Limit-point conjecture for the top eigenvalue vectors of regular graphs

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Let d≥3d\geq 3 and kk be fixed. For a dd-regular graph GG, write λ1(G)≥λ2(G)≥⋯\lambda_1(G)\geq\lambda_2(G)\geq\cdots for its ordered eigenvalues. Define B(d,k)B(d,k) by

B(d,k)={(μ1,μ2,…,μk):d=μ1≥μ2≥μ3≥⋯≥μk≥2d−1}.B(d,k)=\{(\mu_1,\mu_2,\ldots,\mu_k):d=\mu_1\geq\mu_2\geq\mu_3\geq\cdots\geq\mu_k\geq 2\sqrt{d-1}\}.

Top-eigenvalue vector limit-point conjecture. For any d≥3d\geq 3 and any fixed kk, the set of all limit points of the vectors

(λ1(Gi),λ2(Gi),…,λk(Gi))(\lambda_1(G_i),\lambda_2(G_i),\ldots,\lambda_k(G_i))

along infinite sequences GiG_i of dd-regular graphs is exactly B(d,k)B(d,k). This extends the proved one-coordinate result for the second eigenvalue, while the realizability of all admissible vectors remains open.

References

Primary source

Noga Alon and Fan Wei, “The limit points of the top and bottom eigenvalues of regular graphs”, arXiv:2304.01281 (2023).

Additional references

5 papers in this index state this conjecture (2011–2023). The statement above is taken from the most recent of them; the others are arXiv:2204.05287, arXiv:1711.07996, arXiv:1608.08931, arXiv:1108.3805.

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