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arXiv 1205.2017math.RT
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博雷尔子群的余伴随结构及其零根

The coadjoint structure of Borel subgroups and their nilradicals

Bertram Kostant

摘要:

设$G$是复单连通半单李群,$\frak{g}= Lie G$ .令$\frak{g} = \frak{n}_- +\frak{h} + \frak{n}$是$\frak{g}$的三角分解。人们很容易得到$Cent\,U({\frak n})$同构于对称不变量的环$S({\frak n})^{\frak\n}$。利用强正交根的级联${\cal B}$,我们证明了$S({\frak n})^{{\frak n}$是多项式环$\Bbb C [ξ_1,...,ξ_m]$,其中$m$是${\cal B}$的基数.利用这个结果,我们建立了$N = exp \frak {n}$的最大余伴随有余维数$m$。设$\frak {b}= \frak {h} + \frak {n}$,使得相应的子群$B$是$G$的Borel子群。令$\ell = rank \frak{g}$。本文证明了$B$的最大余伴随轨道具有余维数$\ell - m$,使得下列命题(1)和(2)等价:(1)-1在$G$的Weyl群中(即,$\ell = m$),(2)B有一个非空的开余伴轨道.我们注意到幂零群或半单群不能有非空的开余伴随轨道。著名的例子,其中一个可解李群有一个非空的coadjoint轨道是由于Piatetski-夏皮罗在他的反例建设的一个有界复杂的齐次域是不是嘉当型。

英文摘要:

Let $G$ be a complex simply-connected semisimple Lie group and let $\frak{g}= Lie G$. Let $\frak{g} = \frak{n}_- +\frak{h} + \frak{n}$ be a triangular decomposition of $\frak{g}$. One readily has that $Cent\,U({\frak n})$ is isomorphic to the ring $S({\frak n})^{\frak\n}$ of symmetric invariants. Using the cascade ${\cal B}$ of strongly orthogonal roots, some time ago we proved that $S({\frak n})^{{\frak n}$ is a polynomial ring $\Bbb C [ξ_1,...,ξ_m]$ where $m$ is the cardinality of ${\cal B}$. Using this result we establish that the maximal coadjoint of $N = exp \frak {n}$ has codimension $m$. Let $\frak {b}= \frak {h} + \frak {n}$ so that the corresponding subgroup $B$ is a Borel subgroup of $G$. Let $\ell = rank \frak{g}$. Then in this paper we prove the theorem that the maximal coadjoint orbit of $B$ has codimension $\ell - m$ so that the following statements (1) and (2) are equivalent: (1) -1 is in the Weyl group of $G$ (i.e., $\ell = m$), and (2), B has a nonempty open coadjoint orbit. We remark that a nilpotent or a semisimple group cannot have a nonempty open coadjoint orbit. Celebrated examples where a solvable Lie group has a nonempty coadjoint orbit are due to Piatetski--Shapiro in his counterexample construction of a bounded complex homogeneous domain which is not of Cartan type.

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