REVIEW 4 major objections 4 minor 50 references
Reconstruction techniques for inverse Sturm-Liouville problems with complex coefficients
T0 review · 4 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read A direct linear-system method recovers complex-valued potentials and boundary constants from inverse Sturm-Liouville data.
desk verdict A solid, useful extension of the author's NSBF inverse spectral method; the IP4 reduction has a sign error in the printed derivative formula, but the core contribution deserves serious refereeing. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is the Neumann series of Bessel functions (NSBF) representation of solutions and their derivatives, for example $\phi_h(\rho,x)=\cos\rho x+\sum_{n=0}^\infty(-1)^n g_n(x)j_{2n}(\rho x)$, with analogous series for $S$, $\psi_H$, $T$, and their derivatives. These series converge uniformly in strips of the $\rho$-plane, and their first coefficients satisfy $\phi_h(0,x)=g_0(x)+1$ and $\psi_H(0,x)=\psi_0(x)+1$, which gives the recovery formulas for $q$, $h$, and $H$. The argument is carried by identity (3.2), $\psi_H=\Delta_0\phi_h-\Delta S$, which converts the given spectral data into linear equations for the needed coefficients, and by the companion identity (3.3) used for computing additional coefficients.
What would settle it
Construct a complex-valued potential with known spectra, run the method with several truncation orders, and check whether the $N_1$ that minimizes $R(N_1)$ also minimizes the actual reconstruction error in $q$; if the correlation fails on a single smooth example such as a complex Paine-type potential with complex $h$ and $H$, the accuracy-control claim is refuted.
Extended reading notes
Core claim
The central claim is that four standard inverse Sturm-Liouville problems with complex-valued potentials can be solved by the same two-step linear-algebra scheme. In the first step, the characteristic functions $\Delta(\rho)$ and $\Delta_0(\rho)$ are approximated from the given spectral data by solving linear systems for NSBF coefficients evaluated at the endpoint. In the second step, identity (3.2), which relates the solutions $\psi_H$, $\phi_h$, and $S$ through $\Delta_0$ and $\Delta$, is sampled at many points $\rho$ to produce another linear system for the coefficients $g_n(x)$ and $\psi_n(x)$ at each point $x$. The first coefficients then carry the whole answer: $q(x)=\frac{g_0''(x)}{g_0(x)+1}$, $h=g_0'(0)$, and $H=-\psi_0'(b)$. The paper further claims that the same construction works when the input is the Weyl function, a spectrum with multiplier constants, or a spectrum with norming constants, and that the truncation order can be selected by minimizing the residuals $R(N_1)$, $P(N_1)$, or $Q(N_1)$.
Load-bearing premise
The load-bearing premise is that the heuristic minimization of $R(N_1)$, $P(N_1)$, or $Q(N_1)$ picks a truncation order for which the truncated NSBF expansions are accurate over the whole interval; if it does not, the recovered $g_0$ and $\psi_0$ are wrong, and so is $q=g_0''/(g_0+1)$.
Editorial extensions
If this is right
- The boundary constants $h$ and $H$ are produced by the same computation as the potential, so inverse problems no longer have to be posed with known boundary conditions.
- Complex-valued potentials and complex boundary constants are within scope, not just real self-adjoint problems.
- Two-spectrum data, the Weyl function, spectrum with multiplier constants, and spectrum with norming constants are all handled by the same linear-algebra machinery.
- The residuals $R(N_1)$, $P(N_1)$, and $Q(N_1)$ give a practical, parameter-free way to choose the truncation order in place of ad hoc choices.
- The method is direct and non-iterative, and the reported examples run in seconds even with noisy input data.
Reading between the lines
- Because the recovery formulas use only the first NSBF coefficient, the same scheme should be testable on other boundary-value problems where NSBF representations exist, such as impedance-form equations or systems on graphs, without re-deriving the whole inverse theory.
- The accuracy of $q$ will be limited where $g_0$ is differentiated; for noisy data, recovering $q$ from $\psi_0$ or from a smoothed combination of $g_0$ and $\psi_0$ may reduce endpoint errors and is a natural numerical extension.
- The $R(N_1)$ heuristic could be validated statistically on random complex potentials; if it fails, the residual could be combined with cross-validation on a subset of the spectral data.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops a unified numerical method for several inverse Sturm-Liouville problems with complex-valued potentials: the two-spectrum problem (IP1), recovery from the Weyl function (IP2), recovery from eigenvalues and multiplier constants (IP3), and recovery from eigenvalues and norming constants (IP4). The method uses Neumann series of Bessel functions (NSBF) representations of solutions, derives linear algebraic systems for the first NSBF coefficients from identities (3.2) and (3.3), and then recovers the potential and boundary constants from the first coefficients via q(x)=g0''(x)/(g0(x)+1), h=g0'(0), H=-ψ0'(b). Numerical examples illustrate accuracy, including complex potentials and noisy data.
Significance. If the method is valid, it is a useful contribution: it treats complex-valued potentials and unknown boundary constants, unifies several inverse problems within one linear-algebra framework, and avoids iterative solvers. The derivation is transparent and mostly based on exact identities; the recovery formulas in Remark 3.3 and the reduction of IP4 to IP3 are elegant. The paper also proposes explicit residual-based criteria for choosing the truncation order, which is a practical step beyond earlier NSBF work. However, the numerical validation is partially self-referential, the truncation criterion is heuristic, and the recovery of q requires a second derivative whose numerical treatment is not specified. One displayed formula, Eq. (5.8), has a sign error that is load-bearing for IP4.
major comments (4)
- [§5.3, Eq. (5.8)] The derivative of the h_n term is printed with the wrong sign. Since d/dρ [h_n j_{2n}(ρb)] = h_n[(2n/ρ)j_{2n}(ρb) - b j_{2n+1}(ρb)], the last sum in Eq. (5.8) should be preceded by a minus sign; equivalently, the parentheses should read (2n/ρ)j_{2n}(ρb) - b j_{2n+1}(ρb). As printed, every n≥1 term of Δ̇(ρ_k) has the wrong sign, so β_k computed from Eq. (5.7) changes sign. Since β_k enters the linear system (5.5), a literal implementation of Eq. (5.8) would make the IP4 reconstruction fail. The agreement reported in §6.4 (difference 2.8·10^-10) therefore strongly suggests that the actual computation used a corrected formula. This is a load-bearing point for one of the four claimed inverse problems and must be corrected in the manuscript.
- [§4.1.3 and §4.2] The truncation-order selection is heuristic and is not tied to the actual reconstruction error. Minimizing R(N1) or P(N1) checks identity (3.2) at ρ=0 or at a finite set of points r_j, but the paper gives no argument that a small residual implies small errors in g0(x) and ψ0(x) on the whole interval. Since q is recovered by the second derivative g0''/(g0+1), any error in g0 is amplified, and the numerical differentiation procedure is not described at all. The paper also states that square systems can be ill-conditioned, yet no regularization strategy is given. These omissions do not invalidate the approach, but they are central to the claimed 'efficient accuracy control' and to reproducibility.
- [§6.2–§6.3 and §6.4] Part of the numerical validation is internally generated with the same NSBF machinery used by the reconstruction. In §6.2 the Weyl function is computed from NSBF-based characteristic functions; in §6.3 the multiplier constants β_k are computed from NSBF values of φ_h(ρ_k,b); and in Example 4 the eigenvalues themselves come from an NSBF-based method. These tests demonstrate internal consistency of the reduction formulas, but they are weaker than independent benchmarks. The IP1 examples computed with Matslise and the norming constants in §6.4 are the main independent checks. I recommend adding at least one fully independent benchmark for IP2 and IP3, ideally with data generated by a different numerical method.
- [Remark 3.3 and §4.1.1] The recovery formula q(x)=g0''(x)/(g0(x)+1) requires g0(x)+1 = φ_h(0,x) to be nonzero on (0,b), but no condition excluding zeros of this solution is stated. Similarly, Eq. (4.3) divides by Δ(μ_k), which fails if a singular value μ_k of L0 coincides with a singular value ρ_j of L; the paper assumes simple spectra but not disjointness of the two spectra. The manuscript should either state the needed nonvanishing/disjointness assumptions explicitly or explain how the method is modified in these exceptional cases.
minor comments (4)
- [§3, Theorem 3.1] The theorem states uniform convergence 'for every x ∈ [0,L]' but the interval is elsewhere always [0,b]; this is a typo that should be corrected.
- [§5.3, proof of (5.6)] The displayed derivative d/dx W[ψ_H(ρ,x), φ_h(ρ_k,x)] = (ρ^2 - ρ_k^2)ψ_H φ has the wrong sign; it should be (ρ_k^2 - ρ^2)ψ_H φ. The final identity (5.6) is nevertheless correct, but the intermediate equality as printed is not.
- [§6.1, Example 2] The text notes that N1=N2=13 makes system (4.2) square, while §4.1.3 warns that square systems can be ill-conditioned; a sentence explaining why this particular square case is safe would be helpful.
- [§7 and Data availability] The data availability statement says data are available on request, but no code is provided. Given that the method is numerical, making the MATLAB code available would substantially improve reproducibility.
Circularity Check
Derivation chain is non-circular: exact identities (3.2), (4.2), (4.6), (5.2) and (5.6) connect the input data to NSBF coefficients, and q is recovered from the exact relation q=g0''/(g0+1). The moderate circularity burden lies in the complex-potential numerical benchmarks, whose synthetic input data are generated with the same NSBF forward machinery that the inverse method inverts.
-
other
[Section 6.2, benchmark paragraph before Example 5; analogous in Section 6.1 Example 4 and Section 6.3]
"The Weyl functions of the Sturm-Liouville problems considered in this subsection were computed by the formula M(ρ) = − Δ0(ρ)/Δ(ρ), where in their turn, Δ0(ρ) and Δ(ρ) were computed with the aid of the NSBF representations (3.4), (3.5), (3.11), (3.12), with the coefficients calculated following the recurrent integration procedure from [31]."
For the complex-potential tests, the input data are produced by the same NSBF expansions that the inverse systems (4.2), (5.2), and (5.5) later solve: eigenvalues are zeros of characteristic functions built from NSBF series, Weyl values are ratios of NSBF-based Δ0 and Δ, and multiplier constants are endpoint values of NSBF-computed eigenfunctions. Thus the data lie in the finite-dimensional NSBF model class by construction; the reported agreement largely confirms that the linear systems invert their own generator, rather than independently testing whether a truncated NSBF representation faithfully captures the true potential. This is a validation circularity, not a derivation-level reduction.
full rationale
The derivation chain itself is not circular. Theorem 3.1 is cited from the previously published [31]; although the author overlaps, the theorem is a parameter-free convergence statement that does not include the inverse result. The systems (4.1)-(4.6), (5.2), and (5.5) are obtained by substituting exact NSBF representations into elementary identities (3.2), (3.3), and (5.1), and q is recovered from the exact formula q = g0''/(g0+1) (Remark 3.3), not from a fitted parameter. No uniqueness result is imported from the authors' own work. The main circularity-adjacent issue is that several numerical validations for complex-valued potentials generate their test data with the same NSBF forward machinery (Example 4 uses [30], Section 6.2 uses NSBF characteristic functions, Section 6.3 uses NSBF-computed multiplier constants); this weakens the empirical independence of those examples, but it does not make the mathematical reduction of the inverse problem equivalent to its inputs. I also note, as a correctness issue rather than a circularity, that Eq. (5.8) appears to have the wrong sign for the derivative of the h_n j_{2n}(ρb) terms; as written the IP4 reduction is defective, and the 2.8e-10 agreement in Section 6.4 suggests the actual code used the corrected derivative. This does not affect the circularity assessment.
Assumptions & free parameters
free parameters (5)
- N1 (truncation order) =
7, 13, 31, 22, 33, 49 in the examples
- N2 =
equal to N1 in all examples
- N3 =
equal to N1 in all numerical tests
- Collocation point set {r_j} for system (4.6) =
J = 1501 points, logarithmically spaced on [0.01, 1000] (and analogous sets for IP2)
- Strip height a in NSBF error estimates =
a = 0 in the numerical tests (points chosen real)
assumptions (5)
- domain assumption NSBF representations for solutions and their derivatives (Theorem 3.1 of [31])
- domain assumption The spectra of problems L and L0 are simple
- domain assumption Uniqueness and stability of the two-spectrum problem for complex potentials (results of [8], [10], [11])
- ad hoc to paper The linear systems (4.1)-(4.6), (5.2), (5.5) are nonsingular for the chosen truncation and collocation points
- domain assumption The input spectral data are exact for the reconstructions (noise is only introduced in the dedicated tests)
Cite this review
Pith. "Pith review of Reconstruction techniques for inverse Sturm-Liouville problems with complex coefficients." pith.science (2026). https://pith.science/paper/3BSOHPP2
@misc{pith2026250600670,
author = {Pith},
title = {Pith review of: Reconstruction techniques for inverse Sturm-Liouville problems with complex coefficients},
year = {2026},
howpublished = {\url{https://pith.science/paper/3BSOHPP2}},
note = {Machine review of arXiv:2506.00670}
}
read the original abstract
A variety of inverse Sturm-Liouville problems is considered, including the two-spectrum inverse problem, the problem of recovering the potential from the Weyl function, as well as the recovery from the spectral function. In all cases the potential in the Sturm-Liouville equation is assumed to be complex valued. A unified approach for the approximate solution of the inverse Sturm-Liouville problems is developed, based on Neumann series of Bessel functions (NSBF) representations for solutions and their derivatives. Unlike most existing approaches, it allows one to recover not only the complex-valued potential but also the boundary conditions of the Sturm-Liouville problem. Efficient accuracy control is implemented. The numerical method is direct. It involves only solving linear systems of algebraic equations for the coefficients of the NSBF representations, while eventually the knowledge only of the first NSBF coefficients leads to the recovery of the Sturm-Liouville problem. Numerical efficiency is illustrated by several test examples.
Figures
Figures from the paper (14 more)
Reference graph
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