Generalized Green functions and unipotent classes for finite reductive groups, III

Wavefront Sets of Unipotent Representations of Reductive $p$-adic Groups II

The wavefront set is a fundamental invariant of an admissible representation arising from the Harish-Chandra-Howe local character expansion. In this paper, we give a precise formula for the wavefront set of an irreducible representation of real infinitesimal character in Lusztig's category of unipotent representations in terms of the Deligne-Langlands-Lusztig correspondence. Our formula generalizes the main result of arXiv:2112.14354, where this formula was obtained in the Iwahori-spherical case. We deduce that for any irreducible unipotent representation with real infinitesimal character, the…

有限约化群的广义Green函数与幂幺类，IV

Generalized Green functions and unipotent classes for finite reductive groups, IV

本文定义了连通约化群幂幺类的分裂元，将广义Green函数的Lusztig算法计算归结为分裂元存在性，并证明典型群及除E7型一个类外的例外群中分裂元均存在。

Distinguished strata in a reductive group

The set of strata of a reductive group can be viewed as an enlargement of the set of unipotent classes. In this paper the notion of distinguished unipotent class is extended to this larger set. The strata of a Weyl group are introduced and studied.

Some Unipotent Arthur Packets for Reductive $p$-adic Groups

Let $k$ be a $p$-adic field and let $\mathbf{G}(k)$ be the $k$-points of a connected reductive group, inner to split. The set of Aubert-Zelevinsky duals of the constituents of a tempered L-packet form an Arthur packet for $\mathbf{G}(k)$. In this paper, we give an alternative characterization of such Arthur packets in terms of the wavefront set, proving in some instances a conjecture of Jiang-Liu and Shahidi. Pursuing an analogy with real and complex groups, we define some special unions of Arthur packets which we call \emph{weak} Arthur packets and describe their constituents in terms of thei…

Fields of invariants for unipotent radicals of parabolic subgroups

The paper is devoted to the problem of finding free generators in the fields of invariants for actions of unipotent groups on affine varieties. We consider the case when the unipotent group is the unipotent radical in an arbitrary parabolic subgroup in the reductive group of classical type GL(n), SL(n), O(n) or Sp(2n). In the explicit form, we present a system of free generators in the field of invariants for the action of the unipotent radical on the reductive group by conjugation.

On Unipotent Supports of Reductive Groups with a Disconnected Centre

Let $G$ be a simple algebraic group defined over a finite field of good characteristic, with associated Frobenius endomorphism $F$. In this article we extend an observation of Lusztig, (which gives a numerical relationship between an ordinary character of $G^F$ and its unipotent support), to the case where $Z(G)$ is disconnected. We then use this observation in some applications to the ordinary character theory of $G^F$.

Wavefront Sets of Unipotent Representations of Reductive $p$-adic Groups I

The wavefront set is a fundamental invariant arising from the Harish-Chandra-Howe local character expansion of an admissible representation. We prove a precise formula for the wavefront set of an irreducible Iwahori-spherical representation with `real infinitesimal character' and determine a lower bound for this invariant in terms of the Deligne-Langlands-Lusztig parameters. In particular, for the Iwahori-spherical representations with real infinitesimal character, we deduce that the algebraic wavefront set is a singleton, as conjectured by Moeglin and Waldspurger. As a corollary, we obtain an…

On character sheaves and characters of reductive groups at unipotent classes

With a view to determining character values of finite reductive groups at unipotent elements, we prove a number of results concerning inner products of generalised Gelfand-Graev characters with characteristic functions of character sheaves, here called Lusztig functions. These are used to determine projections of generalised Gelfand-Graev characters to the space of unipo- tent characters, and to the space of characters with a given wave front set. Such projections are expressed largely in terms of Weyl group data. We show how the values of characters at their unipotent support or wave front se…

Character sheaves and generalizations

This paper contains an exposition of the theory of character sheaves for reductive groups and some attempts to extend it to other cases: unipotent groups, reductive groups modulo the unipotent radical of a parabolic.

On the exponent of geometric unipotent radicals of pseudo-reductive groups

Let $k'/k$ be a finite purely inseparable field extension and let $G'$ be a reductive $k'$-group. We denote by $G=\R_{k'/k}(G')$ the Weil restriction of $G'$ across $k'/k$, a pseudo-reductive group. This article gives bounds for the exponent of the geometric unipotent radical $\RR_{u}(G_{\bar{k}})$ in terms of invariants of the extension $k'/k$, starting with the case $G'=\GL_n$ and applying these results to the case where $G'$ is a simple group.

Unipotent blocks and weighted affine Weyl groups

We show that various properties of unipotent elements in a reductive group over the complex numbers can be recovered purely in terms of the affine Weyl group of the dual group.

Reducing mod p complex representations of finite reductive groups

We state a conjecture on the reduction modulo the defining characteristic of a unipotent representation of a finite reductive group.

Deligne-Lusztig constructions for unipotent and p-adic groups

In 1979 Lusztig proposed a conjectural construction of supercuspidal representations of reductive p-adic groups, which is similar to the well known construction of Deligne and Lusztig in the setting of finite reductive groups. We present a general method for explicitly calculating the representations arising from Lusztig's construction and illustrate it with several examples. The techniques we develop also provide background for the author's joint work with Weinstein on a purely local and explicit proof of the local Langlands correspondence.

约化群中的刚性层

Rigid strata in a reductive group

本文通过两种等价方式定义约化群的刚性层，并将其作为刚性幂幺类概念的推广。

Unipotent Representations Attached to the Principal Nilpotent Orbit

In this paper, we construct and classify the special unipotent representations of a real reductive group attached to the principal nilpotent orbit. We give formulas for the $\mathbf{K}$-types, associated varieties, and Langlands parameters of all such representations.

The nonabelian Fourier transform for elliptic unipotent representations of exceptional $p$-adic groups

We define an involution on the space of elliptic unipotent Langlands parameters of a reductive $p$-adic group $G$ and verify that when $G$ is split adjoint exceptional, the composition of this involution with the hyperspecial parahoric restriction map agrees with Lusztig's nonabelian Fourier transform for unipotent representations of the finite reductive quotient. This is inspired by recent works of Lusztig on the almost unipotent characters of $p$-adic groups and of Moeglin and Waldspurger on the elliptic Fourier transform of odd orthogonal groups.

From conjugacy classes in the Weyl group to unipotent classes

Let G be a connected reductive algebraic group over an algebraic closed field. We define a (surjective) map from the set of conjugacy classes in the Weyl group to the set of unipotent classes of G.

本质上一元的超 cuspidal 表示的局部朗兰兹对应（对于不连通的半单群）

The local Langlands correspondence of essentially unipotent supercuspidal representations for disconnected reductive groups

本文在刚性内形式框架下构造了本质上一元的超 cuspidal 表示的局部朗兰兹对应，并证明了某种泛函性和兼容性，该结果比 [Sol20] 中的类似结果更强，因为后者不考虑内扭的刚性化。同时，本文将这种对应推广到不连通的半单群。

温和幂幺表示的非阿贝尔傅里叶变换

A nonabelian Fourier transform for tempered unipotent representations

本文定义分裂 $p$-进群内扭形的紧温和幂幺表示空间上的对合，提出其与 Lusztig 非阿贝尔傅里叶变换关系的精确猜想，并给出 $\mathsf{SL}_n$ 和 $\mathsf{PGL}_n$ 的证明。

Overgroups of regular unipotent elements in simple algebraic groups

We investigate positive-dimensional closed reductive subgroups of almost simple algebraic groups containing a regular unipotent element. Our main result states that such subgroups do not lie inside proper parabolic subgroups unless possibly when their connected component is a torus. This extends the earlier result of Testerman and Zalesski treating connected reductive subgroups.

Abelian Unipotent Subgroups of Reductive Groups

Let G be a connected reductive group defined over an algebraically closed field k of characteristic p > 0. The purpose of this paper is two-fold. First, when p is a good prime, we give a new proof of the ``order formula'' of D. Testerman for unipotent elements in G; moreover, we show that the same formula determines the p-nilpotence degree of the corresponding nilpotent elements in the Lie algebra of G. Second, if G is semisimple and p is sufficiently large, we show that G always has a faithful representation (r,V) with the property that the exponential of dr(X) lies in r(G) for each p-nilpote…

Unipotent Ideals for Spin and Exceptional Groups

In the monograph arXiv:2108.03453, we define the notion of a unipotent representation of a complex reductive group. The representations we define include, as a proper subset, all special unipotent representations in the sense of Barbasch-Vogan and form the (conjectural) building blocks of the unitary dual. In arXiv:2108.03453 we provide combinatorial formulas for the infinitesimal characters of all unipotent representations of linear classical groups. In this paper, we establish analogous formulas for spin and exceptional groups, thus completing the determination of the infinitesimal character…

全正性中幂幺元的正则性

Regularity of Unipotent Elements in Total Positivity

本文研究分裂连通约化群全非负幺半群中幂幺元的正则性，证明其在某些 Levi 子群中正则，证实 Lusztig 猜想。

模仿射 Hecke 范畴与正则幂零中心化子

Modular affine Hecke category and regular unipotent centralizer

本文在温和显式假设下，用 Langlands 对偶仿射旗簇上的反常层几何地描述了约化代数群中正则幂零元中心化子的表示范畴，为后续建立仿射 Hecke 代数各几何实现之间的范畴等价奠定基础。

From unipotent classes to conjugacy classes in the Weyl group

Let G be a connected reductive group. We define a map from the set of unipotent classes in G to the set of conjugacy classes in the Weyl group (assuming that the characteristic is not bad). This map is a one sided inverse of a map in the opposite direction constructed in an earlier paper.

Springer's work on unipotent classes and Weyl group representations

In this paper we discuss some of Springer's work on unipotent elements in a reductive groups and on representations of Weyl groups. Among the topics considered are Springer's bijection from the unipotent variety to the nilpotent variety, Springer's example of nonrationality of certain representations of the Hecke algebras associated to a Weyl group and the Springer representation of the Weyl group associated to a unipotent element.

Unipotent elements in small characteristic

We make a study of unipotent elements in a connected reductive group over an algebraically closed field with emphasis on the case where the characteristic is a bad prime. We try to see how much of the theory of Dynkin-Kostant extends to this case. We also study the variety of Borel subgroups containing a unipotent element and prove a purity property for its cohomology in certain cases in bad characteristic.

Unipotent elements in small characteristic, IV

We consider the variety of nilpotent elements in the dual of the Lie algebra of a reductive algebraic group over an algebraically closed field. We propose a definition of a partition of this variety into smooth locally closed smooth subvarieties indexed by the unipotent classes in the corresponding group over complex numbers. We obtain explicit results in type A,C and D.

扩展幂幺特征的合理性

Rationality of extended unipotent characters

本文确定了在 Frobenius 映射下作为不连通约化群不动点产生的有限约化群的幂幺特征的合理性，证明中结合了 Deligne--Lusztig 簇的 $\ell$-adic 上同调群中的特征实现与块理论考虑。

Non-unipotent character sheaves as a categorical centre

Let G be a reductive connected group over the algebraic closure of a finite field. In this paper we give the classification of character sheaves on G in categorical terms (as a categorical centre). Previously such a classification was known for unipotent character sheaves and also in the case where the ground field is replaced by the complex numbers.

Unipotent Representations and Microlocalization

We develop a theory of microlocalization for Harish-Chandra modules, adapting a construction of Losev (\cite{Losev2011}). We explore the applications of this theory to unipotent representations of real reductive groups. For complex groups, we deduce a formula for the $K$-multiplicities of unipotent representations attached to a nilpotent orbit $\OO$, proving an old conjecture of Vogan (\cite{Vogan1991}) in a large family of cases.

有限约化群的特征标重数与Drinfeld对偶

Multiplicity of characters of finite reductive groups and Drinfeld doubles

研究有限约化群\(G^F\)的几乎幂幺特征标和德利涅 - 卢茨蒂格特征标张量积重数，涉及与复不可约特征标环结构关系，还考虑Drinfeld对偶上模的弗罗贝尼乌斯 - 舒尔指标，最后研究相关重数并提出检测问题。

On the definition of unipotent representations

Let G be a connected reductive group over a finite field F_q. A map from irreducible representations of G(F_q) to semisimple classes in the dual group was defined by a cohomological method by Deligne and the author in 1976. Here we show that when q is large enough this map has an alternative (non-cohomological) definition.

Non-unipotent representations and categorical centres

Let G be a connected reductive group defined over a finite field F_q. We give a parametrization of the irreducible representations of G(F_q) in terms of (twisted) categorical centres of various monoidal categories associated to G. (Results of this type were known earlier for unipotent representations. Analogous results for the parametrization of character sheaves on G were also known earlier.)

有限域上非连通约化群表示的内窥

Endoscopy for representations of disconnected reductive groups over finite fields

本文基于 Lusztig 和 Yun 的工作，将有限域上连通约化群不可约表示的 Jordan 分解典范化，并推广到具有约化中性分量的光滑群概形，对有理级数与几何级数建立与内窥群幂幺表示之间的典范等价。

On the representations of disconnected reductive groups over F_q

Let \hat G be the semidirect product of a connected reductive group G over F_q with a finite cyclic group generated by a quasisemisimple automorphism of G defined over F_q. In this paper we prove a conjecture of G. Malle concerning the extensions of unipotent representations of G(F_q) to \hat G(F_q).

Restriction of a character sheaf to conjugacy classes

Let A be a character sheaf on a reductive connected group G over an algebraically closed field. Assuming that the characteristic is not bad, we show that for certain conjugacy classes D in G the restriction of A to D is a local system up to shift; we also give a parametrization of unipotent cuspidal character sheaves on G in terms of restrictions to conjugacy classes. Without restriction on characteristic we define canonical bijections from the set of unipotent character sheaves on G (and from the set of unipotent representations of the corresponding split reductive group over a finite field) …

Quadratic unipotent blocks in general linear, unitary and symplectic groups

An irreducible ordinary character of a finite reductive group is called quadratic unipotent if it corresponds under Jordan decomposition to a semisimple element $s$ in a dual group such that $s^2=1$. We prove that there is a bijection between, on the one hand the set of quadratic unipotent characters of $GL(n,q)$ or $U(n,q)$ for all $n \geq 0$ and on the other hand, the set of quadratic unipotent characters of $Sp(2n,q)$ for all $n \geq 0$. We then extend this correspondence to $\ell$-blocks for certain $\ell$ not dividing $q$.

Finding Characters Satisfying a Maximal Condition for Their Unipotent Support

In this article we extend independent results of Lusztig and Hézard concerning the existence of irreducible characters of finite reductive groups, (defined in good characteristic and arising from simple algebraic groups), satisfying a strong numerical relationship with their unipotent support. Along the way we obtain some results concerning quasi-isolated semisimple elements.

Cohomological representations for real reductive groups

For a connected reductive group $G$ over ${\mathbb R}$, we study cohomological $A$-parameters, which are Arthur parameters with the infinitesimal character of a finite-dimensional representation of $G({\mathbb C})$. We prove a structure theorem for such $A$-parameters, and deduce from it that a morphism of $L$-groups which takes a regular unipotent element to a regular unipotent element respects cohomological $A$-parameters. This is used to give complete understanding of cohomological $A$-parameters for all classical groups. We review the parametrization of Adams-Johnson packets of cohomologic…

On formal degrees of unipotent representations

Let G be a reductive p-adic group which splits over an unramified extension of the ground field. Hiraga, Ichino and Ikeda conjectured that the formal degree of a square-integrable G-representation $π$ can be expressed in terms of the adjoint $γ$-factor of the enhanced L-parameter of $π$. A similar conjecture was posed for the Plancherel densities of tempered irreducible G-representations. We prove these conjectures for unipotent G-representations. We also derive explicit formulas for the involved adjoint $γ$-factors.

From conjugacy classes in the Weyl group to unipotent classes, III

Let G be an affine algebraic group over an algebraically closed field such that the identity component G^0 of G is reductive. Let W be the Weyl group of G and let D be a connected component of G whose image in G/G^0 is a unipotent element. In this paper we define a map from the set of "twisted conjugay classes" in W to the set of unipotent G^0-conjugacy classes in D, generalizing an earlier construction which applied when G is connected.

Solvable, reductive and quasireductive supergroups

This work was inspired by two natural questions. The first question is when Lie(G')=Lie(G)', where G is a connected algebraic supergroup defined over a field of characteristic zero. The second question is whether the unipotent radical of any normal supersubgroup H of G coincides with the intersection of H and G_u, where G_u is the unipotent radical of G. Both questions have affirmative answers in the category of algebraic groups (in the second case one has to assume additionally that G and H are reduced whenever char K >0). Surprisingly, using the technique of Harish-Chandra superpairs and a c…

关于分歧 $p$-进群的单幂表示

On unipotent representations of ramified $p$-adic groups

本文分析了分歧 $p$-进群的单幂表示，为其建立了满足 Borel 期望及形式次数猜想的局部 Langlands 对应，推广了前人工作至非分歧分裂约化群，并用刚性内扭曲解释结果。

On special pieces, the Springer correspondence, and unipotent characters

Let G be a reductive algebraic group over the algebraic closure of a finite field F_q of good characteristic. In this paper, we demonstrate a remarkable compatibility between the Springer correspondence for G and the parametrization of unipotent characters of G(F_q). In particular, we show that in a suitable sense, "large" portions of these two assignments in fact coincide. This extends earlier work of Lusztig on Springer representations within special pieces of the unipotent variety.

单伴随例外群的幂幺 $\ell$-块分解矩阵的单位三角性

Unitriangularity of decomposition matrices of the unipotent ${\ell}$-blocks for simple adjoint exceptional groups

本文针对单伴随例外群，调整 Brunat-Dudas-Taylor 的证明并纳入 Chaneb 的 $\ell$-特殊类，将幂幺 $\ell$-块分解矩阵的单位三角性从好素数推广到坏素数。

Unipotent variety in the group compactification

The unipotent variety of a reductive algebraic group $G$ plays an important role in the representation theory. In this paper, we will consider the closure $\bar{\Cal U}$ of the unipotent variety in the De Concini-Procesi compactification $\bar{G}$ of a connected simple algebraic group $G$. We will prove that $\bar{\Cal U}-\Cal U$ is a union of some $G$-stable pieces introduced by Lusztig in \cite{L4}. This was first conjectured by Lusztig. We will also give an explicit description of $\bar{\Cal U}$. It turns out that similar results hold for the closure of any Steinberg fiber in $\bar{G}$.

Upper Triangularity for Unipotent Representations

Suppose $G$ is a real reductive group. The determination of the irreducible unitary representations of $G$ is one of the major unsolved problem in representation theory. There is evidence to suggest that every irreducible unitary representation of $G$ can be constructed through a sequence of well-understood operations from a finite set of building blocks, called the unipotent representations. These representations are `attached' (in a certain mysterious sense) to the nilpotent orbits of $G$ on the dual space of its Lie algebra. Inside this finite set is a still smaller set, consisting of the u…

Unipotent representations as a categorical centre

Let G(F_q) be the group of rational points of a split connected reductive group G defined over the finite field F_q. In this paper we show that the category of representations of G(F_q) which are finite direct sums of unipotent representations in a fixed two-sided cell is equivalent to the centre of a certain monoidal category of sheaves on the product of two copies of the flag manifold of G. We also consider a version of this for nonsplit groups.

On the Morita Frobenius numbers of blocks of finite reductive groups

We show that the Morita Frobenius number of the blocks of the alternating groups, the finite groups of Lie type in describing characteristic, and the Ree and Suzuki groups is 1. We also show that the Morita Frobenius number of almost all of the unipotent blocks of the finite groups of Lie type in non-defining characteristic is 1, and that in the remaining cases it is at most 2.

幂幺轨道的好位置辫表示与横截面

Good position braid representatives and transversal slices of unipotent orbits

本文利用Weyl群关联辫幺半群中的好元素，为代数闭域上连通约化群的所有幂幺轨道构造横截面，并以此重新表述仿射Springer纤维的维数公式。

Categorical actions on unipotent representations of finite classical groups

We review the categorical representation of a Kac-Moody algebra on unipotent representations of finite unitary groups in non-defining characteristic given by the authors. Then, we extend this construction to finite reductive groups of types B or C, in non-defining characteristic. We show that the decategorified representation is isomorphic to a direct sum of level 2 Fock spaces. We deduce that the Harish-Chandra branching graph coincides with the crystal graph of these Fock spaces. We also obtain derived equivalences between blocks, yielding Broue's abelian defect group conjecture for unipoten…

单幂零 $\ell$-块与连通且单连通的 $p$-进群

Unipotent $\ell$-blocks for simply-connected $p$-adic groups

本文研究连通且单连通约化群的 $F$-点集的单幂零 $\ell$-块，通过引入基于 Harish-Chandra 理论的 $(d,1)$-级数和 Bruhat-Tits 建筑上的一致幂等系来构造 $\ell$-块，并描述了非分歧经典群深度零范畴的稳定 $\ell$-块分解。

On the unipotent support of character sheaves

Let $G$ be a connected reductive group over $F_q$, where $q$ is large enough and the center of $G$ is connected. We are concerned with Lusztig's theory of {\em character sheaves}, a geometric version of the classical character theory of the finite group $G(F_q)$. We show that under a certain technical condition, the restriction of a character sheaf to its {\em unipotent support} (as defined by Lusztig) is either zero or an irreducible local system. As an application, the generalized Gelfand-Graev characters are shown to form a $\Z$-basis of the $\Z$-module of unipotently supported virtual char…