😳 Ops! Scusate per l’errore. Rimedio subito. Ecco il preambolo, ve lo riporto tutto.
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\documentclass[10pt,a4paper]{book}
% Pacchetti
\usepackage[italian]{babel}
\usepackage[T1]{fontenc}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{graphicx}
\usepackage[italian]{varioref}
\usepackage{psfrag}
\usepackage{subfig}
\usepackage{cancel}
\usepackage{mathrsfs}
\usepackage{xcolor}
\usepackage{framed}
% Definizioni comandi e operatori matematici
\newcommand{\N}{\mathbb{N}}
\newcommand{\R}{\mathbb{R}}
\newcommand{\Z}{\mathbb{Z}}
\newcommand{\C}{\mathbb{C}}
\newcommand{\Hol}{\mathcal{H}}
\newcommand{\der}{\mathcal{D}}
\newcommand{\zt}{\mathcal{Z}}
\newcommand{\lt}{\mathcal{L}}
\newcommand{\intL}{\mathscr{L}}
\newcommand{\Cor}{\mathcal{C}}
\newcommand{\Cont}{$\mathscr{C}$}
\newcommand{\cont}{\mathscr{C}}
\DeclareMathOperator{\tg}{tg}
\DeclareMathOperator{\rect}{rect}
\DeclareMathOperator{\triang}{triang}
\DeclareMathOperator{\Log}{Log}
\DeclareMathOperator{\Arg}{Arg}
\DeclareMathOperator{\res}{res}
\DeclareMathOperator{\sinc}{sinc}
\DeclareMathOperator{\sgn}{sgn}
\DeclareMathOperator{\arctg}{arctg}
\let\Re\relax
\DeclareMathOperator{\Re}{Re}
\let\Im\relax
\DeclareMathOperator{\Im}{Im}
\DeclareMathOperator{\dist}{d}
% Stili e ambienti
\theoremstyle{definition}
\newtheorem{defin}{Definizione}[chapter]
\newtheorem{example}{Esempio}[chapter]
\newenvironment{definizione}[1][]
{
\definecolor{shadecolor}{gray}{0.90} \begin{shaded}\begin{defin}[#1]
}
{
\end{defin}\end{shaded}
}
\theoremstyle{plain}
\newtheorem{theorem}{Teorema}[chapter]
\newtheorem{proposition}{Proposizione}[chapter]
\newtheorem{corollary}{Corollario}[chapter]
% Contatori di livello inferiore
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
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