\usepackage{amssymb,amsmath,amsthm,amsfonts,mathrsfs,xypic}
\theoremstyle{plain} \newtheorem{theorem}{Theorem}[section] \newtheorem{lemma}{Lemma}[section] \newtheorem{proposition}{Proposition}[section] \newtheorem{corollary}{Corollary}[section] \newtheorem{question}{Question}[section]
\theoremstyle{definition} \newtheorem{definition}{Definition}[section]
\theoremstyle{remark} \newtheorem{remark}{Remark}[section] \newtheorem{example}{Example}[section]
\newcommand{\Aut}{\operatorname{Aut}}
\newcommand{\co}{\colon\thinspace} \newcommand{\Z}{\mathbb Z} \newcommand{\R}{\mathbb R} \newcommand{\Q}{\mathbb Q} \newcommand{\C}{\mathbb C} \renewcommand{\H}{\mathbb H}
\renewcommand{\hat}[1]{\widehat{#1}} \renewcommand{\bar}[1]{\overline{#1}} \renewcommand{\tilde}[1]{\widetilde{#1}}