Guiduli's spectral dense-neighborhood conjecture

Let GG be a graph on nn vertices, let Tr(n)T_r(n) denote the Turán graph, let N(v)N(v) denote the neighborhood of a vertex vv, let d(v)d(v) denote its degree, and let bb(G)bb(G) denote the spectral radius of GG. Guiduli's spectral dense-neighborhood conjecture. If

λ(G)λ(Tr(n)),\lambda(G)\geq\lambda(T_r(n)),

then either GTr(n)G\cong T_r(n), or there is a vertex vv such that

λ(G[N(v)])λ(Tr1(d(v))).\lambda(G[N(v)])\geq\lambda(T_{r-1}(d(v))).

Furthermore, if

λ(G)>λ(Tr(n)),\lambda(G)>\lambda(T_r(n)),

then the conclusion holds for any vertex vv having maximum weight given by a positive eigenvector for λ(G)\lambda(G). The conjecture is presented as a spectral analogue of the dense-neighborhood theorem of Bollobás and Thomason and Erdős and Sós, which proved the corresponding ordinary Turán statement. The supplied text does not state whether Guiduli's spectral conjecture has been resolved.

Sources & referencesView supporting material

Primary source

Lele Liu and Bo Ning, “On spectral Turán theorems: confirming a conjecture of Guiduli and two problems of Nikiforov”, arXiv:2605.05048 (2026).

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