,1,1,1,}_4 \, \underbrace{2,}_2 \, \underbrace{1,1,}_2 \, \underbrace{2,}_2 \, \underbrace{1,1,}_2 \, \underbrace{2,2,}_4 \, \underbrace{1,1,}_2 \, \underbrace{2,}_2 \, \underbrace{1,1,1,1,}_4 \ldots

Cloitre's density conjecture. The number of 11's appearing in the prefix a1a2⋯ana_1a_2\cdots a_n is 2n/3+o(n)2n/3+o(n).

The paper proves the stronger bound 0≤3gn−2n≤40\leq 3g_n-2n\leq 4, where gng_n counts the 11's in the prefix, so the conjecture is resolved.

References

Primary source

Jeffrey Shallit, “Cloitre's Self-Generating Sequence”, arXiv:2501.00784 (2025).

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