The conjecture of Q\mathbb{Q}-independent spectrum for metric graphs

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Let Γ\Gamma be a metric graph with NN edges and edge-length vector ℓ∈R+N\boldsymbol{\ell}\in\mathbb{R}_{+}^{N}. Write the non-zero square-root eigenvalues in increasing order as

k1≤k2≤k3≤⋯↗∞.k_{1}\le k_{2}\le k_{3}\le\cdots\nearrow\infty.

Q\mathbb{Q}-independent spectrum conjecture. For every graph Γ\Gamma, possibly except for some pathological cases, there is a generic set G⊂R+NG\subset\mathbb{R}_{+}^{N} such that, for every ℓ∈G\boldsymbol{\ell}\in G, the spectrum spec⁡(Γ,ℓ)\operatorname{spec}(\Gamma,\boldsymbol{\ell}) is linearly independent over Q\mathbb{Q}. Equivalently, for every n∈Nn\in\mathbb{N} and every q=(q1,…,qn)∈Qn∖{0}\boldsymbol{q}=(q_{1},\ldots,q_{n})\in\mathbb{Q}^{n}\setminus\{0\},

∑j=1nkjqj≠0.\sum_{j=1}^{n}k_{j}q_{j}\ne0.

Kurasov and Sarnak established that the spectrum has infinite dimension over Q\mathbb{Q} when the edge lengths are Q\mathbb{Q}-independent; the conjecture asks for full linear independence for generic edge lengths. The source presents this as future work and gives no resolution.

References

Primary source

Lior Alon, “Generic Laplace eigenfunctions on metric graphs”, arXiv:2203.16111 (2022).

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