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The density of elliptic curves over $\mathbb{Q}_p$ with a rational 3-torsion point or a rational 3-isogeny
T0 review · 0 major / 4 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read For primes $p \ge 3$, the paper proves exact rational-function formulas for the Haar-measure density of Weierstrass equations over $\mathbb{Z}_p$ that define elliptic curves over $\mathbb{Q}_p$ with a rational 3-torsion point, and…
desk verdict Exact p-adic densities for 3-torsion and 3-isogeny, with two independent proofs in the p>3 case and one externally supported p=3 entry that a referee should check. 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 machinery is the upgrade of the classical level-3 modular parametrizations to spaces of Weierstrass equations equipped with a regular differential. Tate normal form $y^2 + uxy + vy = x^3$ parametrizes curves with a marked nonzero 3-torsion point, the Hesse pencil $u(x^3+y^3+z^3) - 3vxyz = 0$ parametrizes curves with full level-3 structure, and a twisted version with the point defined over a quadratic extension parametrizes the nontrivial characters of a rational 3-isogeny. Forgetting the level structure yields covering maps $\pi_1 : A^2_{X_1(3)} \to A^2_X$ and $\pi_2 : A^2_{X(3)} \to A^2_X$ whose Jacobian determinants are $256v^2$ and $-559872\,u^2(u-v)^2(u^2+uv+v^2)^2$, respectively. The $p$-adic change-of-variables formula turns the counting of integral preimages into integrals of the $p$-adic absolute values of these Jacobians, which are then evaluated by a residue-class calculation. Bad-reduction contributions are supplied by pre-existing Kodaira-type densities, and the case $p = 3$ uses congruences for the 3-division polynomial $\psi_3$ modulo higher powers together with the canonical lift.
What would settle it
Fix $p=5$, for which Theorem 1.1 predicts density $25/62$ and Theorem 1.2 predicts $401/781$, and compute by exhaustive enumeration the fraction of tuples in $(\mathbb{Z}/5^m\mathbb{Z})^5$ for increasing $m$ whose Weierstrass equation has a rational 3-torsion point, respectively a root of $\psi_3$ in $\mathbb{Q}_5$; convergence to the predicted rational numbers would support the theorems, and a definite mismatch at any single $m$ would refute them. Independently, recompute Proposition 4.3's $\mathbb{F}_p$-point counts on $X_1(3)$ and $X(3)$ for $p=5$ or $p=7$ by direct computer algebra to check the good-reduction step.
Extended reading notes
Core claim
The paper's central claim is that two local probabilities are exact rational functions of the prime $p$. For $p \ge 3$, the Haar measure of the set of tuples $[a_1,a_2,a_3,a_4,a_6]$ in $\mathbb{Z}_p^5$ defining an elliptic curve with a nonzero $\mathbb{Q}_p$-rational 3-torsion point is $\frac{p^2(3p^6+4p^2-4p+4)}{8(p^8+p^6+p^4+p^2+1)}$ when $p \equiv 1 \pmod 3$, $\frac{p^2}{2(p^2+p+1)}$ when $p \equiv 2 \pmod 3$, and $\frac{3}{26}$ when $p = 3$. For $p > 3$, the measure of the set defining a $\mathbb{Q}_p$-rational cyclic 3-isogeny is $\frac{3p^4+3p^3+4p^2+4}{4(p^4+p^3+p^2+p+1)}$ when $p \equiv 1 \pmod 3$ and $\frac{p^4+p^3+2p^2+2}{2(p^4+p^3+p^2+p+1)}$ when $p \equiv 2 \pmod 3$. The proof treats good and bad reduction separately: for good reduction, the density is obtained from $\mathbb{F}_p$-point counts on the modular curves $X_1(3)$ and $X(3)$; for bad reduction, it is obtained from the Kodaira type, using pre-existing densities for each type. A second proof via $p$-adic integration computes the same torsion density from the Jacobian determinants of the forgetful maps, and the case $p = 3$ requires a separate analysis of the 3-division polynomial and the canonical lift.
Load-bearing premise
The load-bearing premise is that the Kodaira-type densities quoted from the external local-density computation (for split multiplicative reduction of type $I_m$ and for additive types $IV$ and $IV^*$) are correct; the final formulas inherit these numbers, and the paper does not re-derive them.
Editorial extensions
If this is right
- For $p \ge 3$, the complete system of congruences characterizing elliptic curves over $\mathbb{Q}_p$ with a rational 3-torsion point is known, and its total Haar volume is the explicit rational function in Theorem 1.1.
- If the density theorems are correct, the large-sieve argument gives an upper bound $\#S \ll N_1N_2/\sqrt{\min\{N_1,N_2\}}$ for elliptic curves with rational 3-torsion in an arbitrary box $[M_1,M_1+N_1] \times [M_2,M_2+N_2]$, and a nontrivial saving for short intervals.
- For a fixed ordinary elliptic curve $\bar{E}$ over $\mathbb{F}_p$ with an $\mathbb{F}_p$-rational $p$-torsion point, exactly one lift in $p$ of the coefficient pairs modulo $p^2$ gives a $\mathbb{Q}_p$-rational $p$-torsion point, so the lifting probability is $1/p$; these lifts are cut out by congruences modulo $p^2$ and coincide with the canonical lift.
- For a prime $\ell > 3$ with $p \neq \ell$, the density of curves with a $\mathbb{Q}_p$-rational $\ell$-torsion point tends to $\frac{1}{\ell-1} - \frac{\delta}{\ell^2-1}$ as $p \to \infty$ through a fixed residue class modulo $\ell$, where $\delta = 1$ for $p \equiv 1 \pmod \ell$ and $\delta = 0$ otherwise; the paper also expresses the finite-$p$ density in terms of $\mathbb{F}_p$-point counts on
Reading between the lines
- Editorial inference: the same $p$-adic integration scheme should yield exact densities for $\ell = 2, 3, 4, 5$, where $X(\ell) \cong \mathbb{P}^1$, with formulas depending on $p$ through the residue class modulo $\ell$; the paper expects this but does not compute it.
- Editorial inference: since the torsion density is assembled from local point counts on modular curves, one should expect the density of curves with a prescribed local Galois structure on $E[\ell]$ to be a rational number whenever the relevant moduli space is rational over $\mathbb{Q}_p$, and not otherwise; testing $\ell = 7$ would discriminate between these regimes.
- Editorial inference: combining the exact $p$-adic densities with the large sieve suggests that the short-interval count of elliptic curves with a rational 3-torsion point satisfies the improved bound in Corollary 7.2(2), a regime where the trivial geometry-of-numbers estimate is worse and the local $p$-adic information carries the argument.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper determines, for each prime p≥3, the Haar measure of the subset of Z_p^5 consisting of coefficients [a1,...,a6] for which the corresponding Weierstrass equation defines an elliptic curve over Q_p with a nonzero Q_p-rational 3-torsion point (Theorem 1.1), and for p>3 the analogous density for rational cyclic 3-isogenies (Theorem 1.2). The densities are explicit rational functions of p, depending only on p mod 3, with the exceptional value 3/26 at p=3. The proof strategy has three parts: (i) reduction to short Weierstrass form via measure-preserving changes of variables, (ii) counting F_p-points on modular curves X(3), X_1(3), and twisted versions, and (iii) a p-adic integration method based on Igusa's change-of-variables formula applied to the Jacobians of the covering maps. The paper also contains a self-contained treatment of p-torsion over Q_p (Proposition 6.13), an asymptotic for ℓ-torsion as p→∞, a large-sieve counting application for elliptic curves over Q with rational 3-torsion, and a family-of-twists analysis.
Significance. If correct, the two main theorems give exact unconditional local densities rather than asymptotics, and they are the first such results for torsion of order greater than 2 in this family. The paper is particularly strong in that Theorem 1.1 for p>3 is proved in two independent ways that agree: a reduction-type count (§4) and a p-adic integration computation (§5.2). The modular-curve parametrizations are explicit and the Jacobian determinants are stated in closed form, making the calculations reproducible. All numerical constants arise from a finite set of explicit inputs; no fitted parameters appear. The main external input is the set of Kodaira-type probabilities from Cremona–Sadek [11], which is used in the p=3 part of Theorem 1.1 and in the bad-reduction cases for p>3; the other parts of the argument are self-contained. The paper is clearly written and will be of interest to arithmetic statisticians and to researchers studying local-global questions for elliptic curves.
minor comments (4)
- [Section 6, first paragraph] The citation '[31, Theorem .6.4]' has a missing theorem number; it should refer to a valid statement in Silverman's book, for example the formal logarithm or the structure of E_1(Q_p).
- [Theorem 6.9] The typo 'Propostion' should read 'Proposition'. In the same theorem, the additive and split-multiplicative contributions are imported from [11, Proposition 2.2 and Proposition 2.5]; since the p=3 value 3/26 is the only part of Theorem 1.1 not corroborated by the independent p-adic integration proof, please add a sentence identifying the exact numerical densities taken from [11] (for Kodaira types IV, IV^*, and split I_m) so that this dependence can be checked without consulting the external paper.
- [Lemma 6.11] The phrase 'the smoothness of a map End(E_can) → End(E) is an isomorphism' is mangled; presumably the intended statement is that the reduction homomorphism on endomorphism rings is an isomorphism. Please rephrase.
- [Proposition 7.1 and Corollary 7.2] Proposition 7.1 assumes M_1 and M_2 are positive integers, but Corollary 7.2(1) applies it with M_1 = M_2 = 0; the hypotheses should be amended to allow nonnegative M_i.
Circularity Check
No significant circularity: the main densities are derived by independent modular-curve counting and p-adic integration, with the only self-citation being contextual and not load-bearing.
full rationale
The paper's central claims, Theorems 1.1 and 1.2, are derived through two independent routes: reduction-type counting using modular curves X1(3) and X(3), and p-adic integration via explicit bijections with Tate and Hesse normal forms. The p-adic integration proof does not import the conclusion of Theorem 1.1; instead it computes the same quantity from Jacobian determinants and Igusa's change-of-variables theorem, and Remark 5.11 notes agreement as a sanity check. No fitted parameters appear, and no quantity is defined in terms of the target density. The reliance on Cremona and Sadek [11] for Kodaira-type densities is an external, cited benchmark with no author overlap, and the paper explicitly states when it is used; this is ordinary mathematical citation, not circularity. The only self-citation is to [8] by Bhargava, Cremona, Fisher, and the first author, but it appears solely as context for related p-adic density results and is not load-bearing for the proofs here. The p=3 case uses [11] for IV and IV* densities, but those are independent external inputs, so any sensitivity there is a correctness/fragility concern, not circularity.
Assumptions & free parameters
assumptions (6)
- standard math Igusa's p-adic change of variables formula (Theorem 5.1), including the Jacobian determinant rule for Haar measure.
- domain assumption Cremona-Sadek local density formulas for Kodaira types, especially split multiplicative type I_m density (p-1)^2/(2 p^{m+2}) and the densities of types IV and IV^* with Tamagawa number 3.
- domain assumption The bijections between A^2_{X1(3)}, A^2_{X(3)}, A^2_{Xpsi(3)}, A^2_{XF(3)} and moduli of (E,omega,P) from Tate and Hesse normal forms, valid over perfect fields with char not 2 or 3.
- standard math Hensel's lemma and Nagell-Lutz integrality for torsion points over Z_p.
- domain assumption Serre-Tate canonical lift and Serre's ramification criterion for ordinary elliptic curves (Theorem 6.12).
- standard math Geometric Chebotarev equidistribution of Frobenius conjugacy classes in GL2(F_l) as p varies.
Cite this review
Pith. "Pith review of The density of elliptic curves over $\mathbb{Q}_p$ with a rational 3-torsion point or a rational 3-isogeny." pith.science (2026). https://pith.science/paper/7Y3R37MP
@misc{pith2026250208583,
author = {Pith},
title = {Pith review of: The density of elliptic curves over $\mathbbQ_p$ with a rational 3-torsion point or a rational 3-isogeny},
year = {2026},
howpublished = {\url{https://pith.science/paper/7Y3R37MP}},
note = {Machine review of arXiv:2502.08583}
}
abstract
We determine the probability that a random Weierstrass equation with coefficients in the $p$-adic integers defines an elliptic curve with a non-trivial $3$-torsion point, or with a degree $3$ isogeny, defined over the field of $p$-adic numbers. We determine these densities by calculating the corresponding $p$-adic volume integrals and analyzing certain modular curves. Additionally, we explore the case of $\ell$-torsion for $\ell>3$ prime.
Forward citations
Cited by 1 Pith paper
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Counting odd genus $2$ curves with a marked rational $3$-torsion point
The number of minimal monic odd-degree genus-2 Weierstrass models of height ≤ X with a marked rational Jacobian 3-torsion point is c X^10 + o(X^10) for an effectively computable constant c.
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