per maggior chiarezza vi copio incollo il preambolo
\documentclass[a4paper,oneside,11pt,]{amsbook}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[sc]{mathpazo}
\linespread{1.05}
\usepackage[italian]{babel}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{mnsymbol}
\usepackage[square]{natbib}
\usepackage{amsmath}
\usepackage{amscd}\makeindex
\makeatletter
\renewcommand{\tocchapter}[3]{%
\indentlabel{\bfseries\@ifnotempty{#2}{\ignorespaces#2.\quad}}%
\@putspacebelow\textbf{#3}}
\newcommand{\@putspacebelow}{\vadjust{\kern.5ex}}
\makeatother
\let\tocappendix\tocchapter
\renewcommand{\thesection}{\for{toc}{\thechapter.}\arabic{section}}\newtheorem{Teorema}{Teorema}
\newtheorem{Definizione}{Definizione}
\newtheorem{Lemma}{Lemma}
\newtheorem{Esempio}{Esempio}
con il solito
\bibliographystyle{alpha}
\bibliography{tesi}
in coda