REVIEW 2 major objections 3 minor 48 references
Limiting spectral laws for sparse random circulant matrices
T0 review · 2 major / 3 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read Sparse random $G$-circulant spectra converge exactly when element-order distributions do, with explicit roots-of-unity limits.
desk verdict Genuinely new sparse circulant model with a clean iff characterization; the main spectral theorems hold up, but Lemma 3.1(6) is false and the determinant proof needs repair. 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 central object is the $G$-circulant matrix for a finite abelian group $G$: a matrix $(A_{x,y})_{x,y\in G}$ with $A_{x,y}=a(xy^{-1})$, the matrix of convolution by $a$. These matrices are normal and are simultaneously diagonalised by the Fourier transform on $G$, so the eigenvalue multiset is $\{\widehat a(\gamma):\gamma\in\widehat G\}$. In the random sparse model, $\widehat{S_n}(\gamma)=\sum_{j=1}^d \gamma(X_{n,j})$ with $X_{n,j}$ independent uniform elements of $G_n$; this sum has law $\eta_{\mathrm{ord}(\gamma)}^{\ast d}$ and support in $dR_{\mathrm{ord}(\gamma)}$. The proof machinery is the Hermitisation method (as formulated in Proposition 2.3): eigenvalue convergence is deduced from weak convergence of shifted singular value measures plus uniform integrability of the logarithm, and the latter is reduced to small-ball estimates for sums of roots of unity, chiefly Lemma 3.1. The variance computation for even moments of shifted singular value measures (Lemma 5.4) forces the functional equation $\tau(\gcd(a,b))=\tau(a)\tau(b)$ on the limiting order distribution, whose $\{0,1\}$-valued solutions are exactly indicators of the multiples of a fixed $m$.
What would settle it
Take $d=2$, even $m$, $z=0$, and $r\to0$; directly count pairs of $m$-th roots of unity whose sum lies in $B(0,r)$. There is an atom of size $1/m$ at $0$, so $\eta_m^{\ast2}(B(0,r))$ does not decay as $1/m^2$, contradicting Lemma 3.1(6) as stated; the proof's claim that a point is the midpoint of at most one chord of the unit circle is false for the centre, which is the midpoint of every diameter.
Extended reading notes
Core claim
The paper's central claim is that the spectral law of a sparse random circulant matrix is completely encoded by a single group-theoretic statistic: the order of a uniformly random element. Concretely, Theorem 1.2 says $\mu_{C_n}$ converges weakly in expectation if and only if $\rho_{G_n}$ converges weakly to some $\rho$ on $\mathbb{N}^\ast$, with limit $\mu = \sum_{m\in\mathbb{N}^\ast} \rho(\{m\})\eta_m^{\ast d}$. Theorem 1.3 sharpens this: convergence in probability holds if and only if $\rho = \delta_m$, and then the limit is $\eta_m^{\ast d}$ -- the $d$-fold convolution of the uniform distribution on the $m$-th roots of unity, or on the whole unit circle when $m=\infty$. Because the eigenvalues of a $G$-circulant matrix are exactly the Fourier coefficients of its first row, each eigenvalue is a sum of $d$ independent uniform elements of the dual group, and its law is $\eta_{\mathrm{ord}(\gamma)}^{\ast d}$; the theorems follow by tracking how these laws mix as the group grows. Theorem 1.6 adds that, under the natural nonsingularity condition $0\notin dR_{\exp(G_n)}$ and $\rho_{G_n}\to\delta_m$, the normalized logarithm of the determinant converges in probability to $c_{m,d}=\int\log|z|\,d\eta_m^{\ast d}(z)$.
Load-bearing premise
The argument's control of the logarithm near zero rests on Lemma 3.1(6), the estimate $\eta_m^{\ast d}(B(z,r)) \ll ((rm+1)/m)^2$ for $d\ge2$; the proof's geometric justification, that every point is the midpoint of at most one chord of the circle, fails at the centre, where for $d=2$ and even $m$ the sum has an atom of size $1/m$, not $O(1/m^2)$.
Editorial extensions
If this is right
- For $G_n=\mathbb{Z}/n\mathbb{Z}$, the element-order measure $\rho_{G_n}$ converges to $\delta_\infty$, so the ESD converges weakly in probability to $\eta_\infty^{\ast d}$, the $d$-fold convolution of the uniform distribution on the unit circle -- a commutative counterpart of the conjectured sparse i.i.d. limit.
- For $G_n=(\mathbb{Z}/m\mathbb{Z})^n$ with $m$ fixed, the limiting law is $\eta_m^{\ast d}$, so the spectrum is governed by a finite root-of-unity convolution and is supported in the finite set $dR_m$.
- If the order distribution converges to a non-Dirac measure, the ESD converges only in expectation and its random fluctuations do not disappear; the example $G_n=\mathbb{Z}/2\mathbb{Z}\oplus(\mathbb{Z}/3\mathbb{Z})^n$ has limit $\frac12\delta_3+\frac12\delta_6$.
- When $0\notin dR_{\exp(G_n)}$ and $\rho_{G_n}\to\delta_m$, asymptotically almost surely $|\det C_n|=\exp((c_{m,d}+o(1))|G_n|)$; for $m=\infty$, $c_{\infty,d}=\frac12\log d-\frac{\gamma}{2}+o(1)$ with $\gamma\approx0.577$ the usual constant.
Reading between the lines
- The if-and-only-if gives a recipe for constructing group sequences with no limiting spectrum in probability: mix groups whose exponents produce different root-of-unity limits, e.g. a proportion $p$ of cyclic groups of order $3^k$ and $1-p$ of order $5^k$, so that $\rho_{G_n}$ converges to a non-Dirac mixture; then the ESD converges in expectation but keeps a random component.
- A repair of Lemma 3.1(6) appears necessary: for $d=2$, even $m$, and $z=0$, the probability $\eta_m^{\ast2}(\{0\})=1/m$ is not of order $1/m^2$, so the bound as stated cannot be correct; a corrected version that removes or separately handles the atom at zero would preserve the uniform-integrability conclusion used by Theorem 1.6.
- The same Fourier-coefficient representation points to finer spectral statistics -- the spectral gap of the underlying random Cayley graph, or the proportion of eigenvalues near zero -- as natural next targets, since they reduce to concentration and small-ball properties of sums of $d$ roots of unity.
- The non-abelian version (Problem 7.1) lacks simultaneous diagonalisation, so the present order-statistic characterisation cannot transfer verbatim; if a limiting law exists for symmetric-group circulants, it would need new tools to be identified.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper studies the empirical spectral distribution (ESD) of d-regular G_n-circulant matrices for a sequence of finite abelian groups G_n with |G_n| tending to infinity. The main results are a complete characterization of weak convergence in expectation (Theorem 1.2) and in probability (Theorem 1.3) in terms of the limiting distribution rho of the order of a uniform element of G_n, with explicit limit mu = sum_m rho({m}) eta_m^{*d}. Theorem 1.6 gives an asymptotic log-determinant law under a non-singularity assumption. The proofs use Fourier diagonalisation, Hermitisation, moment computations, Smith normal form, and small-ball estimates for sums of roots of unity.
Significance. If Theorems 1.2 and 1.3 are correct, they provide a sharp and elegant counterpart to the sparse i.i.d. problem of Problem 1.1 for circulant (Cayley) matrices, identifying exactly when randomness in the group order produces fluctuation in the ESD. The limits are explicit, the characterization is necessary as well as sufficient (with Remark 1.4 showing that convergence in expectation and in probability genuinely differ), and the determinant asymptotics give a concrete constant c_{m,d}. The proofs are self-contained modulo standard results and contain no fitted parameters. Theorems 1.2 and 1.3 appear to be sound. However, the proof of Theorem 1.6 is incomplete because it relies on a false small-ball estimate, so the full set of claims in the abstract is not established as written.
major comments (2)
- [Section 3, Lemma 3.1(6)] Lemma 3.1(6) is false as stated. Take d = 2, m even, z = 0, and r = c/m with c > 0 sufficiently small so that B(0,r) contains no nonzero element of 2R_m. Then the atom at 0 has eta_m^{*2}({0}) = m/m^2 = 1/m, while the claimed bound O((rm + 1/m)^2) is O(m^{-2}) in this regime. The proof's geometric assertion that every point is the midpoint of at most one chord of the unit circle fails at the center, which is the midpoint of m/2 chords when m is even.
- [Section 3, Proposition 3.3; Section 6] The false estimate in Lemma 3.1(6) is load-bearing for Theorem 1.6. In the proof of Proposition 3.3, the 'complementary case' z = 0, d >= 2 uses Lemma 3.1(6) to bound the third summand by (4C/ord^2) * ord log(3d), which is what shows that 0 is good under the final hypothesis of the proposition. Since Lemma 3.1(6) is not available, the proof that 0 is good is incomplete, and consequently the derivation of Theorem 1.6 in Section 6 does not go through as written. Lemma 3.2 cannot repair this: its lower bound d^{1-phi(m)} is typically far smaller than the radius m^{-3d}, so it does not make the relevant event empty. The author should state and prove a restricted small-ball estimate for B^*(0,r) under the condition 0 notin dR_m, or give an alternative bound for P(|xSn(gamma)| < ord^{-3d}), and then rerun the argument of Proposition 3.3.
minor comments (3)
- [Section 3, Proposition 3.3] The displayed lower bound |xSn(gamma) - z| >= (3d)^{-ord} is not a direct restatement of Lemma 3.2; please include the short derivation using phi(m) <= m.
- [Section 5, Lemma 5.1] The notation rds^P (or [d]^P) for tuples indexed by P is not defined in the text; please define it explicitly before Lemma 5.1.
- [Throughout] There are several typographical artifacts in the extracted text (e.g., 'f ollowing-up' in the first paragraph and 'matrice s' in the title on the first page); a careful proofreading pass is recommended.
Circularity Check
No circularity found: the spectral laws are derived from explicit Fourier diagonalization and standard external criteria, with no fitted inputs, no self-citation loops, and no definitional reduction.
full rationale
The derivation is self-contained and non-circular. The central identity in Section 4 expresses E∫f dμ_{C_n} as ∫_{N*} Φ(f) dρ_{G_n}, where Φ(f)(m)=∫f dη_m^{*d}; this identity is a direct consequence of the Fourier diagonalization of G-circulant matrices and the fact that the eigenvalue distribution attached to a character depends only on the character's order. It is not an input that assumes the target result. Theorem 1.2 is then proved by a genuine measure-theoretic separation argument using tent functions and the Möbius inversion lemma, while Theorem 1.3 is proved through the variance computation of Lemma 5.1 and a functional equation for τ, again without assuming the conclusion. The Hermitisation criterion (Proposition 2.3) is quoted from Sah–Sahasrabudhe–Sawhney, but it is a general external input and is not used to smuggle in the paper's specific limit. There are no fitted parameters, no parameter-free claim that is actually a renamed fit, and no load-bearing self-citations: the bibliography contains no prior work by the author, and the cited Lam–Leung and small-ball results are independent. The apparent defect in Lemma 3.1(6), which is invoked at z=0 in Proposition 3.3 and hence in the proof of Theorem 1.6, is a correctness gap rather than a circularity: the claimed O(m^{-2}) bound can fail at the atom of a sum of two roots of unity, but this does not make the derivation circular and does not affect the verdict for Theorems 1.2 and 1.3.
Assumptions & free parameters
assumptions (6)
- standard math Hermitisation criterion (Proposition 2.3) from Sah-Sahasrabudhe-Sawhney [41]
- standard math Smith normal form for integer matrices acting on finite abelian groups
- domain assumption Contiguity of the with-replacement model to the exactly-d model via event (4)
- domain assumption Eigenvalue magnitudes are bounded by d
- ad hoc to paper Lemma 3.1(6) small-ball estimate for sums of roots of unity
- standard math Lam-Leung theorem on vanishing sums of roots of unity
Cite this review
Pith. "Pith review of Limiting spectral laws for sparse random circulant matrices." pith.science (2026). https://pith.science/paper/DF53XYVV
@misc{pith2026250413833,
author = {Pith},
title = {Pith review of: Limiting spectral laws for sparse random circulant matrices},
year = {2026},
howpublished = {\url{https://pith.science/paper/DF53XYVV}},
note = {Machine review of arXiv:2504.13833}
}
abstract
Fix a positive integer $d$ and let $(G_n)_{n\geq1}$ be a sequence of finite abelian groups with orders tending to infinity. For each $n \geq 1$, let $C_n$ be a uniformly random $G_n$-circulant matrix with entries in $\{0,1\}$ and exactly $d$ ones in each row/column. We show that the empirical spectral distribution of $C_n$ converges weakly in expectation to a probability measure $\mu$ on $\mathbb{C}$ if and only if the distribution of the order of a uniform random element of $G_n$ converges weakly to a probability measure $\rho$ on $\mathbb{N}^*$, the one-point compactification of the natural numbers. Furthermore, we show that convergence in expectation can be strengthened to convergence in probability if and only if $\rho$ is a Dirac mass $\delta_m$. In this case, $\mu$ is the $d$-fold convolution of the uniform distribution on the $m$-th roots of unity if $m\in\mathbb{N}$ or the unit circle if $m = \infty$. We also establish that, under further natural assumptions, the determinant of $C_n$ is $\pm\exp((c_{m,d}+o(1))|G_n|)$ with high probability, where $c_{m,d}$ is a constant depending only on $m$ and $d$.
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