REVIEW 4 major objections 3 minor 15 references
Boundedness of average rank of elliptic curves ordered by the coefficients
T0 review · 4 major / 3 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Ordering elliptic curves by the coefficient height $\max(|A|,|B|)$, this paper proves the average 2-Selmer size is at most 3, hence the average rank is at most 1.5.
desk verdict Fresh method for counting quartic orbits under a non-homogeneous height, but the paper's main constant is off by 27 in the displayed volume computation and one algebraic identity does not check out. 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 machine is the change of coordinates $\upsilon$ from a quartic form $(a,b,c,d,e)$ to the semi-form $(a,b,c,R,I)$, with cubic semi-invariant $R = b^3 + 8a^2d - 4abc$ and $H = 8ac - 3b^2$; for $a \neq 0$ this map is a bijection onto an explicitly described lattice $\Lambda \subset \mathbb{Z}^5$ (Lemma 2.6). Through the syzygy $H^3 - 48a^2HI + 64a^3J = -27R^2$, fixing $(a,b,c,R)$ constrains $I$ to an interval of length $O(|aX/H|)$, so the height condition $|J| < X$ becomes a one-dimensional interval condition carrying a congruence condition modulo $12a$ of the same size. Over each fiber the admissible $(R,I)$ form a lattice of covolume $|8a^2 \cdot 12a|$ whose dual Fourier transform vanishes unless $\alpha \equiv 3b\beta \pmod{12a}$ (Proposition 3.6); Poisson summation on this lattice yields the equidistribution theorem (Theorem 3.1) with per-fiber error $O(X^{1/4+12\delta_s+50\delta})$. A second fibering, over the unique rational root of a quartic form, runs the sieve: Lemma 5.4 expresses $J$ in terms of the root and the semi-invariants, and Lemma 5.5 bounds the number of such forms of height at most $X$ by $O_\epsilon(X^{7/4+\epsilon})$, giving the uniformity estimate behind Theorem 5.2.
What would settle it
Run the count in Theorem 2.2 computationally at a moderately large $X$: enumerate integral $(a,b,c)$ with $|a|,|c| \le X^{1/2}$, $|b| \le X^{1/2}$, $H = 8ac - 3b^2 \neq 0$, enumerate over each fiber the lattice points $\Lambda_{(a,b,c)}$ inside $\mathcal{H}'_{(a,b,c)}$, and compare the totals for each real-root type against $(C\zeta(2)/27)X^2$; if the difference is not $o(X^2)$, for instance if it grows like $X^{7/4}$, the central asymptotic fails. A more surgical test targets Theorem 3.1 directly: for fibers with $|H|$ near $X^{1-\delta_H}$, compute the summed Fourier error and check it truly behaves as $O(X^{1/4+12\delta_s+50\delta})$; an error of size $X^{1/2}$ per fiber, multiplied across $X^{3/2}$ fibers, would already exceed the budget.
Extended reading notes
Core claim
The central discovery is Theorem 1.1 with Corollary 1.2: when elliptic curves over $\mathbb{Q}$ are ordered by $h(E_{A,B}) = \max(|A|,|B|)$, the average size of the 2-Selmer group is at most 3 and the average rank is at most 1.5. Since rank is at most half the 2-Selmer size, the rank bound is an immediate corollary; the Selmer bound is the real target. Its engine is Theorem 2.2, an asymptotic count of irreducible integral binary quartic forms of height $h_C(f) = \max(|I(f)|, |J(f)|/C)$: there are $(C\zeta(2)/27)X^2$ orbits with 4 real roots, $(2C\zeta(2)/27)X^2$ with 2 real roots, and $(C\zeta(2)/27)X^2$ for each of the two zero-real-root families, up to $o(X^2)$. Theorem 1.4 reads the same result as constancy on average: the mean number of orbits per eligible invariant pair is $2\zeta(2)/n_i$ with $n_0 = 4$ and $n_1 = n_2 = 2$. The Selmer bound follows by sieving locally soluble orbits, using the uniformity estimate of Theorem 5.2: curves of height at most $X$ whose discriminant is divisible by $p^2$ for some $p > Q$ number $O_\epsilon(X^{2+\epsilon}/Q + X^{7/4+\epsilon})$, which keeps the sieve error below the main term. The paper states plainly that only the upper bound is proved; equality with the heuristically predicted value 3 would require a tail estimate it does not supply (Remark 4.10).
Load-bearing premise
The proof rests on exponent estimates that are, on the paper's own account, tight: the per-fiber equidistribution error $O(X^{1/4+12\delta_s+50\delta})$ of Theorem 3.1 must remain $o(X^2)$ after summing over the roughly $X^{3/2}$ fibers, and the replacement of the true height region by its simplified copy must lose at most $O_\epsilon(X^{7/4+\delta_a/2+\epsilon})$ points; if either exponent fails by a small amount, the error swallows the main term.
Editorial extensions
If this is right
- The bound of 3 on the average 2-Selmer size, and of 1.5 on the average rank, holds for the coefficient-height ordering whose discriminant range reaches $O(X^3)$ — the first such result for this ordering, where the standard orbit-counting argument breaks down.
- The same average-rank bound holds uniformly for every large family of elliptic curves, meaning all curves or any family cut out by finitely many congruence conditions on $A$ and $B$, under the same ordering (Theorem 5.1).
- The average number of quartic orbits per eligible invariant pair is $2\zeta(2)/n_i$, a constancy-on-average statement (Theorem 1.4) that matches the heuristically predicted mean 2-Selmer size of 3, at least as an upper bound.
- The paper argues that the fibering method is not specific to quartics: because semi-invariants and syzygies exist for binary $n$-ic forms generally, the same strategy may handle other orbit spaces where classical lattice-point counting performs poorly.
Reading between the lines
- Beyond the paper: if, as the author expects in Remark 4.10, the upper bound is actually equality, then the random-matrix heuristic for Selmer sizes survives in a family whose discriminants reach $X^3$; that would point to a cause other than discriminant range for the heuristic failures seen in other thin families.
- Beyond the paper: the core trick — replacing an Archimedean flat-projection difficulty by a congruence-modulus problem of the same size, then applying Fourier summation — reads as a general recipe for counting integral points in regions cut out by invariant inequalities, with sextic forms and higher-degree genus-one models as natural next targets.
- Beyond the paper: the sieve estimate's exponent $7/4$ echoes half-integer exponents that appear in conductor-ordered counting; checking whether the two share the root-fibering mechanism would be a concrete test of whether conductor-order analogues of this average-rank bound are within reach.
- Beyond the paper: the stated exponents make a checkable prediction — with $\delta$ fixed small, the summed error after fibering should be $O(X^{7/4+\epsilon})$, so a numerical experiment at $X \approx 10^8$ comparing the true count against $\zeta(2)X^2/27$ should reveal a clean $7/4$ error law if the estimates are sharp.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that when elliptic curves E_{A,B}: y^2 = x^3 + Ax + B over Q are ordered by the coefficient height h(E_{A,B}) = max(|A|, |B|), the average size of the 2-Selmer group is at most 3, and hence the average rank is at most 1.5 (Theorems 1.1 and 1.2). The route is to count irreducible GL2(Z)-orbits of integral binary quartic forms with invariants |I| < X and |J| < CX (Theorem 2.2), using a new semi-invariant space (a,b,c,R,I), a fiberwise Poisson-summation argument, and an equidistribution statement (Theorem 3.1), followed by a volume computation (Section 4.2) and a Selmer-group argument (Section 5). The paper also states an average-per-pair version (Theorem 1.4) and a uniformity estimate for large families (Theorem 5.2).
Significance. If correct, Theorem 1.1 would be a genuinely new boundedness result: the coefficient height h(E_{A,B}) = max(|A|,|B|) has discriminant O(X^3), so the standard naive-height methods, which rely on the homogeneity of the height function, do not apply. The paper introduces a promising technique of passing to semi-invariants and reducing the Archimedean counting difficulty to a non-Archimedean congruence problem over fibers, and it explicitly identifies the role of equidistribution rather than Davenport-type lattice-point estimates. These ideas are valuable and could have applications to other thin families. However, the proof as written contains load-bearing algebraic and numerical inconsistencies that prevent verification of the main theorem: a factor-27 discrepancy in the volume computation, an incorrect syzygy rewriting in the reducibility estimate, and inconsistencies in the constants of the final Selmer average.
major comments (4)
- [Section 4.2] The displayed conclusion of the volume computation, NC = 4Cζ(2)X^2/n_i + O(...), drops the factor 1/27 that is present in the immediately preceding identity NC = (Vol(F_PGL2)/(27 n_i)) ∫∫ ... . With Vol(F_PGL2) = 2ζ(2) and the computed volume integral 2CX^2, the correct conclusion is 4Cζ(2)X^2/(27 n_i), which is what Theorem 2.2 states. This is not cosmetic: the final Selmer computation in Section 5.2 cancels the orbit-count constant against local densities to obtain the Tamagawa number 2. If the true orbit count were 27 times the stated value, the same computation would give an average of #Sel2 - 1 equal to 54, and Corollary 1.2 would not follow. The factor 1/27 must be restored and all downstream constants re-checked.
- [Lemma 2.14] The displayed 'rewriting' of the syzygy, H^3 - 3(12a)^2 I H - (12a)^3 J = R^2, does not follow from equation (6), which states H^3 - 48a^2 I H + 64a^3 J = -27R^2. Substituting (6) gives H^3 - 432a^2IH - 1728a^3J = -27R^2 - 384a^2IH - 1792a^3J, not R^2. The subsequent appeal to Theorems 5.1 and 5.2 of [5] is therefore unjustified, and the claimed bound O_ε(X^{1/2+ε}) for the number of classes with reducible cubic resolvent is not established. This bound is used in Lemma 2.13, equations (14)-(15), to discard reducible and non-generic forms, and hence it is load-bearing for Proposition 2.7 and Theorem 2.2.
- [Theorem 1.4] Theorem 1.4 is inconsistent with Theorems 1.3 and 2.2. For example, for i=1, Theorem 1.3(b) gives ∑ h_{I,J}^{(1)} ~ 2ζ(2)/27 · X^2, while the denominator in Theorem 1.4, the number of pairs (I,J) with h(I,J)<X and Δ(I,J)<0, is asymptotic to (2+o(1))X^2 (the left half-square |I|<X, |J|<X contributes area 2X^2). The ratio is therefore ζ(2)/27, not ζ(2) as stated by the formula 2ζ(2)/n_i with n_1=2. The correct averaging constant appears to be 2ζ(2)/(27 n_i). This is a second manifestation of the missing factor 1/27 and must be corrected consistently.
- [Section 5.2] In the proof of Theorem 1.1, the asymptotic for M∞(F;X) is stated as 3^4 X^2 ∏ M_p(F). But h_e(I,J) < X is equivalent to |I| < 3X and |J| < 27X, whose area is (2·3X)(2·27X) = 324X^2 = 4·3^4 X^2. The displayed count of elliptic curves is consequently off by a factor 4. Since the final bound of 2 for the average of #Sel2 - 1 is obtained by comparing the Selmer count with this curve count, the missing factor 4 would change the final average to 8 if uncorrected. The entire numerical chain in this subsection should be re-derived, especially because the transition from the orbit-count formula to the expression involving M∞(V,F;X) is not reproducible without specifying the normalization of the local and archimedean measures.
minor comments (3)
- [Lemma 2.4] The statement of Lemma 2.4 bounds the number of e with |I|<X and |J|<X, but the proof uses the interval 4C/3 |aX/H| and mentions J < CX via the constant C. The statement should either include C or clarify that a fixed C is understood throughout.
- [Theorem 1.4] The symbol n_i in Theorem 1.4 is defined again (n_0=4, n_1=2, n_2=2) and is not the same as the PGL2(R)-stabilizer sizes n_i introduced before Proposition 2.3, where n_{2+}=n_{2-}=4. The dual use of the same notation for different quantities is confusing and should be changed.
- [Section 3.2] In Corollary 3.11 and the proof of Theorem 3.1, the parameters δ, δ_s, δ_a, δ_H, δ_e are only required to be 'sufficiently small.' Since the exponents must satisfy specific inequalities for the summed error to be o(X^2), the paper should state one explicit admissible choice of these parameters (for example, δ_s = 22δ as used later in Section 4.2) at the point where the error is bounded.
Circularity Check
No significant circularity: the orbit-counting and Selmer-bound arguments are derived from independent external theorems and internal estimates, not from the target result.
full rationale
The paper's central claim—an upper bound of 3 for the average size of the 2-Selmer group under coefficient-height ordering—is obtained by counting GL2(Z)-orbits of integral binary quartic forms with bounded invariants. Sections 2–4 carry out this count self-containedly: the semi-invariant parametrization, the fiberwise equidistribution theorem (Theorem 3.1), the Poisson-summation estimates, and the volume computation are all derived from stated assumptions, with no fitted constant or assumed bound on Selmer groups entering the argument. The constants ζ(2)/27 and the eventual Tamagawa number 2 are outputs of the calculation, not inputs. The paper's heavy reliance on Bhargava–Shankar [3], Bhargava–Shankar–Swaminathan [4], Shankar–Shankar–Wang [13], Shankar–Taniguchi [15], and Cremona–Fisher [9] is external support: those works are independent, established results and do not presuppose the present theorem. The author is not an author of any cited work, so there is no self-citation chain. The apparent omission of the factor 1/27 in the displayed conclusion of Section 4.2 is an arithmetic inconsistency with the preceding line and with Theorem 2.2, but it does not make the derivation circular; it is a correctness risk in exposition. Remark 4.10 notes that the tail estimate needed to upgrade Theorem 4.9 from an upper bound to an exact asymptotic is missing, but this affects sharpness only, not the circularity of the argument. No equation in the paper reduces by construction to its own inputs.
Assumptions & free parameters
assumptions (4)
- standard math Bhargava's smooth averaging identity (Theorem 5.1 of [4]) for converting orbit sums to integrals over the fundamental domain
- domain assumption Counting bounds for integral points on elliptic curves (Theorems 5.1 and 5.2 of [5])
- domain assumption Embedding of minimal irreducible cubic polynomials into PGL2(Z)-orbits of quartic forms with a unique rational root (Proposition 5.3 of [13])
- domain assumption Evaluated constants from [3]: Vol(PGL2(Z)\PGL2(R)) = 2ζ(2), local densities and Selmer mass ratios in Propositions 3.6, 3.9, 3.18 of [3]
Cite this review
Pith. "Pith review of Boundedness of average rank of elliptic curves ordered by the coefficients." pith.science (2026). https://pith.science/paper/Y4SHN6WE
@misc{pith2026250607089,
author = {Pith},
title = {Pith review of: Boundedness of average rank of elliptic curves ordered by the coefficients},
year = {2026},
howpublished = {\url{https://pith.science/paper/Y4SHN6WE}},
note = {Machine review of arXiv:2506.07089}
}
abstract
We study the average rank of elliptic curves $E_{A,B} : y^2 = x^3 + Ax + B$ over $\mathbb{Q}$, ordered by the height function $h(E_{A,B}) := \text{max}(|A|, |B|)$. Understanding this average rank requires estimating the number of irreducible integral binary quartic forms under the action of $\mathrm{GL}_2(\mathbb{Z})$, where the invariants $I$ and $J$ are bounded by $X$. A key challenge in this estimation arises from working within regions of the quartic form space that expand non-uniformly, with volume and projection of the same order. To address this, we develop a new technique for counting integral points in these regions, refining existing methods and overcoming the limitations of Davenport's lemma. This leads to a bound on the average size of the 2-Selmer group, yielding an upper bound of 1.5 for the average rank of elliptic curves ordered by $h$.
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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