Searcharxiv⌕ Search

arXiv subjects

Dmitri I. Panyushev

Publications and source records attributed to Dmitri I. Panyushev.

62 records · Page 4Linked to original sources
arXiv2003

Ad-nilpotent ideals of a Borel subalgebra: generators and duality

It was shown by Cellini and Papi that an ad-nilpotent ideal determines certain element of the affine Weyl group, and that there is a bijection between the ad-nilpotent ideals and the integral points of a simplex with rational vertices. We give a description of the generators of ad-nilpotent ideals in terms of these elements, and show that an ideal has $k$ generators if and only it lies on the face of this simplex of codimension $k$. We also consider two combinatorial statistics on the set of ad-nilpotent ideals: the number of simple roots in the ideal and the number of generators. Considering the first statistic reveals some relations with the theory of clusters (Fomin-Zelevinsky). The distribution of the second statistic suggests that there should exist a natural involution (duality) on the set of ad-nilpotent ideals. Such an involution is constructed for the series A,B,C.

math.RT↗
arXiv2002

Abelian ideals of a Borel subalgebra and long positive roots

Let $\b$ be a Borel subalgebra of a simple Lie algebra $\g$ and let $\Ab$ denote the set of all Abelian ideals of $\b$. We consider $\Ab$ as poset with respect to inclusion, the zero ideal being the unique minimal element of $\Ab$. It was shown in my paper with G.Roehrle (Adv. Math. v.159 (2001)) that there is a one-to-one correspondence between the maximal Abelian ideals and the long simple roots of $\g$. But the very existence of it was demonstrated in a case-by-case fashion. Here a conceptual explanation for that empirical observation is given. The main results are: 1) there is a natural mapping $τ$ from the set of all nontrivial Abelian ideals to the set of long positive roots; 2) If $I$ is a maximal Abelian ideal, then $τ(I)$ is a long simple root. Restricting $τ$ to the set of maximal Abelian ideals yields the above-mentioned correspondence; 3) Each fibre of $τ$ is a poset in its own right, and we prove that this fibre has a unique maximal and a unique minimal element. 4) An explicit description of the minimal and the maximal ideal corresponding to a long root is given.

math.RT↗
arXiv2002

Some amazing properties of spherical nilpotent orbits

Let $\g$ be simple Lie algebra. We give a conceptual proof for the fact that the nilpotent orbits of height 3 are spherical. It is shown that if the highest root of $\g$ is fundamental, then $\g$ has a specific nilpotent orbit of height 3. This orbit satisfies several interesting relations. Moreover, it can be used for an intrinsic construction of a $G_2$ grading in $\g$. We also discuss an approach to describing the algebra of covariants on a nilpotent orbit. For the nilpotent orbits of height 2, it is shown that the algebra of regular functions is a free module over the algebra of covariants. For the nilpotent orbits of height 3, a conjectural description of the algebra of covariants is given. This description is compatible with all previously known examples.

math.AG↗
arXiv2002

Weight multiplicity free representations, $\frak g$-endomorphism algebras, and Dynkin polynomials

$\frak g$-endomorphism algebras form an interesting class of associative algebras related to the adjoint representation of a semisimple Lie algebra $\frak g$. These algebras were recently introduced by A.Kirillov, who used the term `family algebras'. Let $C_λ$ denote the $\frak g$-endomorphism algebra associated with a simple $\frak g$-module $V_λ$. Most of our results concern the case in which $C_λ$ is commutative, i.e., $V_λ$ is a weight multiplicity free $\frak g$-module. It is proved that $C_λ$ is a polynomial algebra if and only if $λ$ is minuscule. We also characterise in general the number of the irreducible components of the corresponding affine variety. The main result is that the commutative $\frak g$-endomorphism algebra is always Gorenstein. We explicitly compute the Poincare series of $C_λ$ for any $λ$, and show that in the commutative case the numerator coincides with the polynomial that was introduced by E.B.Dynkin in 1950. We also discuss a connection between commutative $\frak g$-endomorphism algebras and equivariant cohomology.

math.AG↗
arXiv2001

The index of a Lie algebra, the centraliser of a nilpotent element, and the normaliser of the centraliser

In this paper, we study the index for several natural classes of non-reductive subalgebras of semisimple Lie algebras. Namely, we look at parabolic subalgebras, centralisers of nilpotent elements, and the normalisers of the centralisers. We discuss a conjecture of Elashvili to the effect that the index of any centraliser is equal to the rank of the semisimple algebra in question. It is shown that Elashvili's conjecture is true for `small' and `large' orbits. Some properties of the index for the normaliser of the centraliser are proved. In particular, we prove that, for a regular nilpotent element, the normaliser of the centraliser is a Frobenius Lie algebra.

math.AG↗
arXiv2000

The exterior algebra and `Spin' of an orthogonal g-module

A well-known result of Kostant gives a description of the G-module structure for the exterior algebra of Lie algebra $\frak g$. We give a generalization of this result for the isotropy representations of symmetric spaces. If $\frak g={\frak g}_0+{\frak g_1}$ is a Z_2-grading of a simple Lie algebra, we explicitly describe a ${\frak g}_0$-module $Spin_0({\frak g}_1)$ such that the exterior algebra of ${\frak g}_1$ is the tensor square of this module times some power of 2. Although $Spin_0({\frak g}_1)$ is usually reducible, we show that a Casimir element for ${\frak g}_0$ always acts scalarly on it. We also a give classification of all orthogonal representations of simple algebraic groups having an exterior algebra of skew-invariants.

math.AG↗
arXiv1999

Nilpotent pairs in semisimple Lie algebras and their characteristics

Recently, V.Ginzburg introduced and studied in depth the notion of a principal nilpotent pair in a semisimple Lie algebra \g. Our aim is to contribute to the general theory of nilpotent pairs. Roughly speaking, a nilpotent pair (e_1,e_2) consists of two commuting elements in \g that can independently be contracted to the origin. A principal nilpotent pair is a double counterpart of a regular nilpotent element. Consequently, the theory of nilpotent pairs should stand out as double counterpart of the theory of nilpotent orbits. We show that any nilpotent pair has a characteristic (h_1,h_2), which is unique within to conjugacy. Generalizing Dynkin's approach to (sl_2)-triples, we prove that the number of G-orbits of characteristics of nilpotent pairs is finite and provide some estimates for the numerical labels α_j(h_i), where {α_j} are suitable simple roots of \g. It was observed by Ginzburg that the number of G-orbits of nilpotent pairs is infinite. This means this class is too wide to have a reasonable theory. To resolve this difficulty, we introduce `wonderful' nilpotent pairs. We prove that if two wonderful pairs have the same characteristic, then these are conjugate. This implies that there are finitely many G-orbits of wonderful pairs. A number of nice properties of wonderful pairs shows that these can be regarded as right double analogue of nilpotent orbits. We also consider several natural classes of wonderful pairs and describe characteristics for principal and almost principal nilpotent pairs.

math.AG↗