arXiv subjects

Ugo Bruzzo

Publications and source records attributed to Ugo Bruzzo.

At least 19 recordsLinked to original sources

arXiv2026

Higgs Grassmannians

We review and study the notion of Higgs Grassmannians, which are schemes parametrizing the Higgs subbundles of a given Higgs bundle over a smooth variety. We write their equations as closed subschemes of the usual Grassmann bundles and investigate their geometry. Often the Higgs Grassmannians generically have 0-dimensional fibers over the base variety, thus implying that Higgs subbundles are "scarce." We characterize the structure of the Higgs Grassmannians by analyzing the local Jordan type of the Higgs field. A refined analysis of the rank 2 case is also provided in terms of the discriminant of the characteristic polynomial. We apply our characterizations to the Simpson system of a smooth variety to provide a streamlined proof of its semistability, and we establish a structural relationship between the rank 1 Higgs Grassmannian and the spectral cover of the Higgs bundle. Finally, we introduce the schemes of flags of Higgs subbundles of a given Higgs bundle, and the Quot schemes parametrizing Higgs quotients; we conclude with some examples.

math.AG↗

arXiv2025

Moduli of stable supermaps

We review the notion of stable supermap from SUSY curves to a fixed target superscheme, and prove that when the target is (super)projective, stable supermaps are parameterized by a Deligne-Mumford superstack with superschematic and separated diagonal. We characterize the bosonic reduction of this moduli superstack and see that it has a surjective morphism onto the moduli stack of stable maps from spin curves to the bosonic reduction of the target, whose fibers are linear schemes; for this reason, the moduli superstack of stable supermaps is not proper unless such linear schemes reduce to a point. Using Manin-Penkov-Voronov's super Grothendieck-Riemann-Roch theorem we also make a formal computation of the virtual dimension of the moduli superstack, which agrees with the characterization of the bosonic reduction just mentioned and with the dimension formula for the case of bosonic target existing in the literature.

math.AG↗

arXiv2025

Foundations of superstack theory

In view of applications to the construction of moduli spaces of objects in algebraic supergeometry, we start a systematic study of stacks in that context. After defining a superstack as a stack over the \'etale site of superschemes, we define quotient superstacks, and, based on previous literature, we see that, in analogy with superschemes, every superstack has an underlying ordinary stack, which we call its bosonic reduction. Then we progressively introduce more structure, considering algebraic superspaces, Deligne-Mumford superstacks and algebraic superstacks. We study the topology of algebraic superstacks and several properties of morphisms between them. We introduce quasi-coherent sheaves, and the sheaves of relative differentials. An important issue is how to check that an algebraic superstack is Deligne-Mumford, and we generalize to this setting the usual criteria in terms of the unramifiedness of the diagonal of the stack. Two appendices are devoted to collecting the basic definitions of group superschemes and principal superbundles, and to stating and analyzing some properties of morphisms of superschemes, that are at the basis of the study of morphisms of superstacks in the main text.

math.AG↗

arXiv2025

On the exceptional set of crepant resolutions of abelian singularities

Let G be a finite abelian subgroup of SL(n,C), and suppose there exists a toric crepant resolution phi: X -- > C^n/G. We prove that for each component E of the exceptional set of phi there exists an open subset U of X that contains E and is isomorphic to the total space of the canonical bundle of E. This contributes to the collection of results aimed at solving a classical problem, i.e., to determine which submanifolds of a complex manifold have a neighborhood isomorphic to a neighborhood of the zero section of their normal bundle.

math.AG↗

arXiv2025

Notes on fundamental algebraic supergeometry. Hilbert and Picard superschemes

These notes aim at providing a complete and systematic account of some foundational aspects of algebraic supergeometry, namely, the extension to the geometry of superschemes of many classical notions, techniques and results that make up the general backbone of algebraic geometry, most of them originating from Grothendieck's work. In particular, we extend to algebraic supergeometry such notions as projective and proper morphisms, finiteness of the cohomology, vector and projective bundles, cohomology base change, semicontinuity theorems, relative duality, Castelnuovo-Mumford regularity, flattening, Hilbert and Quot schemes, faithfully flat descent, quotient étale relations (notably, Picard schemes), among others. Some results may be found elsewhere, and, in particular, there is some overlap with a recent preprint by Moosavian and Zhou. However, many techniques and constructions are presented here for the first time, notably, a first development of Grothendieck relative duality for proper morphisms of superschemes, the construction of the Hilbert superscheme in a more general situation than the one already known (which in particular allows one to treat the case of sub-superschemes of supergrassmannians), and a rigorous construction of the Picard superscheme for a locally superprojective morphism of noetherian superschemes with geometrically integral fibres. Moreover, some of the proofs given here are new as well, even when restricted to ordinary schemes. In a final section we construct a period map from an open substack of the moduli of proper and smooth supercurves to the moduli stack of principally polarized abelian superchemes.

math.AG↗

arXiv2025

The paint group Tits Satake theory of hyperbolic symmetric spaces: the distance function, paint invariants and discrete subgroups

The present paper, which is partially a review, but also contains several completely new results, aims at presenting, in a unified mathematical framework, a complex and articulated lore regarding non-compact symmetric spaces, with negative curvature, whose isometry group is a non-compact, real simple Lie group. All such manifolds are Riemannian normal manifolds, according to Alekseevsky's definition, in the sense that they are metrically equivalent to a solvable Lie group manifold. This identification provides a vision in which, on one side one can derive quite explicit and challenging formulae for the unique distance function between points of the manifold, on the other one, one can organize the entire set of the available manifolds in universality classes distinguished by their common Tits Satake submanifold and, correspondingly, by their non-compact rank. The members of the class are distinguished by their different Paint Groups, the latter notion having been introduced by two of the present authors in an earlier collaboration. In relation to the construction of neural networks, these mathematical structures offer unique possibilities of replacing ad hoc activation functions with the naturally defined non-linear operations that relate Lie algebras to Lie Groups and vice-versa. The Paint Group invariants offer new tokens both to construct algorithms and inspect (hopefully to control) their working. A conspicuous part of the paper is devoted to the study and systematic construction of parabolic/elliptic discrete subgroups of the Lie groups SO(r,r+q), in view of discretization and/or tessellations of the space to which data are to be mapped. Furthermore, it is shown how the ingredients of Special K\"ahler Geometry and the c-map, well known in the supergravity literature, provide a unified classification scheme of the relevant Tits Satake universality classes with non-compact rank r<5.

math.DG↗

arXiv2025

Splitting of supervector bundles on projective superspaces

We provide a splitting criterion for supervector bundles over the projective superspaces $\mathbb{P}^{n|m}$. More precisely, we prove that a rank $p|q$ supervector bundle on $\mathbb{P}^{n|m}$ with vanishing intermediate cohomology is isomorphic to the direct sum of even and odd line bundles, provided that $n \geq 2$. For $n=1$ we provide an example of a supervector bundle that cannot be written as a sum of line bundles.

math.AG↗

arXiv2024

Donagi-Markman cubics for Hitchin systems of type A2, B2. G2

We obtain explicit formulae for the Donagi-Markman (Bryant-Griffiths, Yukawa) cubic for Hitchin systems of type $A_2$, $B_2$ and $G_2$. This is achieved by evaluating the quadratic residues in the Balduzzi-Pantev formula, using a previous result of ours. For $G_2$ we also recover earlier results of Hitchin.

math.AG↗

arXiv2024

Cox-Gorenstein algebras

We study G-graded Artinian algebras having Poincar\'e duality, considering in particular their Lefschetz properties. We also prove a correspondence between the toric setup and the G-graded one, provide an application to toric geometry, and prove a Hessian criterion in the G-graded setup

math.AC↗

arXiv2024

Nested Hilbert Schemes on Hirzebruch surfaces and quiver varieties

For $n\ge 1$ we show that the length 1 nested Hilbert scheme of the total space $X_n$ of the line bundle $\mathcal O_{\mathbb P^1}(-n)$, parameterizing pairs of nested 0-cycles in $X_n$, is a quiver variety associated with a suitable quiver with relations. This generalizes previous work about nested Hilbert schemes on $\mathbb C^2$ in one direction, and about the Hilbert schemes of points of $X_n$ in another direction.

math.AG↗

arXiv2024

Seiberg-Witten differentials on the Hitchin base

In this note we describe explicitly, in terms of Lie theory and cameral data, the covariant (Gauss--Manin) derivative of the Seiberg--Witten differential defined on the weight-one variation of Hodge structures that exists on a Zariski open subset of the base of the Hitchin fibration. Dedicated to Tony Pantev on the occasion of his 60th birthday.

math.AG↗

arXiv2023

On a conjecture about Higgs bundles for rank 2 and some inequalities

We briefly review an open conjecture about Higgs bundles that are semistable with after pulling back to any curve, and prove it in the rank 2 case. We also prove a set of inequalities holding for H-nef Higgs bundles that generalize some of the Fulton-Lazarsfeld inequalities for numerically effective vector bundles.

math.AG↗

arXiv2023

Positivity for Higgs vector bundles: criteria and applications

Working in the category of smooth projective varieties over an algebraically closed field of characteristic 0, we review notions of ampleness and numerical nefness for Higgs bundles which "feel" the Higgs field and formulate criteria of the Barton-Kleiman type for these notions. We give an application to minimal surfaces of general type that saturate the Miyaoka-Yau inequality, showing that their cotangent bundle is ample. This will use results by Langer that imply that also for varieties over algebraically closed field of characteristic zero the so-called Simpson system is stable.

math.AG↗

arXiv2023

D3-brane supergravity solutions from Ricci-flat metrics on canonical bundles of Kähler-Einstein surfaces

D3-brane solutions of type IIB supergravity can be obtained by means a classical ansatz involving a harmonic warp factor and two summands, the first being the flat Minkowskian metric of the D3 brane world-sheet and the second a Ricci flat metric on a suitable 6-dimensional transverse space, both twisted by the warp factor. Of particular interest is the case of the total space of thecanonical bundle over a complex Kähler 2-fold. This situation emerges in many cases while considering the resolution of finite quotient singulaties. When the group is $\mathbb{Z}_4$, the complex 2-fold is the second Hirzebruch surface endowed with a Kähler metric having SU(2)xU(1) isometry. There is actually an entire class of such metrics parameterized by a single function, and best described in the AMSY symplectic formalism. We recover the existence of a two parameter subclass of Kähler-Einstein metrics on manifolds that are homeorphic to $S^2\times S^2$, and study in detail this class. The K"ahler-Einstein nature of these manifolds allows the construction of the Ricci flat metric on their canonical bundle via the Calabi Ansatz, which we recast in the AMSY formalism deriving some new elegant formulae. Furthermore we show the full integrability of the differential system of geodesics equations thanks to an additional conserved quantity that we unveil and which is similar to the Carter constant in the case of the Kerr metric.

math-ph↗

arXiv2022

Supercycles, stable supermaps and SUSY Nori motives

We define stable supercurves and stable supermaps, and based on these notions we develop a theory of Nori motives for the category of stable supermaps of SUSY curves with punctures. This will require several preliminary constructions, including the development of a basic theory of supercycles.

math.AG↗

arXiv2022

Deformation of pairs and Noether-Lefschetz loci in toric varieties

We continue our study of the Noether-Lefschetz loci in toric varieties and investigate deformation of pairs (V,X) where V is a complete intersection subvariety and X a quasi-smooth hypersurface in a odd dimensional simplicial projective toric variety, with V\subset X. Under some assumptions, we prove that the cohomological class in H^{k,k}(X) associated to V remains of type (k,k) under an infinitesimal deformation if and only if V remains algebraic. Actually we prove that locally the Noether-Lefschetz locus is an irreducible component of a suitable Hilbert scheme. This generalizes Theorem 4.2 in our previous work [4] and the main theorem proved by Dan in [10].

math.AG↗

arXiv2021

Moduli of rank 2 Higgs sheaves on elliptic surfaces

We study torsion-free, rank 2 Higgs sheaves on genus one fibered surfaces, (semi)stable with respect to suitable polarizations in the sense of Friedman and O'Grady. We prove that slope-semistability of a Higgs sheaf on the surface implies semistability on the generic fiber. In the case of Higgs sheaves of odd fiber degree on elliptic surfaces in characteristic $\neq 2$, we prove that any moduli space of Higgs sheaves with fixed numerical invariants splits canonically as the product of the moduli space of ordinary sheaves (with the same invariants), and the space of global regular $1$-forms on the surface. For elliptic surfaces with section in characteristic zero, and in the case arbitrary fiber degree, we prove that if a Higgs sheaf has reduced Friedman spectral curve, or is regular on a general fiber with non-reduced spectral cover, then its Higgs field takes values in the saturation of the pull-back of the canonical bundle of the base curve in the cotangent bundle of the surface.

math.AG↗

arXiv2021

The supermoduli of SUSY curves with Ramond punctures

We construct local and global moduli spaces of supersymmetric curves with Ramond-Ramond punctures. We assume that the underlying ordinary algebraic curves have a level n structure and build these supermoduli spaces as algebraic superspaces, i.e., quotients of étale equivalence relations between superschemes.

math.AG↗