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On Orbits in Double Flag Varieties for Symmetric Pairs

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Abstract

Let G be a connected, simply connected semisimple algebraic group over the complex number field, and let K be the fixed point subgroup of an involutive automorphism of G so that (G, K) is a symmetric pair. We take parabolic subgroups P of G and Q of K, respectively, and consider the product of partial flag varieties G/P and K/Q with diagonal K-action, which we call a double flag variety for a symmetric pair. It is said to be of finite type if there are only finitely many K-orbits on it. In this paper, we give a parametrization of K-orbits on G/P × K/Q in terms of quotient spaces of unipotent groups without assuming the finiteness of orbits. If one of PG or QK is a Borel subgroup, the finiteness of orbits is closely related to spherical actions. In such cases, we give a complete classification of double flag varieties of finite type, namely, we obtain classifications of K-spherical flag varieties G/P and G-spherical homogeneous spaces G/Q.

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Correspondence to XUHUA HE.

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(XUHUA HE) Partially supported by HKRGC grant 602011

(KYO NISHIYAMA) Supported by JSPS Grant-in-Aid for Scientific Research (B) #21340006

(HIROYUKI OCHIAI) Supported by JSPS Grant-in-Aid for Scientific Research (A) #19204011, and JST CREST.

(YOSHIKI OSHIMA) Supported by Grant-in-Aid for JSPS Fellows (10J00710).

Dedicated to Jiro Sekiguchi on the occasion of his sixtieth birthday

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HE, X., OCHIAI, H., NISHIYAMA, K. et al. On Orbits in Double Flag Varieties for Symmetric Pairs. Transformation Groups 18, 1091–1136 (2013). https://doi.org/10.1007/s00031-013-9243-8

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