\usepackage{amssymb,amsmath,amsthm,amsfonts,mathrsfs,xypic}
\theoremstyle{plain} \newtheorem{theorem}{Theorem} \newtheorem{question}{Question}
\theoremstyle{definition} \newtheorem{definition}{Definition}
\theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{example}{Example}
\newcommand{\R}{\mathbb R}