The Hankel determinant periodicity and recurrence conjecture for q-deformed metallic numbers

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Let kk be a positive integer, and for ll∈Z≥0ll\in\mathbb{Z}_{\geq 0} let

Δn(ℓ):=Δn(ℓ)([yk]q)\Delta_n^{(\ell)}:=\Delta_n^{(\ell)}\bigl(\left[y_k\right]_q\bigr)

denote the ℓ\ell-shifted sequence of Hankel determinants associated with the qq-deformation of the metallic number yky_k.

Hankel determinant periodicity and recurrence conjecture. The k+2k+2 sequences Δn(0),Δn(1),…,Δn(k+1)\Delta_n^{(0)},\Delta_n^{(1)},\dots,\Delta_n^{(k+1)} take only the values −1,0,1-1,0,1. They are 2k(k+1)2k(k+1)-periodic when kk is even and 2k(k+1)2k(k+1)-antiperiodic, hence 4k(k+1)4k(k+1)-periodic, when kk is odd:

Δn+2k(k+1)(ℓ)=(−1)kΔn(ℓ)for all n∈Z≥0.\Delta_{n+2k(k+1)}^{(\ell)}=(-1)^k\Delta_n^{(\ell)}\quad\text{for all }n\in\mathbb{Z}_{\geq 0}.

Moreover, all these sequences satisfy

Δn+2k+2(ℓ)Δn(ℓ)=Δn+2k+1(ℓ)Δn+1(ℓ)−(Δn+k+1(ℓ))2for all n∈Z≥0.\Delta_{n+2k+2}^{(\ell)}\Delta_n^{(\ell)}=\Delta_{n+2k+1}^{(\ell)}\Delta_{n+1}^{(\ell)}-\bigl(\Delta_{n+k+1}^{(\ell)}\bigr)^2\quad\text{for all }n\in\mathbb{Z}_{\geq 0}.

For ℓ=1,2,…,k+1\ell=1,2,\ldots,k+1, the consecutive shifted sequences are interconnected by

Δn(ℓ)=(−1)n+k(k+2ℓ+1)2Δn+k+1(ℓ−1)for all n∈Z≥0.\Delta_n^{(\ell)}=(-1)^{n+\frac{k(k+2\ell+1)}{2}}\Delta_{n+k+1}^{(\ell-1)}\quad\text{for all }n\in\mathbb{Z}_{\geq 0}.

The conjecture arose from computer experimentation and concerns the simultaneous integrality, periodicity, Somos-type recurrence, and relations among shifted Hankel determinant sequences associated with qq-deformed metallic numbers. The supplied context does not state whether it has been proved or disproved.

References

Primary source

Valentin Ovsienko and Emmanuel Pedon, “Continued fractions for q-deformed real numbers, \-1,0,1\-Hankel determinants, and Somos-Gale-Robinson sequences”, arXiv:2312.17009 (2024).

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