The conjectural formulas for earliest Thue–Morse progressions in particular families

From papers

Let t{\mathbf t} be the Thue–Morse word. For a positive integer difference dd, let A(d)A(d) be the length of the longest monochromatic arithmetic progression of difference dd, and let i(d)i(d) be the starting position of the first such progression of length A(d)A(d). For the infinite families of differences considered below, the authors conjecture that

The conjectural formulas for earliest Thue–Morse progressions.

i(2n+1)=322n2n1,i(2^n+1)=3\cdot 2^{2n}-2^n-1, i(22n1)=324n22n+1,i(2^{2n}-1)=3\cdot 2^{4n}-2^{2n}+1, i(22n+11)=22n+11.i(2^{2n+1}-1)=2^{2n+1}-1.

These identities are supported by empirical observations and are stated after known formulas for the corresponding values of A(d)A(d), but a proof remains elusive.

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Sources & referencesView supporting material

Primary source

Gandhar Joshi and Dan Rust, “Monochromatic arithmetic progressions in the Fibonacci, Thue-Morse, and Rudin-Shapiro words”, arXiv:2501.05830 (2025).

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