\usepackage{amssymb,amsmath,amsthm,amsfonts,mathrsfs,xypic}
\def\D{\partial} \def\al{\alpha} \def\la{\lambda} \def\be{\beta} \def\si{\sigma} \def\<{\langle} \def\>{\rangle} \def\Del{\Delta} \def\Om{\Omega} \def\La{\Lambda}
\newcommand{\Isom}{\operatorname{Isom}} \newcommand{\diam}{{\rm diam}} \newcommand{\acts}{\curvearrowright} \newcommand{\geo}{\partial_\infty} \newcommand{\R}{\mathbb{R}} \newcommand{\Ga}{\Gamma} \newcommand{\Z}{\mathbb{Z}} \newcommand{\N}{\mathbb{N}} \newcommand{\Q}{\mathbb{Q}} \newcommand{\C}{\mathbb{C}} \renewcommand{\H}{\mathbb{H}}