scholarly journals GRADED UNIPOTENT GROUPS AND GROSSHANS THEORY

2017 ◽  
Vol 5 ◽  
Author(s):  
GERGELY BÉRCZI ◽  
FRANCES KIRWAN
Keyword(s):  
Projective Variety ◽  
Multiplicative Group ◽  
Ample Line Bundle ◽  
Adjoint Action ◽  
Projective Line ◽  
Complex Numbers ◽  
Unipotent Group ◽  
Unipotent Groups ◽  
Rational Character ◽  
Projective Completion

Let $U$ be a unipotent group which is graded in the sense that it has an extension $H$ by the multiplicative group of the complex numbers such that all the weights of the adjoint action on the Lie algebra of $U$ are strictly positive. We study embeddings of $H$ in a general linear group $G$ which possess Grosshans-like properties. More precisely, suppose $H$ acts on a projective variety $X$ and its action extends to an action of $G$ which is linear with respect to an ample line bundle on $X$. Then, provided that we are willing to twist the linearization of the action of $H$ by a suitable (rational) character of $H$, we find that the $H$-invariants form a finitely generated algebra and hence define a projective variety $X/\!/H$; moreover, the natural morphism from the semistable locus in $X$ to $X/\!/H$ is surjective, and semistable points in $X$ are identified in $X/\!/H$ if and only if the closures of their $H$-orbits meet in the semistable locus. A similar result applies when we replace $X$ by its product with the projective line; this gives us a projective completion of a geometric quotient of a $U$-invariant open subset of $X$ by the action of the unipotent group $U$.

scholarly journals Shintani descent for algebraic groups and almost characters of unipotent groups

2016 ◽  
Vol 152 (8) ◽  
pp. 1697-1724 ◽  
Author(s):  
Tanmay Deshpande
Keyword(s):  
Finite Field ◽  
Algebraic Group ◽  
Algebraic Groups ◽  
Unipotent Group ◽  
Connected Component ◽  
Unipotent Groups ◽  
Character Sheaves ◽  
Treat All ◽  
Trace Of Frobenius

In this paper, we extend the notion of Shintani descent to general (possibly disconnected) algebraic groups defined over a finite field $\mathbb{F}_{q}$. For this, it is essential to treat all the pure inner $\mathbb{F}_{q}$-rational forms of the algebraic group at the same time. We prove that the notion of almost characters (introduced by Shoji using Shintani descent) is well defined for any neutrally unipotent algebraic group, i.e. an algebraic group whose neutral connected component is a unipotent group. We also prove that these almost characters coincide with the ‘trace of Frobenius’ functions associated with Frobenius-stable character sheaves on neutrally unipotent groups. In the course of the proof, we also prove that the modular categories that arise from Boyarchenko and Drinfeld’s theory of character sheaves on neutrally unipotent groups are in fact positive integral, confirming a conjecture due to Drinfeld.

A remark on elementary contractions

1995 ◽  
Vol 118 (1) ◽  
pp. 183-188
Author(s):  
Qi Zhang
Keyword(s):  
Projective Variety ◽  
Intersection Number ◽  
Smooth Projective Variety ◽  
Complex Numbers ◽  
Canonical Bundle ◽  
Small Contraction ◽  
Dimension One ◽  
Extremal Ray

Let X be a smooth projective variety of dimension n over the field of complex numbers. We denote by Kx the canonical bundle of X. By Mori's theory, if Kx is not numerically effective (i.e. if there exists a curve on X which has negative intersection number with Kx), then there exists an extremal ray ℝ+[C] on X and an elementary contraction fR: X → Y associated with ℝ+[C].fR is called a small contraction if it is bi-rational and an isomorphism in co-dimension one.

scholarly journals Effective Faithful Tropicalizations Associated to Adjoint Linear Systems

2018 ◽  
Vol 2019 (19) ◽  
pp. 6089-6112
Author(s):  
Shu Kawaguchi ◽  
Kazuhiko Yamaki
Keyword(s):  
Linear System ◽  
Line Bundle ◽  
Projective Variety ◽  
Ample Line Bundle ◽  
Smooth Projective Variety ◽  
Discrete Valuation Ring ◽  
Complete Discrete Valuation Ring ◽  
System L ◽  
Semistable Model ◽  
Adjoint Bundle

Abstract Let R be a complete discrete valuation ring of equi-characteristic zero with fraction field K. Let X be a connected smooth projective variety of dimension d over K, and let L be an ample line bundle over X. We assume that there exist a regular strictly semistable model ${\mathscr {X}}$ of X over R and a relatively ample line bundle ${\mathscr {L}}$ over ${\mathscr {X}}$ with $\left .{{\mathscr {L}}}\right \vert_{{X}} \cong L$. Let $S({\mathscr {X}})$ be the skeleton associated to ${\mathscr {X}}$ in the Berkovich analytification Xan of X. In this article, we study when $S({\mathscr {X}})$ is faithfully tropicalized into tropical projective space by the adjoint linear system |L⊗m ⊗ ωX|. Roughly speaking, our results show that if m is an integer such that the adjoint bundle is basepoint free, then the adjoint linear system admits a faithful tropicalization of $S({\mathscr {X}})$.

scholarly journals Unitary representations of unipotent groups associated with theta series

1992 ◽  
Vol 127 ◽  
pp. 167-174
Author(s):  
Hisasi Morikawa
Keyword(s):  
Heisenberg Group ◽  
Theta Series ◽  
Unitary Representations ◽  
Unipotent Group ◽  
Split Extension ◽  
Unipotent Groups

1. Unipotent group of real (g + 2) × (g + 2) -matricesmay be regarded as a split extension of Ng (R) by Heisenberg group of real (g + 2) × (g + 2)-matrices

scholarly journals Formality of -objects

2019 ◽  
Vol 155 (5) ◽  
pp. 973-994
Author(s):  
Andreas Hochenegger ◽  
Andreas Krug
Keyword(s):  
Projective Variety ◽  
Smooth Projective Variety ◽  
Complex Numbers ◽  
Cohomology Algebra ◽  
Triangulated Category ◽  
Endomorphism Algebra ◽  
Structure Sheaf

We show that a$\mathbb{P}$-object and simple configurations of$\mathbb{P}$-objects have a formal derived endomorphism algebra. Hence the triangulated category (classically) generated by such objects is independent of the ambient triangulated category. We also observe that the category generated by the structure sheaf of a smooth projective variety over the complex numbers only depends on its graded cohomology algebra.

scholarly journals A note on cubic equivalences

1986 ◽  
Vol 101 ◽  
pp. 1-26
Author(s):  
Hiroshi Saito
Keyword(s):  
Projective Variety ◽  
Present Note ◽  
Smooth Projective Variety ◽  
Complex Numbers

The present note is intended to be a supplement to [9], in which the following is proven: Let V be a smooth projective variety over the field of complex numbers C, T a smooth quasi-projective variety, Z a cycle in T × V of codimension p. If Z(t) is l-cube equivalent to zero for general t e T, then, setting r = dim V − p,vanishes for l′ < l, where {tZ} is the correspondence defined by Z.

Projective manifolds of sectional genus three as zero loci of sections of ample vector bundles

2008 ◽  
Vol 144 (1) ◽  
pp. 109-118 ◽  
Author(s):  
ANTONIO LANTERI ◽  
HIDETOSHI MAEDA
Keyword(s):  
Vector Bundles ◽  
Projective Variety ◽  
Ample Line Bundle ◽  
Global Section ◽  
Sectional Genus ◽  
Ample Vector Bundle ◽  
Complex Projective Variety ◽  
Projective Manifolds ◽  
Complex Projective ◽  
Zero Locus

AbstractLet ϵ be an ample vector bundle of rank r ≥ 2 on a smooth complex projective variety X of dimension n such that there exists a global section of ϵ whose zero locus Z is a smooth subvariety of dimension n-r ≥ 2 of X. Let H be an ample line bundle on X such that the restriction HZ of H to Z is very ample. Triplets (X, ϵ, H) with g(Z, HZ) = 3 are classified, where g(Z, HZ) is the sectional genus of (Z, HZ).

scholarly journals Derived length of zero entropy groups acting on projective varieties in arbitrary characteristic — A remark to a paper of Dinh-Oguiso-Zhang

2020 ◽  
Vol 31 (08) ◽  
pp. 2050059
Author(s):  
Sichen Li
Keyword(s):  
Projective Variety ◽  
Type Theorem ◽  
Closed Field ◽  
Unipotent Group ◽  
Projective Varieties ◽  
Arbitrary Characteristic ◽  
Derived Length ◽  
Dimension Formula ◽  
Zero Entropy ◽  
Finite Index Subgroup

Let [Formula: see text] be a projective variety of dimension [Formula: see text] over an algebraically closed field of arbitrary characteristic. We prove a Fujiki–Lieberman type theorem on the structure of the automorphism group of [Formula: see text]. Let [Formula: see text] be a group of zero entropy automorphisms of [Formula: see text] and [Formula: see text] the set of elements in [Formula: see text] which are isotopic to the identity. We show that after replacing [Formula: see text] by a suitable finite-index subgroup, [Formula: see text] is a unipotent group of the derived length at most [Formula: see text]. This result was first proved by Dinh et al. for compact Kähler manifolds.

On Certain Pairs of Matrices which Generate free Groups

1958 ◽  
Vol 10 ◽  
pp. 279-284 ◽  
Author(s):  
Bomshik Chang ◽  
S. A. Jennings ◽  
Rimhak Ree
Keyword(s):  
Multiplicative Group ◽  
Free Groups ◽  
Complex Numbers ◽  
Image Position

Denote by Fα,β the multiplicative group generated by the two matrices where α and β are complex numbers. Sanov (3) proved that F α,β is free, and Brenner (1) showed that F m,m is free if m ⩾ 2.

scholarly journals Hamiltonian actions of unipotent groups on compact K\"ahler manifolds

2018 ◽  
Vol Volume 2 ◽  
Author(s):  
Daniel Greb ◽  
Christian Miebach
Keyword(s):  
Lie Groups ◽  
Invariant Theory ◽  
Symplectic Reduction ◽  
Stability Conditions ◽  
Unipotent Group ◽  
Geometric Invariant ◽  
Projective Varieties ◽  
Final Version ◽  
Unipotent Groups ◽  
Hamiltonian Actions

We study meromorphic actions of unipotent complex Lie groups on compact K\"ahler manifolds using moment map techniques. We introduce natural stability conditions and show that sets of semistable points are Zariski-open and admit geometric quotients that carry compactifiable K\"ahler structures obtained by symplectic reduction. The relation of our complex-analytic theory to the work of Doran--Kirwan regarding the Geometric Invariant Theory of unipotent group actions on projective varieties is discussed in detail. Comment: v2: 30 pages, final version as accepted by EPIGA

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