\usepackage{amssymb,amsmath,amsthm,amsfonts,mathrsfs,xypic}
\theoremstyle{plain} \newtheorem*{theorem}{Theorem} \newtheorem*{problem}{Problem} \newtheorem*{lemma}{Lemma} \newtheorem*{corollary}{Corollary}
\theoremstyle{definition} \newtheorem*{definition}{Definition}
\theoremstyle{remark} \newtheorem*{remark}{Remark} \newtheorem*{example}{Example}