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arXiv 1008.4544math.RTmath-phmath.COmath.MP
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广义韦尔马模对对称对的限制

Restrictions of generalized Verma modules to symmetric pairs

Toshiyuki Kobayashi

摘要:

本文对复约化对称对(g,k)的广义韦尔马模的分支问题进行了新的研究.在这里我们注意到,g的韦尔马模在一般情况下被限制为一个约化子代数k时可能不包含任何单模。本文利用广义旗簇G_C/P_C上K_C轨道的几何性质,给出了三元组(g,k,p)上的一个充要条件,使得限制X|对于任意的g-模$X$,_k总是包含简单的k-模,该g-模位于附着于g的抛物子代数p的抛物BGG范畴O^p中。给出了g的简单广义韦尔马模中任意简单k-模的Gelfand-Kirillov维数的计算公式.然后我们证明限制X|_k对任意一般g-模X \in 0是无重数的当且仅当(g,k)同构于(A_n,A_{n-1}),(B_n,D_n)或(D_{n+1},B_n)的直和.我们还看到,限制X|_k对任意对称对(g,k)、任意具有交换诣零根的抛物子代数p和任意一般g-模X \in O^p都是多重性自由的.

英文摘要:

We initiate a new line of investigation on branching problems for generalized Verma modules with respect to complex reductive symmetric pairs (g,k). Here we note that Verma modules of g may not contain any simple module when restricted to a reductive subalgebra k in general. In this article, using the geometry of K_C orbits on the generalized flag variety G_C/P_C, we give a necessary and sufficient condition on the triple (g,k, p) such that the restriction X|_k always contains simple k-modules for any g-module $X$ lying in the parabolic BGG category O^p attached to a parabolic subalgebra p of g. Formulas are derived for the Gelfand-Kirillov dimension of any simple k-module occurring in a simple generalized Verma module of g. We then prove that the restriction X|_k is multiplicity-free for any generic g-module X \in O if and only if (g,k) is isomorphic to a direct sum of (A_n,A_{n-1}), (B_n,D_n), or (D_{n+1},B_n). We also see that the restriction X|_k is multiplicity-free for any symmetric pair (g, k) and any parabolic subalgebra p with abelian nilradical and for any generic g-module X \in O^p. Explicit branching laws are also presented.

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