A Hilbert--Mumford criterion for polystability in Kaehler geometry

Abstract

Consider a Hamiltonian action by biholomorphisms of a compact Lie group $K$ on a Kaehler manifold $X$, with moment map $\mu:X\to\klie^*$. We characterize which orbits of the complexified action of $G=K^{\CC}$ in $X$ intersect $\mu^{-1}(0)$ in terms of the maximal weights $\lim_{t\to\infty}\la\mu(e^{\imag ts}\cdot x),s\ra$, where $s$ belongs to the Lie algebra of $K$. We do not impose any a priori restriction on the stabilizer of $x$. Assuming some mild growth conditions on the action of $K$ on $X$, we view the maximal weights as defining a maps $\lambda_x$ from the boundary at infinity of the symmetric space $K\backslash G$ to $\RR\cup\{\infty\}$. We prove that $G\cdot x$ meets $\mu^{-1}(0)$ if: (1) $\lambda_x$ is everywhere nonnegative, (2) any boundary point $y$ such that $\lambda_x(y)=0$ can be connected with a geodesic in $K\backslash G$ to another boundary point $y'$ satisfying $\lambda_x(y')=0$. We also prove that $\lambda_{g\cdot x}(y)=\lambda_x(y\cdot g)$ for any $g\in G$ and $y\in \partial_{\infty}(K\backslash G)$.