287 research outputs found
Centralizers of distinguished nilpotent pairs and related problems
In this paper, by establishing an explicit and combinatorial description of
the centralizer of a distinguished nilpotent pair in a classical simple Lie
algebra, we solve in the classical case Panyushev's Conjecture which says that
distinguished nilpotent pairs are wonderful, and the classification problem on
almost principal nilpotent pairs. More precisely, we show that disinguished
nilpotent pairs are wonderful in types A, B and C, but they are not always
wonderful in type D. Also, as the corollary of the classification of almost
principal nilpotent pairs, we have that almost principal nilpotent pairs do not
exist in the simply-laced case and that the centralizer of an almost principal
nilpotent pair in a classical simple Lie algebra is always abelian.Comment: 25 page
Semi-direct products of Lie algebras and their invariants
The goal of this paper is to extend the standard invariant-theoretic design,
well-developed in the reductive case, to the setting of representation of
certain non-reductive groups. This concerns the following notions and results:
the existence of generic stabilisers and generic isotropy groups for
finite-dimensional representations; structure of the fields and algebras of
invariants; quotient morphisms and structure of their fibres. One of the main
tools for obtaining non-reductive Lie algebras is the semi-direct product
construction. We observe that there are surprisingly many non-reductive Lie
algebras whose adjoint representation has a polynomial algebra of invariants.
We extend results of Takiff, Geoffriau, Rais-Tauvel, and Levasseur-Stafford
concerning Takiff Lie algebras to a wider class of semi-direct products. This
includes -contractions of simple Lie algebras and generalised Takiff
algebras.Comment: 49 pages, title changed, section 11 is shortened, numerous minor
corrections; accepted version, to appear in Publ. RIMS 43(2007
On divisible weighted Dynkin diagrams and reachable elements
Let D(e) denote the weighted Dynkin diagram of a nilpotent element in
complex simple Lie algebra \g. We say that D(e) is divisible if D(e)/2 is
again a weighted Dynkin diagram. (That is, a necessary condition for
divisibility is that is even.) The corresponding pair of nilpotent orbits
is said to be friendly. In this note, we classify the friendly pairs and
describe some of their properties. We also observe that any subalgebra sl(3) in
\g determines a friendly pair. Such pairs are called A2-pairs. It turns out
that the centraliser of the lower orbit in an A2-pair has some remarkable
properties. Let be such an orbit and a characteristic of . Then
determines the Z-grading of the centraliser . We prove that is
generated by the Levi subalgebra and two elements in . In
particular, (1) the nilpotent radical of is generated by and (2)
. The nilpotent elements having the last property are called
reachable.Comment: 17 pages; v2 minor corrrections; final version, to appear in
Transformation Groups (2010
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