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The remaining genus 2 levels, 107, 167, 191 , are more challenging because there are not enough rational points to run the algorithm. Balakrishnan,{ Dogra, M¨uller,} Tuitman, and Vonk [BDMTV21] compute the rational points on the remaining curves of genus 2 and prime level as well as those of genus 3 and prime level using models computed by Galbraith [Gal96] and Elkies. Quadratic Chabauty refers to the technique of depth-two Chabauty-Kim. The Chabauty–Kim method [Kim05; Kim09] can be seen as a generalization of the usual Chabauty–Coleman method. The usual Chabauty–Coleman method is a useful tool in computing rational points on curves (see [Cha41; Col85; MP12; Sto19]), however there are still large classes of curves that do not satisfy the rank condition r := rk Jac(X)(Q) + + Theorem 1.1. The only genus 4 curves X0 (N) where N is prime that have exceptional rational points are X0 (137) + + and X0 (311). There are no exceptional rational points on curves X0 (N) of genus 5 and 6 for any prime N. Therefore, this work confirms Galbraith’s conjecture for prime levels. In particular, this, combined with the results of [BDMTV21] on genus 2 and 3 curves, combined with works of [AM10; BBBLMTV19; BDMTV19; BGX19; Mom86; + Mom87], and work in progress of the second author and M¨uller on curves X0 (N) with composite level will resolve Galbraith’s conjecture. For genus 6, this work shows that the subset of rational points that Mercuri found is in fact all of them, and for those curves with N = 163, 197, 359 we confirm [Mer18, Result 1.2] without any restrictions on height. This paper is organized as follows. In Section 2, we give an overview of the quadratic Chabauty method. Section 3 contains a description of the method we used, namely how we found the models that we used to run the quadratic + Chabauty algorithm starting from the canonical model of X0 (N) and how we chose suitable primes. In Section 4, we + + bound the genus of X0 (N) from below and give a list of all N such that the genus of X0 (N) is less than or equal to 6. 1 + The canonical model for X0 (157) given in Section 5 corrects a small typo in the third equation in Galbraith’s model for N = 157 (the leading term should be 2w2 instead of w2). 2 + In Section 5, there are tables for genus 4, 5, 6 containing the canonical models of X0 (N), the known rational points, and our models and primes for which we run quadratic Chabauty. Acknowledgments This project was initiated as part of a 2020 Arizona Winter School project led by Jennifer Balakrishnan and Netan Dogra. The authors are grateful to Jennifer Balakrishnan and Netan Dogra for the project idea and for their valuable guidance throughout the process. The authors are also grateful to Steffen M¨uller, Padmavathi Srinivasan, and Floris Vermeulen for their support during the project and to Jennifer Balakrishnan, Netan Dogra, and Steffen M¨uller for their comments on an earlier draft. N. A. was supported by the Croatian Science Foundation under the project no. IP2018-01-1313. 2 Overview of Quadratic Chabauty for Modular Curves: theory and algorithm Let X/Q be a smooth projective geometrically connected curve of genus g > 2 with Jacobian J whose Mordell-Weil group J(Q) has rank r = g. For a prime p of good reduction, the abelian logarithm induces a homomorphism 0 1 ∨ log : J(Qp) H (XQ , Ω ) . → p 0 1 ∨ We assume that the p-adic closure of the image of J(Q) J(Qp) in H (XQp , Ω ) has rank g (otherwise classical Chabauty-Coleman applies). We also assume that X(Q)⊂ is non-empty, so we can choose a rational base point b to map X into its Jacobian J. If the N´eron-Severi rank ρ of J is larger than 1, then there exists a non-trivial Z Ker(NS(J) NS(X)) inducing a correspondence on X Q X. Balakrishnan and Dogra in [BD18; BDMTV19] explain∈ how to attach→ to any such Z a locally analytic quadratic× Chabauty function ρZ : X(Qp) Qp → as follows: using Nekov´aˇr’s theory of p-adic heights [Nek93], one can construct a global p-adic height which decomposes as a sum of local height functions. The quadratic Chabauty function ρZ is defined as the difference between the global p-adic height and the local height for the chosen prime p. Even though we do not go into details here, we note that the computation of the global height pairing is easier when X has many rational points (at least 3, which is indeed the case for all modular curves studied in this paper). The crucial property of the quadratic Chabauty function is that there exists a finite set Υ Qp, such that ⊂ ρZ (x) Υ for any x X(Q). Because ρZ has Zariski-dense image on every residue disk and is given by a convergent ∈ ∈ power series, this implies that the set X(Qp)2 of Qp-points of X having values in Υ under ρZ is finite. Because X(Q) is contained in that set, it is finite as well. Since both ρZ and Υ can be explicitly computed by [BBBDLMT+20], this 2 makes the provable determination of X(Qp)2 possible if the quadratic Chabauty condition r • Input: – A modular curve X/Q that satisfies r = g 2 and such that the p-adic closure of the image of J(Q) in 0 1 ∨ ≥ H (XQp , Ω ) under log has rank g. – A prime p which is a prime of good reduction for X/Q such that the Hecke operator Tp generates End(J) Z ⊗ Q. We check this condition in our cases by showing that ap(f) generates the number field generated by the coefficients of f where f is any newform orbit representative associated to X/Q. • Output: A finite set containing X(Qp)2. 2 at least under the assumption that ρZ has no repeated roots; otherwise, we get a finite superset. 3 + The curves of our interest, X0 (N) of genus 4, 5 and 6 where N is a prime, satisfy the condition r = g. This is +,new ′ done by checking that for all N in (1), (2) and (3), f S2(Γ0(N)) satisfies L (f, 1) = 0 (actually, we just need to check this for one arbitrary representative in each Galois∈ orbit). According to [DL20, Proposition6 7.1] (which the + authors attribute to Kolyvagin and Logachev [KL89]), this implies that the (algebraic and analytic) rank of J0 (N)/Q equals dim(Afi )= g where the summation is taken over an arbitrary set of newform orbits representatives. Let us + note here that this equality r = g doesn’t hold for all X0 (N), the smallest genus where r>g happens (for prime N) is g = 206P (and N = 5077) [BDMTV21; DL20]. The implementation (available at [BBBDLMT+20]) of the algorithm in [BDMTV21] is designed to take as input a plane affine patch Y : Q(x, y) = 0 of a modular curve X/Q. The model Q(x, y) = 0 does not need to be smooth, but it should be monic in y with p-integral coefficients. We can sometimes find an affine patch Y such that all rational points on X must be among the points returned by running their program on Y . We have managed to do that for all + genus 4 and 5 curves X0 (N) where N is prime. If no such Y is found, then we find two affine patches such that every rational point on X is contained in at least one patch. Moreover, we need every Fp-point on X (realized as a canonical model) to map to a good point (Definition 3.1) on at least one patch. Finding favorable affine patches turned out to be the most challenging part when applying their algorithm for large genus Atkin-Lehner quotients. We talk about our strategy for generating these affine patches in Section 3. 3 Overview of strategy + As introduced in Section 2, our first task is to find a suitable plane model Q(x, y) of X0 (N) that is monic in y + g−1 and has “small” coefficients. We start with the image of the canonical embedding of X0 (N) in P (as stated in + the introduction, these are non-hyperelliptic for our N). In what follows we shall identify X0 (N) with its canonical + 1 image and use the same notation for both. We find two rational maps τx, τy : X0 (N) P such that the product + 1 1 → 1 1 3 τx τy : X (N) P P is a birational map onto its image. Compose with the Segre embedding P P P , × 0 → × × → [w:x:y:z] and then project from the point [1: 0: 0: 0] onto the plane w = 0; denote by ϕ′ the composite τx×τy Segre projection ϕ′ : X+(N) P1 P1 P3 P2 . 0 −−−−→ × −−−→ [w:x:y:z] −−−−−−→ [x:y:z] Choose x1, x2,y1,y2 Pg−1 (1) such that τx(q) = [x1(q): x2(q)] and τy(q) = [y1(q): y2(q)] in coordinates. Then the equation for ϕ′ is ∈ O ϕ′ : q [(x y )(q): (x y )(q): (x y )(q)]. 7→ 1 2 2 1 2 2 ′ (When x2(q),y2(q) = 0, this is just ϕ : q [(x1/x2)(q) : (y1/y2)(q) : 1].) 6 7→ + ′ Note that on some closed subvarieties of X0 (N), the map ϕ is not defined. Indeed, on the subvariety with additional equations x1 = x2 = 0, the map τx is not defined; with y1 = y2 = 0, the map τy is not defined; with x2 = y2 = 0, (the map τx or τy or) the projection following the Segre embedding is not defined. We denote the union + ′ ′ + ′ of the three subvarieties by X0 (N)ϕ undef, outside which ϕ is well-defined. The set of rational points X0 (N)ϕ undef(Q) can be computed directly using Magma [BCP97], and in our final step, we check that it does not contain any additional + ′ points other than the known rational points. For now, we focus on the generic part of X0 (N) where ϕ is defined. The image ′ := ϕ′(X+(N)) will be a curve given by an equation of the form CN 0 d d−1 Q (x, z)y + Q (x, z)y + + Qd(x, z)=0, 0 1 ··· where Q0(x, 1) is monic. (If not, simply divide the equation throughout by the leading coefficient of Q0(x, 1) to get to d−1 this form.) Multiply the above equation throughout by Q0 , we get d d−1 d−1 (Q (x, z)y) + Q (x, z)(Q (x, z)y) + + Q (x, z) Qd(x, z)=0, 0 1 0 ··· 0 ′ Let deg Q denote the total degree of Q and let N be the image of under the map 0 0 C CN ψ :[x: y : z] xzdeg Q0 : Q (x, z)y : zdeg Q0+1 . 7→ 0 Then the affine patch of N given by z = 1 will have an equation Q(x, y) = 0 where Q(x, y) is a polynomial over Q C + ′ monic in y, suitable for the quadratic Chabauty algorithm. Let ϕ: X (N) N denote the composite ϕ = ψ ϕ . 0 →C ◦ Next, we select a suitable prime p and run QCModAffine on the affine patch of N given by z = 1. However, QCModAffine only computes rational points (and compares with known rational points)C outside the bad residue disks. an Definition 3.1 ([BT20], Definition 2.8 and 2.10). Let X denote the rigid analytic space over Qp associated to a projective curve X/Qp given by (inhomogeneous) equation Q(x, y)=0. Let ∆(x) be the discriminant of Q as a function of y, and let r(x) = ∆/ gcd(∆, d∆/dx). We say that a residue disk (as well as any point inside it) is infinite if it contains a point whose x-coordinate is , is bad if it contains a point whose x-coordinate is or a zero of r(x), and is good if it is not bad. ∞ ∞ 4 Therefore, we may need to repeat this construction multiple times (with the same p), i.e., we need a collection k + (ϕN,i, N,i) i=1 such that for each P X0 (N)(Fp), there exists some i such that ϕN,i(P ) does not map to a bad Fp- { C } ∈ + point. This would imply that for each residue disk D of X (N)(Qp), there exists some i such that ϕN,i(D) N,i(Qp) 0 ⊆C is contained in a good residue disk. If QCModAffine reports that the only rational points in the good disks of N,i(Qp) + C are the images of the known rational points, then we know that X0 (N)(Q) is contained in the finite set k −1 + + ϕ ϕN,i(X (N)(Q) ) X (N)ϕ′ (Q), N,i{ 0 known } ∪ 0 undef i=1 [ + and then we check that this equals X0 (N)(Q)known using direct Magma computations. In practice, we seek to choose x , x ,y ,y g−1 (1) so that dx := deg τx, dy := deg τy are small for faster 1 2 1 2 ∈ OP computations, since the degree of the defining equation Q(x, y) of N is at most dxdy and the maximum power of y C that appears is dx. We would also like to keep k small, and ideally to have k = 1, in which case we need to find a prime p for which Q(x, y) = 0 has no bad disks. In order to do so, we would like to start with Q(x, y) for which r(x) has no linear factor over Q. If the construction does not satisfy this property, we adjust the construction of ϕ′ by 1 1 ′ 2 applying some σ Aut(P Q P ) before the Segre embedding, and/or by post-composing ϕ by some ρ Aut(P ) (the latter only invoked∈ in the× N = 211 case). With suitable adjustments in the construction of ϕ′, we were∈ able to find plane models Q(x, y) for which r(x) has no linear factor over Q for all the cases we aim to solve in this paper. Even so, for four of the genus 6 cases, N = 197, 211, 223, 359, we did not find a model for which there is a prime p without any bad disks, and in each of these four cases, we were able to find two patches and a prime p such that for + each P X0 (N)(Fp), ϕN,i(P ) does not map to a bad Fp-point on at least one of the patches. For all the other prime ∈ + levels where X0 (N) has genus 4, 5, 6, we were able to find a single patch and a prime for which there is no bad disk. That is, we were able to solve the problem with k 2. In this section, we do not use that the curve in≤ question has a modular interpretation. The strategy could possibly be applied to other curves of arbitrary genus g. Currently Magma supports computing plane models and gonal maps for curves up to genus 6. X+ N 4 Modular curves 0 ( ) of genus at most 6 + + We will obtain an explicit lower bound on the genus of X0 (N) in order to find all X0 (N) of genus at most 6. + + Let g0(N) be the genus of X0(N) and let g0 (N) be the genus of X0 (N). Theorem 4.1. For integers N 1, we have g (N) (N 5√N 8)/12. ≥ 0 ≥ − − Proof. See Section 3 of [CWZ00]. Let ν(N) denote the number of fixed points of wN . Let h(D) denote the class number of the quadratic order with discriminant D. Theorem 4.2. For N 5, ≥ h( 4N)+ h( N) if N 3 (mod 4) ν(N)= − − ≡ h( 4N) otherwise. ( − Proof. Apply the formula in Remark 2 of [FH99]. + To bound g0 (N), we will need an upper bound on h(D). Lemma 4.3. For negative D, √ D h(D) − (ln(4 D )+2). ≤ π | | Proof. For negative D, the Dirichlet class number formula states that w√ D h(D)= − L (1,χ ) 2π D D where χD(m)= m is the Kronecker symbol and
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