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Adam Logan

Publications and source records attributed to Adam Logan.

At least 19 recordsLinked to original sources
arXiv2026

Modular curves and bad reduction

We prove results that imply, under various hypotheses, that every elliptic curve over a number field $k$ corresponding to a point on a modular curve has bad reduction at a certain prime $p$ of $\mathcal{O}_k$. For example, every elliptic curve with a cyclic torsion subgroup of order 20 defined over $\mathbb{Q}(\sqrt{-11})$ or $\mathbb{Q}(\sqrt{17})$ has bad reduction at all primes lying over $3$. The proofs of these statements are quite different, since $3$ is split in $\mathbb{Q}(\sqrt{-11})$ and inert in $\mathbb{Q}(\sqrt{17})$.

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arXiv2025

The Degree of Irrationality of del Pezzo Surfaces

For an irreducible variety $X$ over a field $k$, the degree of irrationality $\operatorname{irr}_k X$ is the minimal degree of a dominant rational map $X \dashrightarrow \mathbb{P}_k^{\operatorname{\dim} X}$. When $X$ is a curve, this is simply the gonality of $X$. We determine the possible degrees of irrationality of del Pezzo surfaces over an assortment of field types: number fields, local fields, finite fields, and arbitrary fields.

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arXiv2025

The Kodaira dimension of Hilbert modular threefolds

Following a method introduced by Thomas-Vasquez and developed by Grundman, we prove that many Hilbert modular threefolds of arithmetic genus $0$ and $1$ are of general type, and that some are of nonnegative Kodaira dimension. The new ingredient is a detailed study of the geometry and combinatorics of totally positive integral elements $x$ of a fractional ideal $I$ in a totally real number field $K$ with the property that $\mathop{\mathrm{tr}} xy < \mathop{\mathrm{min}} I \mathop{\mathrm{tr}} y$ for some $y \gg 0 \in K$.

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arXiv2024

Certifying nontriviality of Ceresa classes of curves

The Ceresa cycle is a canonical algebraic $1$-cycle on the Jacobian of an algebraic curve. We construct an algorithm which, given a curve over a number field, often provides a certificate that the Ceresa cycle is non-torsion, without relying on the presence of any additional symmetries of the curve. Under the hypothesis that the Sato--Tate group is the whole of $\operatorname*{GSp}$, we prove that if the Ceresa class (the image of the Ceresa cycle in \'{e}tale cohomology) is non-torsion, then the algorithm will eventually terminate with a certificate attesting to this fact.

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arXiv2024

Rings of Hilbert modular forms, computations on Hilbert modular surfaces, and the Oda-Hamahata conjecture

The modularity of an elliptic curve $E/\mathbb Q$ can be expressed either as an analytic statement that the $L$-function is the Mellin transform of a modular form, or as a geometric statement that $E$ is a quotient of a modular curve $X_0(N)$. For elliptic curves over number fields these notions diverge; a conjecture of Hamahata asserts that for every elliptic curve $E$ over a totally real number field there is a correspondence between a Hilbert modular variety and the product of the conjugates of $E$. In this paper we prove the conjecture by explicit computation for many cases where $E$ is defined over a real quadratic field and the geometric genus of the Hilbert modular variety is $1$.

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arXiv2024

The Basic Theory of Clifford-Bianchi Groups for Hyperbolic n-Space

Let $K$ be a $\mathbb{Q}$-Clifford algebra associated to an $(n-1)$-ary positive definite quadratic form and let $\mathcal{O}$ be a maximal order in $K$. A Clifford-Bianchi group is a group of the form $\operatorname{SL}_2(\mathcal{O})$ with $\mathcal{O}$ as above. The present paper is about the actions of $\operatorname{SL}_2(\mathcal{O})$ acting on hyperbolic space $\mathcal{H}^{n+1}$ via M\"{o}bius transformations $x\mapsto (ax+b)(cx+d)^{-1}$. We develop the general theory of orders exhibiting explicit orders in low dimensions of interest. These include, for example, higher-dimensional analogs of the Hurwitz order. We develop the abstract and computational theory for determining their fundamental domains and generators and relations (higher-dimensional Bianchi-Humbert Theory). We make connections to the classical literature on symmetric spaces and arithmetic groups and provide a proof that these groups are $\mathbb{Z}$-points of a $\mathbb{Z}$-group scheme and are arithmetic subgroups of $\operatorname{SO}_{1,n+1}(\mathbb{R})^{\circ}$ with their M\"{o}bius action. We report on our findings concerning certain Clifford-Bianchi groups acting on $\mathcal{H}^4$, $\mathcal{H}^5$, and $\mathcal{H}^6$ .

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arXiv2023

Rank-based linkage I: triplet comparisons and oriented simplicial complexes

Rank-based linkage is a new tool for summarizing a collection $S$ of objects according to their relationships. These objects are not mapped to vectors, and ``similarity'' between objects need be neither numerical nor symmetrical. All an object needs to do is rank nearby objects by similarity to itself, using a Comparator which is transitive, but need not be consistent with any metric on the whole set. Call this a ranking system on $S$. Rank-based linkage is applied to the $K$-nearest neighbor digraph derived from a ranking system. Computations occur on a 2-dimensional abstract oriented simplicial complex whose faces are among the points, edges, and triangles of the line graph of the undirected $K$-nearest neighbor graph on $S$. In $|S| K^2$ steps it builds an edge-weighted linkage graph $(S, \mathcal{L}, \sigma)$ where $\sigma(\{x, y\})$ is called the in-sway between objects $x$ and $y$. Take $\mathcal{L}_t$ to be the links whose in-sway is at least $t$, and partition $S$ into components of the graph $(S, \mathcal{L}_t)$, for varying $t$. Rank-based linkage is a functor from a category of ``out-ordered'' digraphs to a category of partitioned sets, with the practical consequence that augmenting the set of objects in a rank-respectful way gives a fresh clustering which does not ``rip apart'' the previous one. The same holds for single linkage clustering in the metric space context, but not for typical optimization-based methods. Orientation sheaves play in a fundamental role and ensure that partially overlapping data sets can be ``glued'' together. Open combinatorial problems are presented in the last section.

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arXiv2022

Higher modularity of elliptic curves over function fields

We investigate a notion of "higher modularity" for elliptic curves over function fields. Given such an elliptic curve $E$ and an integer $r\geq 1$, we say that $E$ is $r$-modular when there is an algebraic correspondence between a stack of $r$-legged shtukas, and the $r$-fold product of $E$ considered as an elliptic surface. The (known) case $r=1$ is analogous to the notion of modularity for elliptic curves over $\mathbf{Q}$. Our main theorem is that if $E/\mathbf{F}_q(t)$ is a nonisotrivial elliptic curve whose conductor has degree 4, then $E$ is 2-modular. Ultimately, the proof uses properties of K3 surfaces. Along the way we prove a result of independent interest: A K3 surface admits a finite morphism to a Kummer surface attached to a product of elliptic curves if and only if its Picard lattice is rationally isometric to the Picard lattice of such a Kummer surface.

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arXiv2022

Definite orthogonal modular forms: Computations, Excursions and Discoveries

We consider spaces of modular forms attached to definite orthogonal groups of low even rank and nontrivial level, equipped with Hecke operators defined by Kneser neighbours. After reviewing algorithms to compute with these spaces, we investigate endoscopy using theta series and a theorem of Rallis. Along the way, we exhibit many examples and pose several conjectures. As a first application, we express counts of Kneser neighbours in terms of coefficients of classical or Siegel modular forms, complementing work of Chenevier-Lannes. As a second application, we prove new instances of Eisenstein congruences of Ramanujan and Kurokawa-Mizumoto type.

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arXiv2020

Explicit coverings of families of elliptic surfaces by squares of curves

We show that, for each $n>0$, there is a family of elliptic surfaces which are covered by the square of a curve of genus $2n+1$, and whose Hodge structures have an action by ${\mathbb Q}(\sqrt{-n})$. By considering the case $n=3$, we show that one particular family of K3 surfaces are covered by the square of genus $7$. Using this, we construct a correspondence between the square of a curve of genus $7$ and a general K3 surface in ${\mathbb P}^4$ with $15$ ordinary double points up to isogeny. This gives an explicit proof of the Kuga-Satake-Deligne correspondence for these K3 surfaces and any K3 surfaces isogenous to them, and further, a proof of the Hodge conjecture for the squares of these surfaces. We conclude that the motives of these surfaces are Kimura-finite. Our analysis gives a birational equivalence between a moduli space of curves with additional data and the moduli space of these K3 surfaces with a specific elliptic fibration.

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arXiv2020

Crepant resolutions of double covers: On the Cynk-Hulek criterion for crepant resolutions of double cover

A collection $S = \{D_1,\ldots, D_n\}$ of divisors in a smooth variety $X$ is an {\em arrangement} if intersections of all subsets of $S$ are smooth. We show that a double cover of $X$ ramified on an arrangement has a crepant resolution under additional hypotheses. Namely, we assume that all intersection components that change the canonical divisor when blown up satisfy are {\em splayed}, a property of the tangent spaces of the components first studied by Faber. This strengthens a result of Cynk and Hulek, which requires a stronger hypothesis on the intersection components. Further, we study the singular subscheme of the union of the divisors in $S$ and prove that it has a primary decomposition where the primary components are supported on exactly the subvarieties which are blown up in the course of constructing the crepant resolution of the double cover.

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arXiv2020

Quotient graphs and amalgam presentations for unitary groups over cyclotomic rings

Suppose $4|n$, $n\geq 8$, $F=F_n=\mathbb{Q}(\zeta_n+\bar{\zeta}_n)$, and there is one prime $\mathfrak{p}=\mathfrak{p}_n$ above $2$ in $F_n$. We study amalgam presentations for $\operatorname{PU_{2}}(\mathbb{Z}[\zeta_n, 1/2])$ and $\operatorname{PSU_{2}}(\mathbb{Z}[\zeta_n, 1/2])$ with the Clifford-cyclotomic group in quantum computing as a subgroup. These amalgams arise from an action of these groups on the Bruhat-Tits tree $\Delta =\Delta_{\mathfrak{p}}$ for $\operatorname{SL_{2}}(F_\mathfrak{p})$ constructed via the Hamilton quaternions. We explicitly compute the finite quotient graphs and the resulting amalgams for $8\leq n\leq 48$, $n\neq 44$, as well as for $\operatorname{PU_{2}}(\mathbb{Z}[\zeta_{60}, 1/2])$.

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arXiv2019

Sarnak's conjecture in quantum computing, cyclotomic unitary group coranks, and Shimura curves

Sarnak's conjecture in quantum computing concerns when the groups $\operatorname{PU}_2$ and $\operatorname{PSU}_2$ over cyclotomic rings $\mathbb{Z}[\zeta_n, 1/2]$ with $\zeta_n=e^{2\pi i/n}$, $4|n$, are generated by the Clifford-cyclotomic gate set. We previously settled this using Euler-Poincar\'{e} characteristics. A generalization of Sarnak's conjecture is to ask when these groups are generated by torsion elements. An obstruction to this is provided by the corank: a group $G$ has $\operatorname{corank} G >0$ only if $G$ is not generated by torsion elements. In this paper we study the corank of these cyclotomic unitary groups in the families $n=2^s$ and $n=3\cdot 2^s$, $n\geq 8$, by letting them act on Bruhat-Tits trees. The quotients by this action are finite graphs whose first Betti number is the corank of the group. Our main result is that for $n=2^s$ and $n=3\cdot 2^s$ the corank groups doubly exponentially in $s$ as $s\rightarrow \infty$; it is $0$ precisely when $n=8,12, 16,24$ and indeed the cyclotomic unitary groups are generated by torsion elements (in fact by the Clifford-cyclotomic gates) for these $n$. We give explicit lower bounds for the corank in two different ways. The first is to bound the isotropy subgroups in the action on the tree by explicit cyclotomy. The second is to relate our graphs to Shimura curves over $F_n=\mathbb{Q}(\zeta_n)^+$ via interchanging local invariants and applying a result of Selberg and Zograf. We show that the cyclotomy arguments give the stronger bounds. In a final section we execute a program of Sarnak to show that our results for the $n=2^s$ and $n=3\cdot 2^s$ families are sufficient to give a second proof of Sarnak's conjecture.

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arXiv2019

The Zariski closure of integral points on varieties parametrizing periodic continued fractions

Let $R$ be the ring of $S$-integers in a number field $K$. Let $\mathcal{B}=\{\beta, \beta^{\ast}\}$ be the multi-set of roots of a nonzero quadratic polynomial over $R$. There are varieties $V(\mathcal{B})_{N,k}$ defined over $R$ parametrizing periodic continued fractions $[b_1,\ldots , b_N,\overline{a_1,\ldots ,a_k}]$ for $\beta$ or $\beta^{\ast}$. We study the $R$-points on these varieties, finding contrasting behavior according to whether groups of units are infinite or not. If $R$ is the rational integers or the ring of integers in an imaginary quadratic field, we prove that the $R$-points of $V(\mathcal{B})_{N,k}$ are not Zariski dense. On the other hand, suppose that $\beta\not\in K\cup\{\infty\}$, $R^\times$ is infinite, and that there are infinitely many units in the (left) order $R_\beta$ of $\beta R+R\subseteq K(\beta)$ with norm to $K$ equal to $(-1)^k$. Then we prove that the $R$-points on $V(\mathcal{B})_{1,k}$ are Zariski dense for $k\geq 8$ and the $R$-points on $V(\mathcal{B})_{0,k}$ are Zariski dense for $k\geq 9$. We also prove that $V(\mathcal{B})_{1,k}$ and $V(\mathcal{B})_{0,k}$ are $K$-rational irreducible varieties for $k$ sufficiently large.

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arXiv2019

The Clifford-cyclotomic group and Euler-Poincar\'e characteristics

For an integer $n\geq 8$ divisible by $4$, let $R_n=\mathbb{Z}[\zeta_n,1/2]$ and let $\operatorname{U}_2(R_n)$ be the group of $2\times 2$ unitary matrices with entries in $R_n$. Set $\operatorname{U}_2^\zeta(R_n)=\{\gamma\in\operatorname{U}_2(R_n)\mid \det\gamma\in\langle\zeta_n\rangle\}$. Let $\mathcal{G}_n\subseteq \operatorname{U}_2^\zeta(R_n)$ be the Clifford-cyclotomic group generated by a Hadamard matrix $H=\frac{1}{2}[\begin{smallmatrix} 1+i & 1+i\\1+i &-1-i\end{smallmatrix}]$ and the gate $T=[\begin{smallmatrix}1 & 0\\0 & \zeta_n\end{smallmatrix}]$. We prove that $\mathcal{G}_n=\operatorname{U}_2^\zeta(R_n)$ if and only if $n=8, 12, 16, 24$ and that $[\operatorname{U}_2^\zeta(R_n):\mathcal{G}_n]=\infty$ if $\operatorname{U}_2^\zeta(R_n)\neq \mathcal{G}_n$. We compute the Euler-Poincar\'{e} characteristics of the groups $\operatorname{SU}_2(R_n)$, $\operatorname{PSU}_2(R_n)$, $\operatorname{PU}_2(R_n)$, $\operatorname{PU}^\zeta_2(R_n)$, and $\operatorname{SO}_3(R_n^+)$.

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arXiv2018

Modularity of two double covers of ${\mathbb P}^5$ branched along $12$ hyperplanes

For two varieties of dimension $5$ constructed as double covers of ${\mathbb P}^5$ branched along the union of $12$ hyperplanes, we prove that the number of points over ${\mathbb F}_p$ can be expressed in terms of Artin symbols and the $p$th Fourier coefficients of modular forms. Many analogous results are known in dimension $\le 3$, but very few in higher dimension. In addition, we use an idea of Burek to construct quotients of our varieties for which the point counts mod $p$ are expressible in terms of Artin symbols and the coefficients of a single modular form of weight $6$.

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arXiv2018

Appendix to a paper [arXiv:1809.08623] of B. Williams

In this appendix to a paper [arXiv:1809.08623] by B. Williams, we give birational equivalences between the models of the Hilbert modular surfaces for ${\mathbb Q}(\sqrt{29})$ and ${\mathbb Q}(\sqrt{37})$ given there and those previously found by Elkies and Kumar.

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