scholarly journals Criteria for rational smoothness of some symmetric orbit closures

2012 ◽  
Vol 229 (1) ◽  
pp. 183-200 ◽  
Author(s):  
Axel Hultman
Keyword(s):  
Orbit Closures ◽  
Symmetric Orbit ◽  
Rational Smoothness

scholarly journals Criteria for rational smoothness of some symmetric orbit closures

2010 ◽  
Vol DMTCS Proceedings vol. AN,... (Proceedings) ◽  
Author(s):  
Axel Hultman
Keyword(s):  
Type A ◽  
Linear Algebraic Group ◽  
Schubert Varieties ◽  
Orbit Closure ◽  
Single Vertex ◽  
Orbit Closures ◽  
Symmetric Orbit ◽  
International Audience ◽  
Bruhat Graph ◽  
Rational Smoothness

International audience Let $G$ be a connected reductive linear algebraic group over $ℂ$ with an involution $θ$ . Denote by $K$ the subgroup of fixed points. In certain cases, the $K-orbits$ in the flag variety $G/B$ are indexed by the twisted identities $ι (θ ) = {θ (w^{-1})w | w∈W}$ in the Weyl group $W$. Under this assumption, we establish a criterion for rational smoothness of orbit closures which generalises classical results of Carrell and Peterson for Schubert varieties. That is, whether an orbit closure is rationally smooth at a given point can be determined by examining the degrees in a "Bruhat graph'' whose vertices form a subset of $ι (θ )$. Moreover, an orbit closure is rationally smooth everywhere if and only if its corresponding interval in the Bruhat order on $ι (θ )$ is rank symmetric. In the special case $K=\mathrm{Sp}_{2n}(ℂ), G=\mathrm{SL}_{2n}(ℂ)$, we strengthen our criterion by showing that only the degree of a single vertex, the "bottom one'', needs to be examined. This generalises a result of Deodhar for type A Schubert varieties. Soit $G$ un groupe algébrique connexe réductif sur $ℂ$, équipé d'une involution $θ$ . Soit $K$ le sousgroupe de ses points fixes. Dans certains cas, les orbites des points de la variété de drapeaux $G/B$ sous l'action de $K$ sont indexées par les identités tordues, $ι (θ ) = {θ (w^{-1})w | w∈W}$, du groupe de Weyl $W$. Sous cette hypothèse, on établit un critère pour la lissité rationnelle des adhérences des orbites, qui généralise des résultats classiques de Carrell et Peterson pour les variétés de Schubert. Plus précisément, on peut déterminer si l'adhérence d'une orbite est rationnellement lisse en examinant les degrés dans un "Graphe de Bruhat" dont les sommets forment un sous-ensemble de $ι (θ )$. En outre, l'adhérence d'une orbite est partout rationnellement lisse si et seulement si l'intervalle correspondant dans l'ordre de Bruhat de $ι (θ )$ est symétrique respectivement au rang. Dans le cas particulier $K=\mathrm{Sp}_{2n}(ℂ), G=\mathrm{SL}_{2n}(ℂ)$, nous améliorons notre critère en montrant qu'il suffit d'examiner le degré d'un seul sommet, celui "du bas". Ceci généralise un résultat de Deodhar pour les variétés de Schubert de type A.

scholarly journals POLYNOMIALS FOR SYMMETRIC ORBIT CLOSURES IN THE FLAG VARIETY

2016 ◽  
Vol 22 (1) ◽  
pp. 267-290 ◽  
Author(s):  
B. WYSER ◽  
A. YONG
Keyword(s):  
Flag Variety ◽  
Orbit Closures ◽  
Symmetric Orbit

scholarly journals Governing singularities of symmetric orbit closures

2018 ◽  
Vol 12 (1) ◽  
pp. 173-225 ◽  
Author(s):  
Alexander Woo ◽  
Benjamin Wyser ◽  
Alexander Yong
Keyword(s):  
Orbit Closures ◽  
Symmetric Orbit

scholarly journals Homogeneous orbit closures and applications

2011 ◽  
Vol 32 (2) ◽  
pp. 785-807 ◽  
Author(s):  
ELON LINDENSTRAUSS ◽  
URI SHAPIRA
Keyword(s):  
Real Number ◽  
Unit Volume ◽  
Orbit Closures ◽  
Image Position

AbstractWe give new classes of examples of orbits of the diagonal group in the space of unit volume lattices in ℝd for d≥3 with nice (homogeneous) orbit closures, as well as examples of orbits with explicitly computable but irregular orbit closures. We give Diophantine applications to the former; for instance, we show that, for all γ,δ∈ℝ, where 〈c〉 denotes the distance of a real number c to the integers.

Classification of higher rank orbit closures in ${\mathcal H^{\mathrm{odd}}(4)}$

2016 ◽  
Vol 18 (8) ◽  
pp. 1855-1872 ◽  
Author(s):  
David Aulicino ◽  
Duc-Manh Nguyen ◽  
Alex Wright
Keyword(s):  
Orbit Closures ◽  
Higher Rank

scholarly journals Poincaré polynomials of generic torus orbit closures in Schubert varieties

2021 ◽  
pp. 189-208
Author(s):  
Eunjeong Lee ◽  
Mikiya Masuda ◽  
Seonjeong Park ◽  
Jongbaek Song
Keyword(s):  
Schubert Variety ◽  
Flag Variety ◽  
Schubert Varieties ◽  
Orbit Closure ◽  
Poincaré Polynomial ◽  
Content Type ◽  
Eulerian Polynomial ◽  
Orbit Closures

The closure of a generic torus orbit in the flag variety G / B G/B of type  A A is known to be a permutohedral variety, and its Poincaré polynomial agrees with the Eulerian polynomial. In this paper, we study the Poincaré polynomial of a generic torus orbit closure in a Schubert variety in  G / B G/B . When the generic torus orbit closure in a Schubert variety is smooth, its Poincaré polynomial is known to agree with a certain generalization of the Eulerian polynomial. We extend this result to an arbitrary generic torus orbit closure which is not necessarily smooth.

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