Average $2$-Torsion in Class Groups of Rings Associated to Binary $n$-ic Forms

Let $n geq 3$ be an integer. In this paper, we study the average behavior of the $2$-torsion in class groups of rings cut out by integral binary $n$-ic forms having any fixed odd leading coefficient. Specifically, we compute upper bounds on the average size of the $2$-torsion in class groups of rings and fields arising from such binary forms. Conditional on a uniformity estimate, we further prove that each of these upper bounds is in fact an equality. Our theorems extend recent work of Bhargava-Hanke-Shankar in the cubic case and of Siad in the monic case to binary forms of any degree with any fixed odd leading coefficient. When $n$ is odd, we find that fixing the leading coefficient increases the average $2$-torsion in the class group, relative to the prediction of Cohen-Lenstra-Martinet-Malle. When $n$ is even, such predictions are yet to be formulated; together with Siads results in the monic case, our theorems are the first of their kind to describe the average behavior of the $p$-torsion in class groups of degree-$n$ rings where $p mid n > 2$. To prove these theorems, we first answer a question of Ellenberg by parametrizing square roots of the class of the inverse different of a ring cut out by a binary form in terms of the integral orbits of a certain coregular representation. This parametrization has a range of interesting applications, from studying $2$-parts of class groups to studying $2$-Selmer groups of hyperelliptic Jacobians.

Most Integral Odd-Degree Binary Forms Fail to Properly Represent a Square

Let $F in mathbb{Z}[x,z]$ be a binary form of degree $2n+1 geq 5$. A result of Darmon and Granville known as Faltings plus epsilon states that when $F$ is separable, the superelliptic equation $y^2 = F(x,z)$ has finitely many primitive integer solutions. In this paper, we prove a strong asymptotic version of Faltings plus epsilon which states that in families of superelliptic equations of sufficiently large degree and having a fixed non-square leading coefficient, a positive proportion of members have no primitive integer solutions (put another way, a positive proportion of the corresponding binary forms fail to properly represent a square). Moreover, we show that in these families, a positive proportion of everywhere locally soluble members have a Brauer-Manin obstruction to satisfying the Hasse principle. Our result can be viewed as an analogue for superelliptic equations of Bhargavas result that most even-degree hyperelliptic curves over $mathbb{Q}$ have no rational points.

On the EKL-Degree of a Weyl Cover

More than four decades ago, Eisenbud, Khimv{s}iav{s}vili, and Levine introduced an analogue in the algebro-geometric setting of the notion of local degree from differential topology. Their notion of degree, which we call the EKL-degree, can be thought of as a refinement of the usual notion of local degree in algebraic geometry that works over non-algebraically closed base fields, taking values in the Grothendieck-Witt ring. In this note, we compute the EKL-degree at the origin of certain finite covers $fcolon mathbb{A}^nto mathbb{A}^n$ induced by quotients under actions of Weyl groups. We use knowledge of the cohomology ring of partial flag varieties as a key input in our proofs, and our computations give interesting explicit examples in the field of $mathbb{A}^1$-enumerative geometry.

Hyperelliptic Curves with Maximal Galois Action on the Torsion Points of their Jacobians

In this article, we show that in each of four standard families of hyperelliptic curves, there is a density-$1$ subset of members with the property that their Jacobians have adelic Galois representation with image as large as possible. This result constitutes an explicit application of a general theorem on arbitrary rational families of abelian varieties to the case of families of Jacobians of hyperelliptic curves. Furthermore, we provide explicit examples of hyperelliptic curves of genus $2$ and $3$ over $mathbb Q$ whose Jacobians have such maximal adelic Galois representations.

Inflectionary Invariants for Isolated Complete Intersection Curve Singularities

We investigate the role played by curve singularity germs in the enumeration of inflection points in families of curves acquiring singular members. Let $N geq 2$, and consider an isolated complete intersection curve singularity germ $f colon (mathbb{C}^N,0) to (mathbb{C}^{N-1},0)$. We introduce a numerical function $m mapsto operatorname{AD}_{(2)}^m(f)$ that arises as an error term when counting $m^{mathrm{th}}$-order weight-$2$ inflection points with ramification sequence $(0, dots, 0, 2)$ in a $1$-parameter family of curves acquiring the singularity $f = 0$, and we compute $operatorname{AD}_{(2)}^m(f)$ for various $(f,m)$. Particularly, for a node defined by $f colon (x,y) mapsto xy$, we prove that $operatorname{AD}_{(2)}^m(xy) = {{m+1} choose 4},$ and we deduce as a corollary that $operatorname{AD}_{(2)}^m(f) geq (operatorname{mult}_0 Delta_f) cdot {{m+1} choose 4}$ for any $f$, where $operatorname{mult}_0 Delta_f$ is the multiplicity of the discriminant $Delta_f$ at the origin in the deformation space. Furthermore, we show that the function $m mapsto operatorname{AD}_{(2)}^m(f) -(operatorname{mult}_0 Delta_f) cdot {{m+1} choose 4}$ is an analytic invariant measuring how much the singularity counts as an inflection point. We obtain similar results for weight-$2$ inflection points with ramification sequence $(0, dots, 0, 1,1)$ and for weight-$1$ inflection points, and we apply our results to solve various related enumerative problems.

Surjectivity of Galois Representations in Rational Families of Abelian Varieties

In this article, we show that for any non-isotrivial family of abelian varieties over a rational base with big monodromy, those members that have adelic Galois representation with image as large as possible form a density-$1$ subset. Our results can be applied to a number of interesting families of abelian varieties, such as rational families dominating the moduli of Jacobians of hyperelliptic curves, trigonal curves, or plane curves. As a consequence, we prove that for any dimension $g geq 3$, there are infinitely many abelian varieties over $mathbb Q$ with adelic Galois representation having image equal to all of $operatorname{GSp}_{2g}(widehat{mathbb Z})$.

Lifting Subgroups of Symplectic Groups over $mathbb{Z} / ell mathbb{Z}$

For a positive integer $g$, let $mathrm{Sp}_{2g}(R)$ denote the group of $2g times 2g$ symplectic matrices over a ring $R$. Assume $g ge 2$. For a prime number $ell$, we give a self-contained proof that any closed subgroup of $mathrm{Sp}_{2g}(mathbb{Z}_ell)$ which surjects onto $mathrm{Sp}_{2g}(mathbb{Z}/ellmathbb{Z})$ must in fact equal all of $mathrm{Sp}_{2g}(mathbb{Z}_ell)$. The result and the method of proof are both motivated by group-theoretic considerations that arise in the study of Galois representations associated to abelian varieties.

Permutations that Destroy Arithmetic Progressions in Elementary $p$-Groups

Given an abelian group $G$, it is natural to ask whether there exists a permutation $pi$ of $G$ that destroys all nontrivial 3-term arithmetic progressions (APs), in the sense that $pi(b) - pi(a) eq pi(c) - pi(b)$ for every ordered triple $(a,b,c) in G^3$ satisfying $b-a = c-b eq 0$. This question was resolved for infinite groups $G$ by Hegarty, who showed that there exists an AP-destroying permutation of $G$ if and only if $G/Omega_2(G)$ has the same cardinality as $G$, where $Omega_2(G)$ denotes the subgroup of all elements in $G$ whose order divides $2$. In the case when $G$ is finite, however, only partial results have been obtained thus far. Hegarty has conjectured that an AP-destroying permutation of $G$ exists if $G = mathbb{Z}/nmathbb{Z}$ for all $n eq 2,3,5,7$, and together with Martinsson, he has proven the conjecture for all $n > 1.4 times 10^{14}$. In this paper, we show that if $p$ is a prime and $k$ is a positive integer, then there is an AP-destroying permutation of the elementary $p$-group $(mathbb{Z}/pmathbb{Z})^k$ if and only if $p$ is odd and $(p,k) otin {(3,1),(5,1), (7,1)}$.

Elliptic Curve Variants of the Least Quadratic Nonresidue Problem and Linniks Theorem

Let $E_1$ and $E_2$ be $overline{mathbb{Q}}$-nonisogenous, semistable elliptic curves over $mathbb{Q}$, having respective conductors $N_{E_1}$ and $N_{E_2}$ and both without complex multiplication. For each prime $p$, denote by $a_{E_i}(p) := p+1-#E_i(mathbb{F}_p)$ the trace of Frobenius. Under the assumption of the Generalized Riemann Hypothesis (GRH) for the convolved symmetric power $L$-functions $L(s, mathrm{Sym}^i E_1otimesmathrm{Sym}^j E_2)$ where $i,jin{0,1,2}$, we prove an explicit result that can be stated succinctly as follows: there exists a prime $p mid N_{E_1}N_{E_2}$ such that $a_{E_1}(p)a_{E_2}(p)<0$ and [ p < big( (32+o(1))cdot log N_{E_1} N_{E_2}big)^2. ] This improves and makes explicit a result of Bucur and Kedlaya. Now, if $Isubset[-1,1]$ is a subinterval with Sato-Tate measure $mu$ and if the symmetric power $L$-functions $L(s, mathrm{Sym}^k E_1)$ are functorial and satisfy GRH for all $k le 8/mu$, we employ similar techniques to prove an explicit result that can be stated succinctly as follows: there exists a prime $p mid N_{E_1}$ such that $a_{E_1}(p)/(2sqrt{p})in I$ and [ p < left((21+o(1)) cdot mu^{-2}log (N_{E_1}/mu)right)^2. ]

On Logarithmically Benford Sequences

Let $mathcal{I} subset mathbb{N}$ be an infinite subset, and let ${a_i}_{i in mathcal{I}}$ be a sequence of nonzero real numbers indexed by $mathcal{I}$ such that there exist positive constants $m, C_1$ for which $|a_i| leq C_1 cdot i^m$ for all $i in mathcal{I}$. Furthermore, let $c_i in [-1,1]$ be defined by $c_i = frac{a_i}{C_1 cdot i^m}$ for each $i in mathcal{I}$, and suppose the $c_i$s are equidistributed in $[-1,1]$ with respect to a continuous, symmetric probability measure $mu$. In this paper, we show that if $mathcal{I} subset mathbb{N}$ is not too sparse, then the sequence ${a_i}_{i in mathcal{I}}$ fails to obey Benfords Law with respect to arithmetic density in any sufficiently large base, and in fact in any base when $mu([0,t])$ is a strictly convex function of $t in (0,1)$. Nonetheless, we also provide conditions on the density of $mathcal{I} subset mathbb{N}$ under which the sequence ${a_i}_{i in mathcal{I}}$ satisfies Benfords Law with respect to logarithmic density in every base. As an application, we apply our general result to study Benfords Law-type behavior in the leading digits of Frobenius traces of newforms of positive, even weight. Our methods of proof build on the work of Jameson, Thorner, and Ye, who studied the particular case of newforms without complex multiplication.