Monotonicity conjecture for absolute values of weighted binary-sum Hankel determinants

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Let (nk)(n_k) be the sequence defined earlier in the paper, and let H(n,1)\mathcal H(n,1) be the associated Hankel determinant. Monotonicity conjecture. The sequence of absolute values (∣H(n,1)∣)n≥2(|\mathcal H(n,1)|)_{n\geq 2} is strictly increasing on each interval [nk,2k+1+1][n_k,2^{k+1}+1] and strictly decreasing on each interval [2k+1+1,nk+1][2^{k+1}+1,n_{k+1}]. In particular, for each k∈Nk\in\mathbb{N},

∣H(2k+1,1)∣=max⁡n≤2k+1∣H(n,1)∣.|\mathcal H(2^k+1,1)|=\max_{n\leq 2^k+1}|\mathcal H(n,1)|.

The claim is motivated by experimental computations showing long monotone subsequences and apparent local maxima and minima at the indicated indices. Its validity for all indices remains open.

References

Primary source

Bartosz Sobolewski and Maciej Ulas, “Hankel determinants of weighted binary sums of digits”, arXiv:2607.09376 (2026).

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