**Computing nonsurjective primes associated to Galois representations of genus 2 curves**,
with , Armand Brumer, , Zev Klagsbrun, , and

(pdf) (arXiv)
(code)
(show abstracthide abstract)
For a genus 2 curve C over the rationals whose Jacobian A admits only trivial geometric endomorphisms, Serre's open image theorem for abelian surfaces asserts that there are only finitely many primes ell for which the Galois action on ell-torsion points of A is not maximal. Building on work of Dieulefait, we give a practical algorithm to compute this finite set. The key inputs are Mitchell's classification of maximal subgroups of PSp_4(F_ell), sampling of the characteristic polynomials of Frobenius, and the Khare--Wintenberger modularity theorem. The algorithm has been submitted for integration into Sage, executed on all of the genus 2 curves with trivial endomorphism ring in the LMFDB, and the results incorporated into the homepage of each such curve on a publicly-accessible development branch of the LMFDB.

**Quadratic enrichment of the logarithmic derivative of the zeta function**,
with , , , and

(pdf) (arXiv)
(show abstracthide abstract)
We define an enrichment of the logarithmic derivative of the zeta function of a variety over a finite field to a power series with coefficients in the Grothendieck--Witt group. This enrichment is related to the topology of the real points of a lift. We show a rationality result for cellular schemes over a field, and compute several examples, including toric varieties.

**Subspace configurations and low degree points on curves**,
with

(pdf) (arXiv)
(show abstracthide abstract)
This paper is devoted to understanding curves X over a number field k that possess infinitely many solutions in extensions of k of degree at most d; such solutions are the titular low degree points. For d = 2,3 it is known (by the work of Harris-Silverman and Abramovich-Harris) that such curves, after a base change to kbar, admit a map of degree at most d onto P^1 or an elliptic curve. For d at least 4 the analogous statement was shown to be false by Debarre and Fahlaoui. We prove that once the genus of X is high enough, the low degree points still have geometric origin: they can be obtained as pullbacks of low degree points from a lower genus curve. We introduce a discrete-geometric invariant attached to such curves: a family of subspace configurations, with many interesting properties. This structure gives a natural alternative construction of curves with many low degree points, that were first discovered by Debarre and Fahlaoui. As an application of our methods, we obtain a classification of such curves over k for d = 2,3, and a classification over kbar for d = 4,5.

**Stability of Tschirnhausen bundles**,
with and

*International Mathematics Research Notices*, to appear
(pdf) (arXiv)
(show abstracthide abstract)
Let alpha : X -> Y be a general degree r primitive map of nonsingular, irreducible, projective curves over an algebraically closed field of characteristic zero or larger than r. We prove that the Tschirnhausen bundle of alpha is semistable if g(Y) >= 1 and stable if g(Y) >= 2.

**Curve classes on conic bundle threefolds and applications to rationality**,
with , , and

(pdf) (arXiv)
(code)
(show abstracthide abstract)
We undertake a study of conic bundle threefolds pi: X -> W over geometrically rational surfaces whose associated discriminant covers Deltatilde -> Delta are smooth and geometrically irreducible. First, we determine the structure of the group CH^2 Xbar of rational equivalence classes of curves. Precisely, we construct a Galois-equivariant group homomorphism from CH^2 Xbar to a group scheme associated to the discriminant cover Deltatilde -> Delta of X. The target group scheme is a generalization of the Prym variety of Deltatilde -> Delta and so our result can be viewed as a generalization of Beauville's result that the algebraically trivial curve classes on Xbar are parametrized by the Prym variety.

Next, we use our structural result on curve classes to study rationality obstructions, in particular the refined intermediate Jacobian torsor (IJT) obstruction recently introduced by Hassett--Tschinkel and Benoist--Wittenberg. We show that for conic bundle threefolds there is no strongest (known) rationality obstruction. Precisely, we construct a geometrically rational irrational conic bundle threefold where the IJT obstruction cannot witness irrationality (irrationality is detected through the real topology) and a geometrically rational irrational conic bundle threefold where all classical rationality obstructions vanish and the IJT obstruction is needed to prove irrationality.

**The normal bundle of a general canonical curve of genus at least 7 is semistable**

with and

*Journal of the European Mathematical Society (JEMS)*, to appear
(pdf) (arXiv)
(show abstracthide abstract)
Let C be a general canonical curve of genus g defined over an algebraically closed field of arbitrary characteristic. We prove that if g is not in {4,6}, then the normal bundle of C is semistable. In particular, if g is 1 or 3 mod 6, then the normal bundle is stable.

**Interpolation for Brill--Noether curves**,
with

(pdf) (arXiv)
(show abstracthide abstract)
(Quanta article)
In this paper we determine the number of general points through which a Brill--Noether curve of fixed degree and genus in any projective space can be passed.

**A transcendental Brauer-Manin obstruction to weak approximation on a Calabi-Yau threefold**,
with , , , with an appendix by

*Research in Number Theory*, 2022
(pdf) (arXiv)
(journal)
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In this paper we investigate the Q-rational points of a class of simply connected Calabi-Yau threefolds, originally studied by Hosono and Takagi in the context of mirror symmetry. These varieties are defined as a linear section of a double quintic symmetroid; their points correspond to rulings on quadric hypersurfaces. They come equipped with a natural 2-torsion Brauer class. Our main result shows that under certain conditions, this Brauer class gives rise to a transcendental Brauer-Manin obstruction to weak approximation.
Hosono and Takagi also showed that over C each of these Calabi-Yau threefolds Y is derived equivalent to a Reye congruence Calabi-Yau threefold X. We show that these derived equivalences may also be constructed over Q and give sufficient conditions for X to not satisfy weak approximation. In the appendix, N. Addington exhibits the Brauer groups of each class of Calabi-Yau variety over C.

**Global Brill--Noether theory over the Hurwitz Space**,
with and

*Geometry & Topology*, to appear
(pdf) (arXiv)
(show abstracthide abstract)
Let C be a curve of genus g. A fundamental problem in the theory of algebraic curves is to understand maps C to P^r of specified degree d. When C is general, the moduli space of such maps is well-understood by the main theorems of Brill--Noether theory. Despite much study over the past three decades, a similarly complete picture has proved elusive for curves of fixed gonality. Here we complete such a picture, by proving analogs of all of the main theorems of Brill--Noether theory in this setting. As a corollary, we prove a conjecture of Eisenbud and Schreyer regarding versal deformation spaces of vector bundles on P^1.

**Stability of normal bundles of space curves**,
with and

*Algebra and Number Theory*, 2022
(pdf) (arXiv)
(journal)
(show abstracthide abstract)
In this paper, we prove that the normal bundle of a general Brill-Noether space curve
of degree d and genus g at least 2 is stable if and only if (d, g) is not one of (5, 2) or (6, 4). When g is at most 1 and the
characteristic of the ground field is zero, it is classical that the normal bundle is strictly semistable.
We show that this fails in characteristic 2 for all rational curves of even degree.

**An enriched count of the bitangents to a smooth plane quartic curve**,
with

*Research in the Mathematical Sciences*, 2021
(pdf) (arXiv)
(journal)
(show abstracthide abstract) (video)
Recent work of Kass--Wickelgren gives an enriched count of the 27 lines on a smooth cubic surface over arbitrary fields, generalizing Segre's signed count count of elliptic and hyperbolic lines. Their approach using A^1-enumerative geometry suggests that other classical enumerative problems should have similar enrichments, when the answer is computed as the degree of the Euler class of a relatively orientable vector bundle. Here, we consider the closely related problem of the 28 bitangents to a smooth plane quartic.
However, it turns out the relevant vector bundle is not relatively orientable and new ideas are needed to produce enriched counts.
We introduce a fixed ``line at infinity," which leads to enriched counts of bitangents that depend on their geometry relative to the quartic and this distinguished line.

**Low degree points on curves**, with

*International Mathematics Research Notices*, 2020
(pdf)
(arXiv)
(journal)
(show abstracthide abstract) (video)
In this paper we investigate an arithmetic analogue of the gonality of a smooth projective curve C over a number field k: the minimal e such there are infinitely many points P with [k(P):k] at most e. Developing techniques that make use of an auxiliary smooth surface containing the curve, we show that this invariant can take any value subject to constraints imposed by the gonality. Building on work of Debarre--Klassen, we show that this invariant is equal to the gonality for all sufficiently ample curves on a surface S with trivial irregularity.

**A local-global principle for isogenies of composite degree**,

*Proceedings of the London Mathematical Society*, 2020
(pdf) (arXiv)
(code)
(journal, errata)
(show abstracthide abstract)
Let E be an elliptic curve over a number field K. If for almost all primes of K, the reduction of E modulo that prime has rational cyclic isogeny of fixed degree, we can ask if this forces E to have a cyclic isogeny of that degree over K. Building upon the work of Sutherland, Anni, and Banwait-Cremona in the case of prime degree, we consider this question for cyclic isogenies of arbitrary degree.

**Interpolation for Brill-Noether curves in P^4**, with

*European Journal of Mathematics*, 2020
(pdf) (arXiv)
(journal)
(show abstracthide abstract)
In this paper, we compute the number of general points through which a general Brill-Noether curve in ℙ^4 passes. We also prove an analogous theorem when some points are constrained to lie in a transverse hyperplane.

**Abelian varieties isogenous to a power of an elliptic curve over a Galois extension**,

*Journal de Théorie des Nombres de Bordeaux*, 31(1): 205-213, 2019
(pdf) (arXiv)
(journal)
(show abstracthide abstract)
Given an elliptic curve E/k and a Galois extension k′/k, we construct an exact functor from torsion-free modules over the endomorphism ring End(E/k′) with a semilinear Gal(k′/k) action to abelian varieties over k that are k′-isogenous to a power of E. As an application, we show that every elliptic curve with complex multiplication geometrically is isogenous over the ground field to one with complex multiplication by a maximal order.

**Constants in Titchmarsh divisor problems for elliptic curves**,

with , Cliff Blakestad, , , , and

*Research in Number Theory*, 6(1): Art. 1, 24, 2020,
(pdf) (arXiv)
(journal)
(show abstracthide abstract)
Inspired by the analogy between the group of units 𝔽_p^* of the finite field with p elements and the group of points E(𝔽_p) of an elliptic curve E/𝔽_p, E. Kowalski, A. Akbary & D. Ghioca, and T. Freiberg & P. Kurlberg investigated the asymptotic behaviour of elliptic curve sums analogous to the Titchmarsh divisor sum ∑_{p≤x} τ(p+a) ∼ Cx. In this paper, we present a comprehensive study of the constants C(E) emerging in the asymptotic study of these elliptic curve divisor sums. Specifically, by analyzing the division fields of an elliptic curve E/ℚ, we prove upper bounds for the constants C(E) and, in the generic case of a Serre curve, we prove explicit closed formulae for C(E) amenable to concrete computations. Moreover, we compute the moments of the constants C(E) over two-parameter families of elliptic curves E/ℚ. Our methods and results complement recent studies of average constants occurring in other conjectures about reductions of elliptic curves by addressing not only the average behaviour, but also the individual behaviour of these constants, and by providing explicit tools towards the computational verifications of the expected asymptotics.

**Elliptic fibrations on covers of the elliptic modular surface of level 5**,

with , , , , and

*WINE II: Contributions to Number Theory and Arithmetic Geometry*, AWM Springer Series 11 (2018)

(pdf)
(arXiv)
(book)
(show abstracthide abstract)
We consider the K3 surfaces that arise as double covers of the elliptic modular surface of level 5, R_{5,5}. Such surfaces have a natural elliptic fibration induced by the fibration on R_{5,5}. Moreover, they admit several other elliptic fibrations. We describe such fibrations in terms of linear systems of curves on R_{5,5}. This has a major advantage over other methods of classification of elliptic fibrations, namely, a simple algorithm that has as input equations of linear systems of curves in the projective plane yields a Weierstrass equation for each elliptic fibration. We deal in detail with the cases for which the double cover is branched over the two reducible fibers of type I_5 and for which it is branched over two smooth fibers, giving a complete list of elliptic fibrations for these two scenarios.

**Interpolation for Brill-Noether space curves**,

*manuscripta mathematica* 156(1), (2018) (pdf)
(arXiv)
(journal)
(show abstracthide abstract)
In this note we compute the number of general points through which a general Brill–Noether space curve passes.

**Powers in Lucas sequences via Galois representations**, with

*Proc. Amer. Math. Soc.*, 143 (2015) (pdf)
(arXiv)
(journal)
(show abstracthide abstract)
Let u_n be a nondegenerate Lucas sequence. We generalize the results of Bugeaud, Mignotte, and Siksek, 2006 to give a systematic approach towards the problem of determining all perfect powers in any particular Lucas sequence. We then prove a general bound on admissible prime powers in a Lucas sequence assuming the Frey-Mazur conjecture on isomorphic mod p Galois representations of elliptic curves.