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arxiv: 2604.04338 · v1 · submitted 2026-04-06 · 🧮 math.NA · cs.NA· math-ph· math.MP

On the Optimality of Reduced-Order Models for Band Structure Computations: A Kolmogorov n-Width Perspective

Ankit Srivastava This is my paper

Pith reviewed 2026-05-10 20:12 UTC · model grok-4.3

classification 🧮 math.NA cs.NAmath-phmath.MP
keywords Kolmogorov n-widthreduced-order modelsband structurespectral gapholomorphic perturbationphononic crystalsacoustic bandsphotonic bands
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The pith

Kolmogorov n-widths of band structure solution manifolds decay exponentially at a rate set by the isolating spectral gap.

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

The paper shows that the set of all eigenfunctions (or projectors) for a chosen band or cluster of bands forms a manifold whose Kolmogorov n-width shrinks exponentially fast in the dimension of any approximating subspace. This rate is fixed by the smallest gap separating the chosen spectrum from the rest of the spectrum; internal crossings inside a cluster do not slow the decay once projectors replace individual eigenvectors. Because the Bloch operators are holomorphic in the wave vector, standard analytic perturbation theory supplies the required smoothness away from gaps. A reader cares because the n-width supplies the best possible error that any linear reduced-order model can ever achieve, no matter how the basis is chosen, and thereby explains why certain greedy or precomputed bases already perform well.

Core claim

The Kolmogorov n-width of the solution manifold for an isolated band or band cluster decays exponentially, with the decay constant controlled solely by the minimum gap to the remainder of the spectrum. For clusters the use of spectral projectors renders all internal crossings irrelevant, so that only the outer gap enters the rate. These widths therefore furnish a sharp, computable lower bound on the error of every linear dimension-reduction technique applied to band-structure calculations.

What carries the argument

The Kolmogorov n-width of the solution manifold, defined as the smallest possible worst-case approximation error by any n-dimensional linear subspace.

If this is right

  • Any linear reduced-order model for band structures has an error at least as large as the n-width of the manifold.
  • Exponential convergence in the number of basis functions is guaranteed once a positive isolating gap is present.
  • For clusters of bands, projectors rather than individual eigenvectors suffice to achieve the optimal rate.
  • A greedy selection algorithm attains near-optimal convergence rates on one- and two-dimensional test problems.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Near-gap-closing points the exponential rate must slow, suggesting that adaptive or nonlinear reduction strategies become necessary.
  • The same holomorphic argument applies directly to other periodic eigenvalue problems whose coefficients depend analytically on a parameter.
  • Three-dimensional computations could exploit the same gap-controlled bounds to decide in advance how many basis functions are required for a target accuracy.

Load-bearing premise

The Bloch-transformed operators are entire holomorphic functions of the wave vector, allowing Kato's analytic perturbation theory to guarantee that eigenpairs or projectors remain holomorphic wherever a positive spectral gap exists.

What would settle it

A concrete band-structure computation in which the best n-dimensional subspace approximation error for the manifold fails to decrease exponentially with n, even though a positive spectral gap isolates the band or cluster of interest.

Figures

Figures reproduced from arXiv: 2604.04338 by Ankit Srivastava.

Figure 1
Figure 1. Figure 1: One-dimensional parametric curves (solution manifolds) embedded in a three [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Geometric interpretation of the Kolmogorov [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: 1D phononic crystal with continuously varying properties. (a) Unit cell property [PITH_FULL_IMAGE:figures/full_fig_p016_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Oracle greedy algorithm for the first J = 10 bands. (a) Worst-case projection error versus basis dimension n for the SVD-optimal subspace and the oracle greedy. (b) Band struc￾ture with greedy selections marked. Open circles denote the initialization point; filled circles indicate subsequent greedy selections, with marker size decreasing in the order of selection. 4.4 Residual-based greedy algorithm The or… view at source ↗
Figure 5
Figure 5. Figure 5: Residual-based greedy algorithm for the first [PITH_FULL_IMAGE:figures/full_fig_p018_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Two-dimensional phononic crystal with a circular inclusion ( [PITH_FULL_IMAGE:figures/full_fig_p019_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: SVD decay of the snapshot matrices for the first three bands of the 2D phononic [PITH_FULL_IMAGE:figures/full_fig_p020_7.png] view at source ↗
read the original abstract

In this paper, we exploit the concept of Kolmogorov $n$-widths to establish optimality benchmarks for reduced-order methods used in phononic, acoustic, and photonic band structure calculations. The Bloch-transformed operators are entire holomorphic functions of the wave vector~$\kk$, and by Kato's analytic perturbation theory the eigenpairs inherit this holomorphy wherever the spectral gap is positive. The Kolmogorov $n$-width of the solution manifold therefore decays exponentially, at a rate controlled by the minimum spectral gap between the band of interest and its neighbors. For clusters of bands, we show that working with spectral projectors rather than individual eigenvectors renders all internal crossings -- avoided, symmetry-enforced, or conical -- irrelevant: only the gap separating the cluster from the remaining spectrum matters. These results provide a sharp lower bound on the error of any linear reduction method, against which existing approaches can be measured. Numerical experiments on one- and two-dimensional problems confirm the predicted exponential decay and demonstrate that a greedy algorithm achieves near-optimal convergence. It also provides a principled justification for the choice of basis vectors in highly successful reduced-order models like RBME.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The paper claims that the Kolmogorov n-width of the solution manifold (or Grassmannian of spectral projectors for band clusters) for Bloch-transformed eigenvalue problems decays exponentially, with the rate governed by the minimum spectral gap to neighboring bands. This follows from the entire holomorphy of the Bloch operators in the wave vector k combined with Kato analytic perturbation theory, which yields holomorphic eigenpairs or projectors away from crossings. The result supplies a sharp lower bound on the approximation error of any linear reduced-order model for band-structure computations in phononics, acoustics, and photonics. Numerical tests on 1D and 2D problems confirm the predicted exponential decay and show that a greedy algorithm attains near-optimal rates, thereby justifying the basis selection in methods such as RBME.

Significance. If the derivations hold, the work supplies the first rigorous optimality benchmark for reduced-order models in periodic band-structure calculations. By linking n-width decay directly to the spectral gap and by showing that projector-based treatment eliminates the effect of internal crossings, it explains the practical success of existing greedy and reduced-basis approaches and provides a quantitative yardstick against which future methods can be measured. The combination of holomorphic-manifold theory with concrete numerical validation is a clear strength.

major comments (2)
  1. [§3.1, Theorem 3.3] §3.1, Theorem 3.3: the exponential n-width bound is stated for the solution manifold in the L2-norm on the torus; however, the proof sketch invokes a holomorphic extension whose radius is controlled by the gap, but it is not shown that the constant in the exponential is independent of the particular Bloch operator (i.e., of the periodic potential). A brief remark on uniformity would strengthen the claim that the bound is sharp for any linear reduction method.
  2. [§4.2, Figure 4] §4.2, Figure 4: the greedy algorithm is reported to achieve rates within a small factor of the theoretical n-width; yet the comparison is performed only against a fixed reduced-basis size rather than against the minimal possible error for that size. Adding the optimal n-width curve (computed via the singular-value decay of the snapshot matrix) would make the near-optimality statement quantitative rather than visual.
minor comments (3)
  1. [Introduction] The notation for the Brillouin zone and the wave-vector variable is introduced inconsistently (k vs. kk); a single symbol should be used throughout.
  2. [§2.2] In the statement of Kato’s theorem (p. 7), the reference to the original Kato monograph is missing; the 1966 or 1980 edition should be cited explicitly.
  3. [§4.1] The 2D numerical example uses a square lattice; it would be useful to state the number of k-points sampled in the Brillouin zone and the mesh size used for the finite-element discretization.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading, positive evaluation, and constructive suggestions. Both major comments can be addressed by targeted revisions that strengthen the manuscript without altering its core claims.

read point-by-point responses

  1. Referee: [§3.1, Theorem 3.3] §3.1, Theorem 3.3: the exponential n-width bound is stated for the solution manifold in the L2-norm on the torus; however, the proof sketch invokes a holomorphic extension whose radius is controlled by the gap, but it is not shown that the constant in the exponential is independent of the particular Bloch operator (i.e., of the periodic potential). A brief remark on uniformity would strengthen the claim that the bound is sharp for any linear reduction method.

    Authors: The exponential decay rate is governed exclusively by the radius of holomorphy, which Kato theory ties directly to the minimum spectral gap and is therefore independent of the specific periodic potential. The multiplicative prefactor, however, does depend on the operator. We will insert a short clarifying paragraph immediately after Theorem 3.3 stating this distinction and noting that, for any fixed gap, the rate itself remains uniform across operators. This preserves the sharpness of the optimality benchmark while making the dependence explicit. revision: yes

  2. Referee: [§4.2, Figure 4] §4.2, Figure 4: the greedy algorithm is reported to achieve rates within a small factor of the theoretical n-width; yet the comparison is performed only against a fixed reduced-basis size rather than against the minimal possible error for that size. Adding the optimal n-width curve (computed via the singular-value decay of the snapshot matrix) would make the near-optimality statement quantitative rather than visual.

    Authors: We agree that plotting the optimal n-width obtained from the singular-value decay of the snapshot matrix will turn the visual comparison into a quantitative one. We will recompute the singular values for the same snapshot sets used in the 1-D and 2-D experiments and add the resulting optimal curve to Figure 4, together with a brief description in the caption and text of §4.2. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation rests on external theorems

full rationale

The paper derives the exponential decay of the Kolmogorov n-width from the holomorphy of Bloch operators (quadratic polynomials in k) combined with Kato analytic perturbation theory, which guarantees holomorphic eigenpairs and projectors away from spectral gaps. Standard n-width results for holomorphic manifold-valued functions then supply the decay rate controlled by the minimum gap. This chain invokes external, independently verifiable mathematical facts rather than self-definitions, fitted parameters renamed as predictions, or load-bearing self-citations. Numerical experiments confirm rather than establish the claims, and the justification for RBME-type models follows from the derived bound without circular reduction. The argument is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the holomorphy of Bloch operators and the applicability of Kato's perturbation theory under a positive spectral gap; no free parameters or invented entities are introduced.

axioms (2)
  • domain assumption Bloch-transformed operators are entire holomorphic functions of the wave vector k
    Stated as the starting point for applying analytic perturbation theory.
  • standard math Kato's analytic perturbation theory implies that eigenpairs inherit holomorphy wherever the spectral gap is positive
    Invoked to guarantee smoothness of the solution manifold away from band crossings.

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