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REVIEW 2 major objections 1 minor 5 references

Local Hessian Spectral Filtering for Robust Intrinsic Dimension Estimation

T0 review · 2 major / 1 minor · reviewed 2026-07-01 · grok-4.3

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Pith's one-line read Spectral filtering of the log-density Hessian cuts off large eigenvalues from normal directions to count zero-curvature tangent directions and estimate local intrinsic dimension.

desk verdict LHSD adds a Hessian spectral cutoff plus SLQ to estimate local intrinsic dimension in high-dim diffusion models, but the approach rests on an unverified gap between normal and tangent eigenvalues. read the letter →

arxiv 2605.01221 v2 pith:IEFRZGWU submitted 2026-05-02 cs.LG

Genki Osada This is my paper · ORCID
classification cs.LG
keywords localintrinsicdimensionhessianspectralfilteringlog-densitydiffusionmodelsstochasticlanczosquadraturehigh-dimensionalmanifoldsmemorizationdetectionmanifoldlearning
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Existing local intrinsic dimension estimators lose accuracy in high-dimensional spaces because noise along the many normal directions swamps the tangent-space signal. The paper shows that applying an explicit spectral cutoff to the largest eigenvalues of the log-density Hessian leaves only the near-zero eigenvalues that mark the intrinsic dimension. This filtering step is carried out with Stochastic Lanczos Quadrature so that the method scales linearly with ambient dimension instead of building full Hessians. Tests on synthetic manifolds, real datasets, and large diffusion models confirm that the filtered count remains reliable where prior techniques fail, including for spotting memorization.

What carries the argument

Spectral filtering on the log-density Hessian that partitions eigenvalues into large normal and near-zero tangent groups to count the zero-curvature directions.

What would settle it

A controlled high-dimensional dataset whose known intrinsic dimension is mismatched by the filtered eigenvalue count because the log-density Hessian spectrum lacks a clear gap between normal and tangent eigenvalues.

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Extended reading notes

Core claim

The central claim is that an explicit cutoff applied to the large eigenvalues of the log-density Hessian, which correspond to normal directions, isolates the count of near-zero eigenvalues that mark the zero-curvature tangent directions, thereby recovering the local intrinsic dimension even when normal noise dominates.

Load-bearing premise

The eigenvalues of the log-density Hessian can be partitioned into two clean groups, large values from normal directions and near-zero values from tangent directions, so that an explicit cutoff isolates the intrinsic dimension without distorting the count.

Editorial extensions

If this is right

  • The method achieves linear scaling with dimension D by using Stochastic Lanczos Quadrature without constructing full Hessians.
  • LHSD shows superior robustness over prior LID estimators on both synthetic and real data.
  • The filtered dimension estimates can be used to detect memorization inside large-scale diffusion models.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The same eigenvalue-partitioning step could be inserted into other curvature-based manifold algorithms that currently rely on raw Hessians.
  • Performance would degrade if the eigenvalue gap between normal and tangent directions narrows, which could be checked by gradually increasing ambient dimension while holding intrinsic dimension fixed.
  • The approach might also serve as a diagnostic for the effective dimensionality of learned representations inside other generative models.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 1 minor

Summary. The manuscript proposes Local Hessian Spectral Dimension (LHSD) for estimating local intrinsic dimension (LID) in high-dimensional data, particularly diffusion models. It claims existing LID methods fail when noise from normal directions overwhelms the tangent signal, and resolves this by spectral filtering on the log-density Hessian that explicitly cuts off large eigenvalues to count zero-curvature tangent directions. The approach is implemented via Stochastic Lanczos Quadrature (SLQ) for linear scalability in dimension D. Experiments on synthetic and real data are reported to confirm superior robustness and utility for detecting memorization in large-scale diffusion models, with code released at github.com/geosada/LHSD.

Significance. If the eigenvalue filtering step reliably isolates the intrinsic dimension, LHSD would provide a scalable alternative for LID estimation in regimes where prior methods degrade, with direct applications to analyzing generative models and memorization. The open-source code release is a clear strength supporting reproducibility.

major comments (2)
  1. [Abstract] Abstract: The central claim that 'explicitly cutting off large eigenvalues associated with normal directions to count zero-curvature tangent directions' requires a reliable spectral gap in the eigenvalues of the log-density Hessian separating large normal-space values from near-zero tangent values; no derivation, conditions for gap existence, or analysis of cutoff sensitivity is indicated, which is load-bearing for the robustness guarantee.
  2. [Method] Method description (SLQ implementation): The filtered count approximated by SLQ inherits error from any misclassification due to absent or noise-induced gaps; the manuscript provides no error bounds, parameter-free justification for the cutoff, or ablation showing the count remains undistorted when the gap is marginal.
minor comments (1)
  1. [Abstract] The abstract states experiments confirm 'superior robustness' but does not name the specific baselines, datasets, or quantitative metrics (e.g., error vs. dimension or noise level) used for comparison.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments. We address each major comment below and will revise the manuscript to strengthen the presentation of the spectral gap assumption and related analysis.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim that 'explicitly cutting off large eigenvalues associated with normal directions to count zero-curvature tangent directions' requires a reliable spectral gap in the eigenvalues of the log-density Hessian separating large normal-space values from near-zero tangent values; no derivation, conditions for gap existence, or analysis of cutoff sensitivity is indicated, which is load-bearing for the robustness guarantee.

    Authors: We agree that the method's robustness claim rests on the existence of a spectral gap between normal and tangent eigenvalues of the log-density Hessian. The manuscript invokes the standard manifold hypothesis but does not derive conditions for the gap or analyze cutoff sensitivity. In revision we will add a dedicated subsection deriving the gap under additive isotropic Gaussian noise (showing separation scales with the normal-space curvature) and include a sensitivity study varying the cutoff around the observed gap. revision: yes

  2. Referee: [Method] Method description (SLQ implementation): The filtered count approximated by SLQ inherits error from any misclassification due to absent or noise-induced gaps; the manuscript provides no error bounds, parameter-free justification for the cutoff, or ablation showing the count remains undistorted when the gap is marginal.

    Authors: The current SLQ implementation approximates the count of eigenvalues below a fixed threshold chosen from the median of the spectrum; no formal error bounds or adaptive (parameter-free) rule are supplied. We will revise the Method section to state the cutoff heuristic explicitly, add a brief discussion of approximation error when the gap narrows, and include new ablation experiments that inject controlled noise to shrink the gap and measure distortion in the estimated dimension. revision: yes

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: LHSD is a direct algorithmic proposal with no self-referential reduction

full rationale

The paper presents LHSD as a computational procedure that applies spectral filtering to the log-density Hessian and uses SLQ for implementation. No equations, derivations, or claims are shown that reduce the output dimension count to a fitted parameter, self-citation chain, or input by construction. The method is defined independently via the filtering step on the Hessian spectrum, and the abstract and description contain no load-bearing self-citations or renamings of known results. This is a standard case of a self-contained proposal.

Assumptions & free parameters 0 free parameters · 1 assumptions · 0 invented entities

Abstract-only review; the ledger records the single domain assumption required for the filtering step to function as described.

assumptions (1)
  • domain assumption Eigenvalues of the log-density Hessian separate into large values corresponding to normal directions and near-zero values corresponding to tangent directions, allowing an explicit spectral cutoff to recover intrinsic dimension.
    This separation is the explicit premise of the spectral filtering procedure stated in the abstract.

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Cite this review

Pith. "Pith review of Local Hessian Spectral Filtering for Robust Intrinsic Dimension Estimation." pith.science (2026). https://pith.science/paper/IEFRZGWU

@misc{pith2026260501221,
  author       = {Pith},
  title        = {Pith review of: Local Hessian Spectral Filtering for Robust Intrinsic Dimension Estimation},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/IEFRZGWU}},
  note         = {Machine review of arXiv:2605.01221}
}
abstract

While diffusion models enable new approaches for estimating Local Intrinsic Dimension (LID), existing methods fail in high-dimensional spaces where noise from vast normal directions overwhelms the tangent signal. We propose Local Hessian Spectral Dimension (LHSD), which resolves this by applying spectral filtering to the log-density Hessian, explicitly cutting off large eigenvalues associated with normal directions to count zero-curvature tangent directions. Implemented using Stochastic Lanczos Quadrature (SLQ), LHSD avoids full Hessian construction, achieving linear scalability with dimension $D$. Experiments on synthetic and real data confirm LHSD's superior robustness and its utility in detecting memorization in large-scale diffusion models. The code is available at github.com/geosada/LHSD

Figures

Figures reproduced from arXiv: 2605.01221 by the authors.

Figure 1
Figure 1. Synthetic Manifold: Moon (left), Funnel (center), and L 1+2+3 (right). The L 1+2+3 dataset consists of 3D cube, 2D square, and 1D line submanifolds. The overlaid colors indicate the LID estimated by our LHSD, accurately recovering the underlying manifold dimensionalities. leading to severe degradation in estimation accuracy. To address this, approaches utilizing deep generative mod￾els, particularly diffusion models… view at source ↗
Figure 2
Figure 2. LHSD Filter Behavior (Eq. (9)). Filter responses f(λ) with varying parameters are overlaid on the Hessian spectrum of L 900 ⊂ R 3072 dataset. Increasing c shifts the cutoff κ rightward, while increasing p steepens the transition. The cutoff consistently falls within the spectral gap (noise scale t = 0.04). becomes isotropic and the spectral gap collapses; the spec￾tral collapse and its detectability are examined in … view at source ↗
Figure 3
Figure 3. Selection of t on L 900 ⊂ R 3072 for filter with c = 0.1 and p = 4. (a): Transition mass M(t) identifies a “safe zone” (blue bar): the valley where M(t) ≈ 0 between two collision peaks. See view at source ↗
Figures from the paper (19 more)
Figure 4
Figure 4. Figure 4: LID estimation by LHSD on (Left) L 900 ⊂ R 3072 with MAE 11.53, and (Right) F 10+80+200 ⊂ R 3072 with MAE 5.19. lies within the spectral gap (
Figure 7
Figure 7. Figure 7
Figure 6
Figure 6. Figure 6: Robustness to noise scale t. LHSD (blue) maintains consistently low estimation error (MAE) across a wide range of noise scales on all datasets. settings: (1) the original 3D space (D = 3), and (2) a high￾dimensional embedding (D = 784) obtained by nonlinearly mapping t…
Figure 8
Figure 8. Figure 8: Comparison of generated samples with the lowest and highest LID values estimated by LHSD. SVHN, CIFAR-10 (‘airplane’ class), and LAION-Aesthetics are shown from top to bottom. LHSD successfully distinguishes between visually simple images (e.g., flat backgrounds, singl…
Figure 10
Figure 10. Figure 10: The PNG complexity distributions (left) for the Mem￾orized and Non-memorized sets overlap significantly, ruling out image complexity as a confounding factor. Under this controlled condition, the Estimated LID (right) shows a distinct separation, demonstrating that LHS…
Figure 11
Figure 11. Figure 11: Visualization of Hessian spectral separation computed on 20 randomly sampled data points for each dataset. In all shown cases, the noise scale t is chosen such that the filter cutoff κ (dashed line) is correctly positioned within the spectral gap between tangent and n…
Figure 12
Figure 12. Figure 12: Selection of t on F 10+25+50 ⊂ R 100 (Top) and F 10+80+200 ⊂ R 1024 (Bottom). (a),(e): Transition mass M(t) identifies a “safe zone” (blue bar). (b)–(d) and (f)–(h): Cases with t selected outside the safe zone. See Figs. 11b and 11e for selected safe setting. E. Spect…
Figure 13
Figure 13. Figure 13: illustrates this phenomenon on the F 900 ⊂ R 3072 and F 10+25+50 ⊂ R 100 datasets. For F 900 ⊂ R 3072, a distinct spectral gap persists at t = 0.60, allowing the filter to isolate tangent components, whereas at t = 0.65, the clusters collide, closing the spectral gap.…
Figure 14
Figure 14. Figure 14: Sensitivity analysis of LHSD with respect to Lanczos steps m. Top panels (a-d) show the MAE across noise scales t for varying m. The bottom panel (e) compares the inference time. Increasing m improves accuracy up to a saturation point around m = 5, beyond which comput…
Figure 15
Figure 15. Figure 15: Histograms of LID estimated by LHSD. The peaks of the distributions closely align with the ground-truth dimensions of the underlying submanifolds (e.g., peaks at 10, 80, and 200 for L 10+80+200), demonstrating the accuracy of the estimation. J. Details of Runtime Meas…
Figure 16
Figure 16. Figure 16: Noise scale sensitivity: LHSD (ours) on Moon. 22
Figure 17
Figure 17. Figure 17: Noise scale sensitivity: FLIPD on Moon. 23
Figure 18
Figure 18. Figure 18: Noise scale sensitivity: NB on Moon. 24
Figure 19
Figure 19. Figure 19: Noise scale sensitivity: LHSD (ours) on Funnel. 25
Figure 20
Figure 20. Figure 20: Noise scale sensitivity: FLIPD on Funnel. 26
Figure 21
Figure 21. Figure 21: Noise scale sensitivity: NB on Funnel. 27
Figure 22
Figure 22. Figure 22: Noise scale sensitivity: LHSD (ours) on L 1+2+3 . 28
Figure 23
Figure 23. Figure 23: Noise scale sensitivity: FLIPD on L 1+2+3 . 29
Figure 24
Figure 24. Figure 24: Noise scale sensitivity: NB on L 1+2+3 . 30

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Works this paper leans on

5 extracted references · 5 canonical work pages

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    URL https://openreview.net/forum? id=ZEf03Uunvk. Ansuini, A., Laio, A., Macke, J. H., and Zoccolan, D. In- trinsic dimension of data representations in deep neural networks. InAdvances in Neural Information Processing Systems, volume 32, pp. 6111–6122. Curran Associates, Inc., 2019. Bac, J., Mirkes, E. M., Gorban, A. N., Tyukin, I., and Zi- novyev, A. Sci...

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    safe zone

    doi: 10.1609/aaai.v39i19.34299. Ubaru, S., Chen, J., and Saad, Y . Fast estimation of $tr(f(a))$ via stochastic lanczos quadrature.SIAM Journal on Ma- trix Analysis and Applications, 38(4):1075–1099, 2017. doi: 10.1137/16M1104974. Ventura, E., Achilli, B., Silvestri, G., Lucibello, C., and Ambrogioni, L. Manifolds, random matrices and spectral gaps: The g...

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    Using a single class ensures the generated images possess a coherent visual structure

    Basis Construction:We compute the Principal Component Analysis (PCA) of a single FMNIST class (Class 7: Sneakers) to extract a mean image µ∈R 784 and a basis matrix of principal components U∈R 784×K. Using a single class ensures the generated images possess a coherent visual structure

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    Random Fourier Feature Mapping:To ensure the embedding is non-linear and smooth, the low-dimensional input x is mapped to a higher-dimensional feature vector ϕ(x)∈R K. This is achieved using Random Fourier Features (RFF) combined with bias terms: ϕ(x) = sin(Wx+b),cos(Wx+b),1,∥x∥ 2 (28) where W∈R K′×d and b∈R K′ are fixed weights drawn from a normal and un...

    work page 2025

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