Spectral support-forest-profile conjecture for trees

From papers

Let T1T_1 and T2T_2 be trees on nn vertices. The (n2,2)(n-2,2) support-forest profile is the collection of universal support-forest-counting quantities obtained from the moments Mr(2)(T)M_r^{(2)}(T) for 1rdimWn1\le r\le \dim W_n. The Laplacian spectrum is the spectrum in the (n1,1)(n-1,1) representation, and the (n2,2)(n-2,2)-spectrum is the spectrum in the (n2,2)(n-2,2) representation.

Spectral support-forest-profile conjecture. If T1T_1 and T2T_2 have the same Laplacian spectrum and the same (n2,2)(n-2,2) support-forest profile, then T1T_1 and T2T_2 are isomorphic. Equivalently, no two non-isomorphic trees have identical (n1,1)(n-1,1)- and (n2,2)(n-2,2)-spectra.

For trees, the (n2,2)(n-2,2)-spectrum determines the support-forest profile, whose first nontrivial information beyond Laplacian data is a cubic combination of embedded forest counts. The conjecture asks whether the resulting hierarchy, together with Laplacian data, suffices for tree reconstruction.

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Sources & referencesView supporting material

Primary source

Boris Shapiro, “The (n-2,2)-Spectrum of a Graph”, arXiv:2605.17501 (2026).

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