arXiv — Machine Learning · · 3 min read

Weisfeiler-Leman Is Incomplete on Simple Spectrum Graphs, so Canonicalize Them

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Computer Science > Machine Learning

arXiv:2605.23446 (cs)
[Submitted on 22 May 2026]

Title:Weisfeiler-Leman Is Incomplete on Simple Spectrum Graphs, so Canonicalize Them

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Abstract:Graphs with a simple spectrum admit cubic-time isomorphism testing, yet we prove that for every natural number $k$, the $k$-Weisfeiler-Leman ($k$-WL) test cannot distinguish all non-isomorphic graphs with a simple spectrum. As the WL hierarchy upper-bounds the distinguishing power of widely-used Graph Neural Networks (GNNs), this incompleteness applies to all such GNNs, ruling out completeness for every $k$-WL-aligned GNN family. To close this gap, we introduce PRiSM (Partition, Refine, Solve, Match), the first provably complete canonicalization of simple-spectrum eigendecompositions. PRiSM obtains the completeness guarantee that prior canonicalizations provably lack, and resolves the open problem of achieving complete expressivity on simple-spectrum graphs. When composed with DeepSets or a Transformer, PRiSM achieves universal approximation on simple-spectrum graphs, justifying the use of canonicalized Laplacian positional encodings. Empirically, PRiSM performs comparably to or outperforms existing spectral canonicalizations on graph regression, classification, and expressivity
Subjects: Machine Learning (cs.LG); Combinatorics (math.CO)
Cite as: arXiv:2605.23446 [cs.LG]
  (or arXiv:2605.23446v1 [cs.LG] for this version)
  https://doi.org/10.48550/arXiv.2605.23446
arXiv-issued DOI via DataCite (pending registration)

Submission history

From: Snir Hordan [view email]
[v1] Fri, 22 May 2026 10:01:28 UTC (114 KB)
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