Ostrowski automaticity conjecture for records of Sn(2ξ)S_n(2\xi)

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For an irrational number ξ\xi, define the deterministic random walk by

Sn(2ξ)=∑j=1n(−1)⌊2jξ⌋.S_n(2\xi)=\sum_{j=1}^n(-1)^{\lfloor 2j\xi\rfloor}.

Call rr a record if Sr(2ξ)S_r(2\xi) has not occurred among the preceding partial sums, including S0(2ξ)=0S_0(2\xi)=0. A sequence is ξ\xi-Ostrowski automatic if a finite-state automaton decides membership in it from the ξ\xi-Ostrowski representation of an integer. Ostrowski automaticity conjecture. For every quadratic irrational ξ\xi, the records of Sn(2ξ)S_n(2\xi) are ξ\xi-Ostrowski automatic. This is presented as a variation of the Van de Lune–Arias de Reyna recurrence conjecture; the source gives explicit quadratic examples but leaves the general assertion open.

References

Primary source

Henk Bruin and Robbert Fokkink, “On the records and zeros of a deterministic random walk”, arXiv:2503.11734 (2025).

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