Clark's infinite-lift conjecture for finite graphs

Let GG be a finite graph, and let HH be a lift of GG. Write spec(G)\mathsf{spec}(G) for the spectrum of GG, and let ρ(UCT(G))\rho(\mathsf{UCT}(G)) denote the relevant universal-cover spectral radius. An eigenvalue of HH in spec(H)spec(G)\mathsf{spec}(H)\setminus\mathsf{spec}(G) is called new. Clark's conjecture. For every finite GG, there are infinitely many lifts HH of GG such that every new eigenvalue λspec(H)spec(G)\lambda\in\mathsf{spec}(H)\setminus\mathsf{spec}(G) satisfies

λρ(UCT(G)).\lambda\leq\rho(\mathsf{UCT}(G)).

This generalizes the Bilu–Linial conjecture from 2-lifts to arbitrary lifts and would produce infinitely many Ramanujan graphs in relevant universal-cover settings. The supplied source does not state a resolution of this conjecture; its status is therefore left open.

Sources & referencesView supporting material

Primary source

Sidhanth Mohanty and Ryan O'Donnell, “X-Ramanujan Graphs”, arXiv:1904.03500 (2019).

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