Cloitre's density conjecture for the self-generating sequence

Let (an)n1(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)n1=1,1,22,21,1,1,1,42,21,1,22,21,1,22,2,41,1,22,21,1,1,1,4(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 a1a2ana_1a_2\cdots a_n is 2n/3+o(n)2n/3+o(n).

The paper proves the stronger bound 03gn2n40\leq 3g_n-2n\leq 4, where gng_n counts the 11's in the prefix, so the conjecture is resolved.

Sources & referencesView supporting material

Primary source

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

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