REVIEW 3 major objections 4 minor 1 cited by
Quadratic Chabauty for Atkin-Lehner quotients of modular curves via weakly holomorphic modular forms: Hodge Filtrations
T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read The paper gives an algorithm that computes the Hodge filtration on X_0^+(N) from q-expansions of modular forms instead of a plane model.
desk verdict Useful model-free Hodge filtration algorithm, but the 'always works' claim is proved only for j=d-1 while the algorithm uses minimal j_dR; the worked examples don't cover the proved case. 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 Hodge filtration matrix λ_Fil, a (2g+2)-block lower-triangular unipotent matrix recording how the Hodge filtration sits inside the universal connection AZ. The innovation is to build the de Rham basis and λ_Fil from weakly holomorphic modular forms: the function f_dR = s_{d−j_dR}/s_d, a quotient of two weight-12 plus-eigen cusp forms, has poles only at ∞ and supplies the missing g differentials; Serre's cup-product formula verifies the basis; the Hecke operator T_p, computed on q-expansions, produces the correspondence Z and the connection Λ; solving the gauge equation G_∞^{-1} dG_∞ = Λ yields ψ_{αβ}, and the universal property determines β_Fil and γ_Fil by the inte
What would settle it
For a prime N not among 67, 97, 193 with genus(X_0^+(N)) ≥ 2, compute the minimal j_dR from (14), form {ω_i, (s_{d−j_dR}/s_d)ω_i}, and evaluate the exact cup product determinant; a zero determinant would refute the claim that the algorithm always produces a basis of H^1_dR.
Extended reading notes
Core claim
The paper claims that for prime N with genus(X_0^+(N)) ≥ 2, the matrix λ_Fil — the data of the Hodge filtration on the rank-(2g+2) unipotent bundle with connection AZ — can be computed from modular form q-expansions alone. Algorithm 3.8 obtains a basis of H^1_dR(X/Q) as {ω_0,...,ω_{g−1}, f_dR ω_0,..., f_dR ω_{g−1}}, where the ω_i come from S_2^+(Γ0(N)) and f_dR = s_{d−j_dR}/s_d is a quotient of weight-12 plus-eigen cusp forms with poles only at the cusp ∞. It then computes the Hecke operator T_p and the nice correspondence Z exactly, solves for the gauge matrix ψ_{αβ}, and determines β_Fil and γ_Fil, with γ_Fil built from the same family of quotients. The worked examples include genus 2 (N=6
Load-bearing premise
The algorithm chooses the smallest j_dR satisfying the valuation condition (14), but the paper proves the basis property only for j = d−1; for the minimal j_dR the invertibility of the cup product matrix is checked numerically, not proved.
Editorial extensions
If this is right
- Algorithm 3.8 computes λ_Fil for any prime N with genus(X_0^+(N)) ≥ 2 using only q-expansions of S_2^+(Γ0(N)) and S_12^+(Γ0(N)), eliminating the plane model as an input.
- Because T_p is computed as exact rational numbers rather than p-adic approximations, the same Hodge filtration computation can be reused for multiple primes p.
- The method verifiably handles genus 7: for X_0^+(193), the q-expansion cup product matrix has nonzero determinant, yielding a symplectic basis, T_3, Z, β_Fil, and γ_Fil.
- The congruence experiments for N=67 and N=97 show a corank difference of g−1 = ρ(J)−1 between matrices built from iterated integrals of plus-eigen cusp forms and single integrals of minus-eigen cusp forms, indicating a systematic relation.
- This is the first step toward a model-free quadratic Chabauty algorithm; the remaining step is the matching Frobenius-structure (λ_ϕ) computation.
Reading between the lines
- If the basis construction extends to every j satisfying (14), the same algorithmic pattern would likely apply to other modular curves with a single cusp; for curves with several cusps the single valuation condition would become a system of pole conditions.
- The corank difference g−1 in the congruence experiments suggests that the iterated integrals of plus-eigen cusp forms and the single integrals of minus-eigen cusp forms are related by a quotient of rank g−1, possibly a manifestation of the correspondence Z; this could be tested for further primes.
- The open gap between the proven choice j = d−1 and the algorithm's minimal j_dR could be closed by proving the cup-product determinant is nonzero for every j satisfying (14), or by a deterministic criterion for choosing j; the algorithm's speed depends on using the minimal choice.
- Since the Hecke matrix is computed exactly and rationally, the choice of prime p can be optimized after the cohomology is computed, unlike model-based approaches where p is effectively fixed when computing Frobenius.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents an algorithm (Algorithm 3.8) for computing the Hodge filtration on the universal connection AZ for Atkin–Lehner quotients X_0^+(N), N prime, genus at least 2, using only q-expansions of the plus eigenspaces S_2^+(Γ_0(N)) and S_12^+(Γ_0(N)), thereby avoiding an explicit plane model. The first half of the de Rham basis is obtained from holomorphic differentials ω_i attached to S_2^+, and the second half from products s_{d-j_dR}/s_d · ω_i, where s_i are ordered basis elements of S_12^+ and j_dR is the smallest integer satisfying the valuation condition (14). The algorithm then computes T_p, the nice correspondence Z, the gauge matrix ψ_{αβ}, and finally β_Fil and γ_Fil. Worked computations are given for X_0^+(67) (genus 2) and X_0^+(193) (genus 7), together with corank computations for N=67,97,193 suggesting congruences between iterated integrals in the plus eigenspace and single integrals in the minus eigenspace.
Significance. If the correctness gap described below is closed, this is a useful technical advance: it replaces a plane-model input with modular-form data, extends the worked range to genus 7, and gives an explicit, checkable route to a Hodge filtration for quadratic Chabauty. The paper provides many detailed q-expansion matrices and cup-product computations, so the numerical content is concrete and partly verifiable. The empirical congruence observations in §4 are interesting, though they are presented as evidence rather than as a proven theorem. The main theoretical weakness is that the basis-producing step used in the actual algorithm is only verified numerically, not proved.
major comments (3)
- [§3.1, Corollary 3.5 and Algorithm 3.8(3)] The central algorithmic claim is not proved for the choice made in Step 3. Proposition 3.2 proves existence of a j satisfying (14) by taking j=d-1, and the argument that the construction 'always works' in §3.1 explicitly uses fdR=s1/sd. Algorithm 3.8 instead chooses jdR to be the smallest j satisfying (14). The paper itself concedes after Corollary 3.5 that the basis property for this minimal choice is checked only numerically in §4. For N=67, jdR=1 while d-1=31, and for N=193, jdR=3 while d-1=89, so the worked examples do not exercise the proved case at all. If a prime N admitted a minimal jdR satisfying (14) but producing a degenerate cup product, Step (7) would not be defined and the claimed λ_Fil would not exist. The algorithm should either be changed to take jdR=d-1, or a proof must be supplied that the minimal admissible j inherits nondegeneracy of the cup product.
- [§3.2, Algorithm 3.8(7)] The normalization γ_Fil(b)=0 requires a rational point b on X in the affine open Y=X\{∞}, but Algorithm 3.8 does not list b (or the hypothesis X(Q)≠∅) among its inputs, and the text merely says 'we can take b to be the upper half plane representative of a rational CM point of X'. No proof is given that such a point exists for every N in the stated range, and the worked examples do not identify the point used. Since the uniqueness of γ_Fil depends on this normalization, the algorithm as stated is incomplete. Please state the nonemptiness assumption explicitly and, for the examples, specify the point b or explain how γ_Fil was normalized.
- [§3.1, after (20)] Even for the value j=d-1 covered by Proposition 3.2, the paper does not give a complete proof that {ω_0,...,ω_{g-1}, (s1/sd)ω_0,...,(s1/sd)ω_{g-1}} has nonzero cup-product determinant. The valuation condition (14) is necessary for the displayed differentials not to lie in H^0(X,Ω^1), but it does not by itself imply that they span a complementary g-dimensional subspace of H^1_dR. The text derives a sufficient pole-order supply of functions from Corollary 3.5, then states that the method 'will always work' and appeals to numerical cup-product checks. A theorem establishing nondegeneracy for the chosen j, or an explicit counterexample, is needed for the central claim.
minor comments (4)
- [Algorithm 3.8(2)] Step (2) says 'd = dim S_2^+(Γ_0(N))' but d is used as the dimension of S_12^+(Γ_0(N)); this is surely a typo, but it is confusing in a pseudocode algorithm.
- [Proposition 3.1] The equality w_N(Δ(z)) = Δ(-1/(Nz)) = Δ(Nz) suppresses the automorphy factor and the normalization of the Atkin–Lehner operator; the statement would be cleaner if the slash action and the resulting constant factor were written explicitly. The subsequent quotient construction is unaffected, but the notation should be corrected.
- [§3.2, equation (22)] In Step (7) of Algorithm 3.8 the text refers to '(22 holds)' and the displayed condition is stated twice with slightly different notations; please clean up the formatting and the references to equations.
- [§4.3] The displayed symplectic basis elements contain denominators written as '22', '32', '33' (e.g. 22·23·101·617). These are clearly powers of 2 and 3 but should be typeset as 2^2, 3^2, 3^3 to avoid misreading the numerical coefficients.
Circularity Check
No significant circularity; the only flagged issue is an unproven minimal-jdR basis claim, which is a proof gap rather than a circular reduction.
full rationale
The paper's central computation, Algorithm 3.8, derives the Hodge filtration matrix lambda_Fil from q-expansions of S_2^+(Gamma_0(N)) and S_12^+(Gamma_0(N)), Hecke operators, and cup products. No target value of the Hodge filtration is used to set constants or choose parameters, so the 'prediction' is not fitted to its own output. The basis construction is verified by an independent criterion: Serre's cup product formula and a nonzero determinant check. The paper explicitly concedes a limitation regarding the minimal jdR choice: after Corollary 3.5 it states, 'Moreover, by computing the cup product, we have found that taking jdR as in (14) always produces a basis in our examples in §4.' This is an admission that the basis property for the minimal jdR is only numerical, whereas Proposition 3.2 proves existence for j = d-1. That is a correctness/completeness gap, not circularity: the cup product test is independent of the desired Hodge filtration, and the minimal-jdR choice is not fitted to any target value. The recipe for the nice correspondence Z is imported from prior work [BDM+23], [BBB+21], but those are not self-citations of the present author and are not used to force the conclusion. The congruence observations in Section 4 are made after the computation and are not used as inputs. No self-definitional, fitted-input, or self-citation-load-bearing pattern is present.
Assumptions & free parameters
free parameters (3)
- jdR =
smallest j in {1,...,d-1} satisfying valuation condition (14); 1 for N=67 and N=97, 3 for N=193
- p =
3 in all examples
- n_N =
60 for N=67, 90 for N=97
assumptions (6)
- standard math Serre's cup product formula computes cup products as sums of residues.
- standard math Riemann-Roch and dimension formulas for modular forms give d = dim S_12^+(Gamma_0(N)) = (N+1)/2 - g and d > 5g-5 for the relevant primes.
- domain assumption The Hecke operator T_p on H^1_dR is computed by the Laurent q-expansion formula (21), including for differentials of the second kind with poles at infinity.
- domain assumption Hadian's universal property, as formulated in [BDM+19] Theorem 4.4, uniquely determines beta_Fil and gamma_Fil through the integrality condition (12).
- domain assumption For X_0^+(N) with N prime and genus at least 2, one has r = g and rho(J) = g > 1, so quadratic Chabauty and the finiteness condition apply.
- ad hoc to paper The smallest jdR satisfying (14) yields a basis of H^1_dR, i.e. the cup product matrix has nonzero determinant.
Cite this review
Pith. "Pith review of Quadratic Chabauty for Atkin-Lehner quotients of modular curves via weakly holomorphic modular forms: Hodge Filtrations." pith.science (2026). https://pith.science/paper/XJAMDJRP
@misc{pith2026250902291,
author = {Pith},
title = {Pith review of: Quadratic Chabauty for Atkin-Lehner quotients of modular curves via weakly holomorphic modular forms: Hodge Filtrations},
year = {2026},
howpublished = {\url{https://pith.science/paper/XJAMDJRP}},
note = {Machine review of arXiv:2509.02291}
}
abstract
For Atkin-Lehner quotients $X_0^+(N)$, of prime level and of genus at least 2, we provide an algorithm for computing one of the main objects in the quadratic Chabauty algorithm in terms of weakly holomorphic modular forms associated to the curve. In particular, the algorithm computes a Hodge filtration on a certain unipotent vector bundle with connection related to $X_0^+(N)$, which is crucial in computing the $p$-adic height which is used to define the finite set of $p$-adic points containing the rational points on $X_0^+(N)$. This improves the current Hodge filtration algorithm by replacing the input of an explicit plane model of the curve with weakly holomorphic modular forms to produce a faster computation. We implement our algorithm on the genus 7 modular curve $X_0^+(193)$, and discover congruences between iterated integrals of weight 2 cusp forms in the plus eigenspace for the Atkin-Lehner involution and single integrals of weight 2 cusp forms in the minus eigenspace for the Atkin-Lehner involution.
Forward citations
Cited by 1 Pith paper
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