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))n2(|\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 kNk\in\mathbb{N},

H(2k+1,1)=maxn2k+1H(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.

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Primary source

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

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