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

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

k1k2k3.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 GR+NG\subset\mathbb{R}_{+}^{N} such that, for every G\boldsymbol{\ell}\in G, the spectrum spec(Γ,)\operatorname{spec}(\Gamma,\boldsymbol{\ell}) is linearly independent over Q\mathbb{Q}. Equivalently, for every nNn\in\mathbb{N} and every q=(q1,,qn)Qn{0}\boldsymbol{q}=(q_{1},\ldots,q_{n})\in\mathbb{Q}^{n}\setminus\{0\},

j=1nkjqj0.\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.

Sources & referencesView supporting material

Primary source

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

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