Stability, analytic stability for real reductive Lie groups

Abstract

We presented a systematic treatment of a Hilbert criterion for stability theory for an action of a real reductive group $G$ on a real submanifold $X$ of a K\"ahler manifold $Z$. More precisely, we suppose the action of a compact connected Lie group $U$ with Lie algebra $\mathfrak{u}$ extends holomorphically to an action of the complexified group $U^{\mathbb C}$ and that the $U$-action on $Z$ is Hamiltonian. If $G\subset U^{\mathbb C}$ is closed and compatible, there is a corresponding gradient map $\mu_\mathfrak{p} : X\longrightarrow \mathfrak{p}$, where $\mathfrak g = \mathfrak k \oplus \mathfrak p$ is a Cartan decomposition of the Lie algebra of $G$. The concept of energy complete action of $G$ on $X$ is introduced. For such actions, one can characterize stability, semistability and polystability of a point by a numerical criteria using a $G$-equivariant function associated with a gradient map, called maximal weight function. We also prove the classical Hilbert-Mumford criteria for semistabilty and polystability conditions. We thank the anonymous referee for carefully reading our paper and for giving such constructive comments which substantially helped improving the quality of the paper.