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Andrea Maffei

Publications and source records attributed to Andrea Maffei.

At least 19 recordsLinked to original sources
arXiv2025

Two dimensional versions of the affine Grassmannian and their geometric description

For a smooth affine algebraic group $G$ over an algebraically closed field, we consider several two-variables generalizations of the affine Grassmannian $G(\!(t)\!)/G[\![t]\!]$, given by quotients of the double loop group $G(\!(x)\!)(\!(y)\!)$. We prove that they are representable by ind-schemes if $G$ is solvable. Given a smooth surface $X$ and a flag of subschemes of $X$, we provide a geometric interpretation of the two-variables Grassmannians, in terms of bundles and trivialisation data defined on appropriate loci in $X$, which depend on the flag.

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arXiv2025

The Factorizable Feigin-Frenkel center

We prove a factorizable version of the Feigin-Frenkel theorem on the center of the completed enveloping algebra of the affine Kac-Moody algebra attached to a simple Lie algebra at the critical level. On any smooth curve C we consider a sheaf of complete topological Lie algebras whose fiber at any point is the usual affine algebra at the critical level and consider its sheaf of completed enveloping algebras. We show that the center of this sheaf is a factorization algebra and establish that it is canonically isomorphic, in a factorizable manner, with the factorization algebra of functions on Opers on the pointed disk for the Langlands dual Lie algebra.

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arXiv2025

Topological sheaves and spaces of distributions in the global case

We extend the theory of fields/distributions developed the paper "A Feigin-Frenkel theorem with n singularities" to a general base scheme. In order to do so we introduce suitable notions of topological sheaves on schemes and study their basic properties. We then construct appropriate analogues of the spaces of fields, consider multiplication of fields between them and rebuild the basic theory of vertex algebras in the setting of global distributions in place of formal power series, which takes the form of chiral algebras introduced Beilinson and Drinfeld in "Chiral Algebras".

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arXiv2023

The semi-infinite cohomology of Weyl modules with two singular points

In their study of spherical representations of an affine Lie algebra at the critical level and of unramified opers, Frenkel and Gaitsgory introduced what they called the Weyl module $\mathbb{V}^{\lambda}$ corresponding to a dominant weight $\lambda$. This object plays an important role in the theory. In arXiv:2012.01858, we introduced a possible analogue $\mathbb{V}^{{\lambda},{\mu}}$ of the Weyl module in the setting of opers with two singular points, and in the case of sl(2) we proved that it has the "correct" endomorphism ring. In this paper, we compute the semi-infinite cohomology of $\mathbb{V}^{{\lambda},{\mu}}$ and we show that it does not share some of the properties of the semi-infinite cohomology of the Weyl module of Frenkel and Gaitsgory. For this reason, we introduce a new module $\tilde{\mathbb{V}}^{{\lambda},{\mu}}$ which, in the case of sl(2), enjoys all the expected properties of a Weyl module.

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arXiv2022

Paving Springer fibers for E7

The purpose of this paper is to show that, for each unipotent element in type E7, the corresponding Springer fiber can be paved by affine spaces.

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arXiv2020

Local opers with two singularities: the case of $\mathfrak{sl}(2)$

We study local opers with two singularities for the case of the Lie algebra sl(2), and discuss their connection with a two-variables extension of the affine Lie algebra. We prove an analogue of the Feigin-Frenkel theorem describing the centre at the critical level, and an analogue of a result by Frenkel and Gaitsgory that characterises the endomorphism rings of Weyl modules in terms of functions on the space of opers.

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arXiv2019

A remark on the Mayer-Vietoris double complex for singular cohomology

Given an open cover of a paracompact topological space X, there are two natural ways to construct a map from the cohomology of the nerve of the cover to the cohomology of X. One of them is based on a partition of unity, and is more topological in nature, while the other one relies on the Mayer-Vietoris double complex, and has a more algebraic flavour. In this paper we prove that these two maps coincide, thus answering a question posed by N. V. Ivanov.

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arXiv2018

The Bruhat order on Hermitian symmetric varieties and on abelian nilradicals

Let $G$ be a simple algebraic group and $P$ a parabolic subgroup of $G$ with abelian unipotent radical $P^u$, and let $B$ be a Borel subgroup of $G$ contained in P. Let $\mathfrak{p}^u$ be the Lie algebra of $P^u$ and let $L$ be a Levi factor of $P$, then $L$ is a Hermitian symmetric subgroup of $G$ and $B$ acts with finitely many orbits both on $\mathfrak{p}^u$ and on $G/L$. In this paper we study the Bruhat order of the $B$-orbits in $\mathfrak{p}^u$ and in $G/L$, proving respectively a conjecture of Panyushev and a conjecture of Richardson and Ryan.

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arXiv2018

The Bruhat order on abelian ideals of Borel subalgebras

Let G be a quasi simple algebraic group over an algebraically closed field k whose characteristic is not very bad for G, and let B be a Borel subgroup of G with Lie algebra b. Given a B-stable abelian subalgebra a of the nilradical of b, we parametrize the B-orbits in a and we describe their closure relations.

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arXiv2015

Projective normality of model varieties and related results

We prove that the multiplication of sections of globally generated line bundles on a model wonderful variety M of simply connected type is always surjective. This follows by a general argument which works for every wonderful variety and reduces the study of the surjectivity for every couple of globally generated line bundles to a finite number of cases. As a consequence, the cone defined by a complete linear system over M or over a closed G-stable subvariety of M is normal. We apply these results to the study of the normality of the compactifications of model varieties in simple projective spaces and of the closures of the spherical nilpotent orbits. Then we focus on a particular case proving two specific conjectures of Adams, Huang and Vogan on an analogue of the model orbit of the group of type E8.

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arXiv2015

Standard monomial theory for wonderful varieties

A general setting for a standard monomial theory on a multiset is introduced and applied to the Cox ring of a wonderful variety. This gives a degeneration result of the Cox ring to a multicone over a partial flag variety. Further, we deduce that the Cox ring has rational singularities.

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arXiv2012

Pfaffians and Shuffling Relations for the Spin Module

We present explicit formulas for a set of generators of the ideal of relations among the pfaffians of the principal minors of the antisymmetric matrices of fixed dimension. These formulas have an interpretation in terms of the standard monomial theory for the spin module of orthogonal groups.

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arXiv2011

A generalized Steinberg section and branching rules for quantum groups at roots of 1

In this paper we construct a generalization of the classical Steinberg section for the quotient map of a semisimple group with respect to the conjugation action. We then give various applications of our construction including the construction of a sort of Gelfand Zetlin basis for a generic irreducible representation of quantum GL(n) at odd roots of unity.

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arXiv2011

Normality and non-normality of group compactifications in simple projective spaces

If $G$ is a complex simply connected semisimple algebraic group and if $λ$ is a dominant weight, we consider the compactification $X_λ$ in the projectivisation of $\End(V(λ))$ obtained as the closure of the $G\times G$-orbit of the identity and we give necessary and sufficient conditions on the support of $λ$ so that $X_λ$ is normal; as well, we give necessary and sufficient conditions on the support of $λ$ so that $X_λ$ is smooth.

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