Trees with exactly three main eigenvalues

arXiv:2607.13577 2026 Architecture 1 ideas extracted · analyzed Aug 30, 2026

What the math gives to ML

The transferable contribution is the characterization of main eigenvalues through the Krylov space generated by the adjacency matrix and the all-ones vector. If a graph has only k main eigenvalues, all global walk-count features j, Aj, A^2j, ... lie in a k-dimensional invariant subspace, allowing long-range structural information to be compressed exactly. This suggests a rank-adaptive positional encoder for graph neural networks that replaces many walk-count channels with an orthonormal basis of the main Krylov space. The strongest initial test is graph classification on trees and small graph benchmarks, comparing this basis against random-walk and Laplacian positional encodings.

Ideas from this paper

Unverified 2026

Main-Krylov Structural Encoder

Add a structural positional channel formed from the Krylov sequence generated by the graph adjacency matrix and the all-ones vector. For graphs with k main eigenvalues, this sequence has rank k, so a GNN can retain all information obtainable from global walk counts using only k node features rather than storing many adjacency powers.

Useful5/10
Difficulty4/10
Novelty7/10
Paper: Trees with exactly three main eigenvalues arXiv:2607.13577