\documentclass[11pt]{amsart}
\setlength{\unitlength}{1cm}
\setlength{\oddsidemargin}{1cm} \setlength{\textwidth}{15cm}
\renewcommand{\theequation}{\thesection.\arabic{equation}}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{prop}[theorem]{Proposition}
\newtheorem{remark}{Remark}[section]
\newtheorem{ass}{Assumption}[section]
\newtheorem{corollary}[theorem]{Corollary}
\newtheorem{definition}[theorem]{Definition}
\pretolerance=10000
\title{}
\begin{document}