CyberSec Research

We study Chow groups and \'etale motivic cohomology groups of smooth complete intersections with Hodge structures of level one, classified by Deligne and Rapoport, with particular attention to fivefolds. We extend these results to an \'etale motivic context and recover an analogous finite-dimensionality in the sense of Kimura. We further analyse algebraic cycles on other smooth Fano manifolds with Hodge structures of level one and, as an application, we prove the integral Hodge conjecture for smooth quartic double fivefolds by means of the \'etale motivic approach.

For a morphism $f : X \to Y$ of schemes, we give a tropical criterion for which points of $Y$ (valued in a field, discrete valuation ring, number ring, or Dedekind domain) lift to $X$. Our criterion extends the firmaments of Abramovich to a wide range of morphisms, even logarithmic stable maps.

We give results on reduced complex-analytic curve germs which relate their indecomposable maximal Cohen-Macaulay (MCM) modules to their lattice homology groups and related invariants, thereby providing a connection between the algebraic theory of MCM modules and techniques arising from low-dimensional topology. In particular, we characterize the germs $(C, o)$ of finite Cohen-Macaulay type in terms of the lattice homology $\mathbb{H}_*(C, o)$, and those of tame type in terms of the lattice homologies and associated spectral sequences of $(C, o)$ and its subcurves, including the distinction between germs of finite and infinite growth. As a consequence of these results, we obtain corresponding characterizations of a germ's Cohen-Macaulay type in terms of the motivic Poincar\'e series.

The objective of this article is to prove the necessity statement in Crawley-Boevey's conjectural solution to the (tame) Deligne-Simpson problem. We use the nonabelian Hodge correspondence, variation of parabolic weights and results of Schedler-Tirelli to reduce to simpler situations, where every conjugacy class is semi-simple and the underlying quiver is (1) an affine Dynkin diagram or (2) an affine Dynkin diagram with an extra vertex. In case (1), a nonexistence result of Kostov applies. In case (2), the key step is to show that simple representations, if exist, lie in the same connected component as direct sums of lower dimensional ones.

The class of the fine moduli space of stable $n$-pointed curves of genus zero, $\overline{\mathcal{M}_{0,n}}$, in the Grothendieck ring of varieties encodes its Poincar\'e polynomial. Aluffi-Chen-Marcolli conjecture that the Grothendieck class of $\overline{\mathcal{M}_{0,n}}$ is real-rooted (and hence ultra-log-concave), and they proved an asymptotic ultra-log-concavity result for these polynomials. We build upon their work, by providing effectively computable bounds for the error term in their asymptotic formula for $\mathrm{rk}\, H^{2l}(\overline{\mathcal{M}_{0,n}})$. As a consequence, we prove that in the range $l \le \frac{n}{10\log n}$, the ultra-log-concavity inequality \[\left(\frac{\mathrm{rk}\, H^{2(l-1)}(\overline{\mathcal{M}_{0,n}})}{\binom{n-3}{l-1}}\right)^2 \ge \frac{\mathrm{rk}\, H^{2(l-2)}(\overline{\mathcal{M}_{0,n}})\mathrm{rk}\, H^{2l}(\overline{\mathcal{M}_{0,n}})}{\binom{n-3}{l-2}\binom{n-3}{l}} \] holds for $n$ sufficiently large.

In this paper we prove a common generalisation of results by \v{S}penko-Van den Bergh and Iyama-Wemyss that can be used to generate non-commutative crepant resolutions (NCCRs) of some affine toric Gorenstein varieties. We use and generalise results by Novakovi\'{c} to study NCCRs for affine toric Gorenstein varieties associated to cones over polytopes with interior points. As a special case, we consider the case where the polytope is reflexive with $\le \dim P+2$ vertices, using results of Borisov and Hua to show the existence of NCCRs.

We discuss how metric limits and rescalings of K\"ahler-Einstein metrics connect with Algebraic Geometry, mostly in relation to the study of moduli spaces of varieties, and singularities. Along the way, we describe some elementary examples, review some recent results, and propose some tentative conjectural pictures.

We prove G\"ortz's combinatorial conjecture \cite{Go01} on dual shellability of admissible sets in Iwahori-Weyl groups, proving that the augmented admissible set $\widehat{\mathrm{Adm}}(\mu)$ is dual shellable for any dominant coweight $\mu$. This provides a uniform, elementary approach to establishing Cohen-Macaulayness of the special fibers of the local models with Iwahori level structure for all reductive groups-including residue characteristic $2$ and non-reduced root systems-circumventing geometric methods. Local models, which encode singularities of Shimura varieties and moduli of shtukas, have seen extensive study since their introduction by Rapoport-Zink, with Cohen-Macaulayness remaining a central open problem. While previous work relied on case-specific geometric analyses (e.g., Frobenius splittings \cite{HR23} or compactifications \cite{He13}), our combinatorial proof yields an explicit labeling that constructs the special fiber by sequentially adding irreducible components while preserving Cohen-Macaulayness at each step, a new result even for split groups.

The Gross-Zagier formula on singular moduli can be seen as a calculation of the intersection multiplicity of two CM divisors on the integral model of a modular curve. We prove a generalization of this result to a Shimura curve.

This note revisits stability conditions on the bounded derived categories of coherent sheaves on irreducible projective curves. In particular, all stability conditions on smooth curves are classified and a connected component of the stability manifold containing all the geometric stability conditions is identified for singular curves. On smooth curves of positive genus, the set of all non-locally-finite stability conditions gives a partial boundary of any known compactification of the stability manifold. To provide the full boundary, a notion of weak stability condition is proposed based on the definition of Collins--Lo--Shi--Yau and is classified for smooth curves of positive genus. On singular curves, the connected component containing geometric stability conditions is shown to be preserved by the two natural actions on the stability manifold.

In this paper, we study a problem that is in a sense a reversal of the Hurwitz counting problem. The Hurwitz problem asks: for a generic target -- $\mathbb P^1$ with a list of $n$ points $q_1,\dots,q_n\in \mathbb P^1$ -- and partitions $\sigma_1,\dots,\sigma_n$ of $d$, how many degree $d$ covers $C\to\mathbb P^1$ are there with specified ramification $\sigma_i$ over $q_i$? We ask: for a generic source -- an $r$-pointed curve $(C,p_1,\dots,p_r)$ of genus $1$ -- and partitions $\mu, \sigma_1,\dots,\sigma_n$ of $d$ with $\ell(\mu)=r$, how many degree $d$ covers $C\to\mathbb P^1$ are there with ramification profile $\mu$ over $0$ corresponding to a fiber $\{p_1,\dots,p_r\}$ and elsewhere ramification profiles $\sigma_1,\dots,\sigma_n$? While the enumerative invariants we study bear a similarity to generalized Tevelev degrees, they are more difficult to express in closed form in general. Nonetheless, we establish key results: after proving a closed form result in the case where the only non-simple unmarked ramification profiles $\sigma_1$ and $\sigma_2$ are ``even'' (consisting of $2,\dots,2$), we go on to establish recursive formulas to compute invariants where each unmarked ramification profile is of the form $(x,1,\dots,1)$. A special case asks: given a generic $d$-pointed genus $1$ curve $(E,p_1,\dots,p_d)$, how many degree $d$ covers $(E,p_1,\dots,p_d)\to(\mathbb P^1,0)$ are there with $d-2$ unspecified points of $E$ having ramification index $3$? We show that the answer is an explicit quartic in $d$.

Let $E$ be a vector bundle over a smooth curve $C$, and $V$ a generating space of sections of $E$. We characterise Mumford linear stability of the associated projective model of $\mathbb{P} E^\vee$ in $\mathbb{P} V^\vee$ in terms of geometric and cohomological properties of the coherent system $(E, V)$, and give some applications. We show that any $\mathbb{P}^{r-1}$-bundle over $C$ has a linearly stable model in $\mathbb{P}^{n-1}$ for any $n \ge r+2$. Furthermore; linear stability of $(E, V)$ is a necessary condition for stability of the kernel bundle $M_{E, V}$ of $(E, V)$, which is predicted by Butler's conjecture for general $C$ and $(E, V)$. We give new examples showing that it is not in general sufficient; in particular, a general bundle $E$ of large degree fits into a linearly stable coherent system $(E, V)$ with nonsemistable kernel bundle. Finally, we use these ideas to show the stability of $M_{E, V}$ for certain $(E, V)$ of type $(r, d, r+2)$ where $E$ is not necessarily stable.

In this note, we show that for a smooth algebraic variety $Y$ and a smooth $m$-secant section $X$ of the $\mathbb{P}^1$-bundle \[ f : \mathbb{P}(\mathcal{O}_Y \oplus \mathcal{O}_Y(E)) \longrightarrow Y, \] where $E$ is an effective divisor on $Y$ satisfying $H^1(Y, \mathcal{O}_Y(kE)) = 0$ for all $k = 1, \ldots, m-1$, the Tschirnhausen module of the induced covering $ f|_X : X \longrightarrow Y $ is completely decomposable. We then apply it to coverings of curves arising in such a way.

We study the expressivity of rational neural networks (RationalNets) through the lens of algebraic geometry. We consider rational functions that arise from a given RationalNet to be tuples of fractions of homogeneous polynomials of fixed degrees. For a given architecture, the neuromanifold is the set of all such expressible tuples. For RationalNets with one hidden layer and fixed activation function $1/x$, we characterize the dimension of the neuromanifold and provide defining equations for some architectures. We also propose algorithms that determine whether a given rational function belongs to the neuromanifold. For deep binary RationalNets, i.e., RationalNets all of whose layers except potentially for the last one are binary, we classify when the Zarisky closure of the neuromanifold equals the whole ambient space, and give bounds on its dimensions.

Hanlon, Hicks and Lazarev constructed resolutions of structure sheaves of toric substacks by certain line bundles on the ambient toric stacks. In this paper, we give a new and substantially simpler proof of their result.

We investigate homological stability for the space of sections of Fano fibrations over curves in the context of weak approximation, and establish it for projective bundles, as well as for conic and quadric surface bundles over curves.

Let $X$ be a Mori dream K3 surface of Picard rank $2$. We compute the nef cone of the Hilbert scheme $X^{[n]}$ of $n$ points on $X$ for large $n$ using Bridgeland stability methods. We also find bounds on the nef cone for small $n$. Studying the geometry of a $2$-to-$1$ cover $X\rightarrow Q$ of a smooth quadric $Q$ in $\mathbb{P}^3$, and its induced rational map $X^{[2]}\dashrightarrow Q^{[2]}$ on the Hilbert schemes, we calculate the nef cone of $X^{[2]}$ in a particular case. Finally, we also find the nef cone of the nested Hilbert scheme $X^{[n,n+1]}$ for large $n$.

We present an algorithm which, given a connected smooth projective curve $X$ over an algebraically closed field of characteristic $p>0$ and its Hasse--Witt matrix, as well as a positive integer $n$, computes all \'etale Galois covers of $X$ with group $\mathbb{Z}/p^n\mathbb{Z}$. We compute the complexity of this algorithm when $X$ is defined over a finite field, and provide a complete implementation in SageMath, as well as some explicit examples. We then apply this algorithm to the computation of the cohomology complex of a locally constant sheaf of $\mathbb{Z}/p^n\mathbb{Z}$-modules on such a curve.