\documentclass[a4paper]{article}
\title{}
\author{}


\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{mathrsfs}
\usepackage{amsmath}
\usepackage[francais]{babel}
\usepackage{a4wide}
\usepackage[all]{xy}


\textwidth=472pt
\textheight=675pt
\oddsidemargin=0cm
\marginparwidth=0cm
\marginparsep=0cm
\leftmargin=0cm
\rightmargin=0cm
\topmargin=0cm
\headsep=0cm

%\pagestyle{empty}

\newtheorem{theo}[section]{\textsf{Th\'eor\`eme}}
\newtheorem{lemm}[section]{\textsf{Lemme}}
\newtheorem{coro}[section]{\textsf{Corollaire}}
\newtheorem{prop}[section]{\textsf{Proposition}}
\theoremstyle{definition}
\newtheorem*{rema}{\textsf{Remarque}}
\theoremstyle{definition}
\newtheorem*{defi}{\textsf{D\'efinition}}
\theoremstyle{definition}
\newtheorem*{nota}{\textsf{Notation}}
\theoremstyle{definition}
\newtheorem{exam}[section]{\textsf{Exemple}}


\DeclareMathOperator{\Card}{\mathsf{Card}} % cardinal
\DeclareMathOperator{\pgcd}{\mathsf{pgcd}}
\DeclareMathOperator{\ppcm}{\mathsf{ppcm}}

\DeclareMathOperator{\supp}{\mathsf{supp}} % support
\DeclareMathOperator{\stab}{\mathsf{stab}} % stabilisateur
\DeclareMathOperator{\Fix}{\mathsf{Fix}} % fixateur
\DeclareMathOperator{\orb}{\mathsf{orb}} % orbite
\DeclareMathOperator{\Z}{Z} % centre
\DeclareMathOperator{\ord}{ord} % ordre
\DeclareMathOperator{\Syl}{Syl} % groupes de Sylow

\DeclareMathOperator{\NN}{\mathbf{N}} % entiers naturels
\DeclareMathOperator{\ZZ}{\mathbf{Z}} % entiers relatifs
\DeclareMathOperator{\QQ}{\mathbf{Q}} % rationnels
\DeclareMathOperator{\FF}{\mathbf{F}} % corps finis
\DeclareMathOperator{\RR}{\mathbf{R}} % réels
\DeclareMathOperator{\CC}{\mathbf{C}} % complexes

\DeclareMathOperator{\Id}{\mathsf{Id}} % identité
\DeclareMathOperator{\I}{I} % matrice identité
\DeclareMathOperator{\diag}{diag} % matrice diagonale

\DeclareMathOperator{\Mat}{\mathsf{M}} % matrices
\DeclareMathOperator{\Matrice}{\mathsf{Mat}} % matrice de changement de base
\DeclareMathOperator{\GL}{\mathsf{GL}} % groupe linéaire
\DeclareMathOperator{\PGL}{\mathsf{PGL}} % groupe linéaire projectif
\DeclareMathOperator{\SL}{\mathsf{SL}} % groupe spécial linéaire
\DeclareMathOperator{\PSL}{\mathsf{PSL}} % groupe spécial linéaire projectif
\renewcommand{\O}{\mathsf{O}} % groupe orthogonal 
\DeclareMathOperator{\SO}{\mathsf{SO}} % groupe spécial orthogonal
\DeclareMathOperator{\U}{\mathsf{U}} % groupe unitaire
\DeclareMathOperator{\SU}{\mathsf{SU}} % groupe spécial unitaire
\DeclareMathOperator{\GA}{\mathsf{GA}} % groupe affine
\DeclareMathOperator{\Aff}{\mathsf{Aff}} % affine

\DeclareMathOperator{\Frac}{Frac} % corps des fractions
\DeclareMathOperator{\Gal}{Gal} % groupe de Galois
\DeclareMathOperator{\Ker}{Ker} % noyau
\DeclareMathOperator{\Coker}{Coker} % conoyau
\DeclareMathOperator{\im}{Im} % image
\DeclareMathOperator{\Hom}{Hom} % homomorphismes
\DeclareMathOperator{\Aut}{Aut} % automorphismes

\DeclareMathOperator{\isomto}{\overset{\sim}{\to}} %isomorphisme
\DeclareMathOperator{\from}{\leftarrow} % flèche vers la gauche

%    Absolute value notation
\newcommand{\abs}[1]{\left\vert#1\right\vert}
%    Norm notation
\newcommand{\norm}[1]{\left\|#1\right\|}
%    Scalar product notation
\newcommand{\scal}[2]{\left\langle #1|#2\right\rangle}

\def \ooverline #1#2#3%
{\mkern #1mu \overline{\mkern -#1mu #3 \mkern -#2mu }\mkern #2mu }

% capitales surmontées par une barre
\def \Abar {\ooverline40A{\mkern 1mu}{}}
\def \Bbar {\ooverline40B{\mkern 1mu}{}}
\def \Cbar {\ooverline40C{\mkern 1mu}{}}
\def \Dbar {\ooverline40D{\mkern 1mu}{}}
\def \Ebar {\ooverline40E{\mkern 1mu}{}}
\def \Fbar {\ooverline40F{\mkern 1mu}{}}
\def \Gbar {\ooverline40G{\mkern 1mu}{}}
\def \Hbar {\ooverline41H{\mkern 1mu}{}}
\def \Ibar {\ooverline40I{\mkern 1mu}{}}
\def \Jbar {\ooverline60J{\mkern 1mu}{}}
\def \Kbar {\ooverline40K{\mkern 1mu}{}}
\def \Lbar {\ooverline40L{\mkern 1mu}{}}
\def \Mbar {\ooverline41M{\mkern 1mu}{}}
\def \Nbar {\ooverline41N{\mkern 1mu}{}}
\def \Obar {\ooverline40O{\mkern 1mu}{}}
\def \Pbar {\ooverline40P{\mkern 1mu}{}}
\def \Qbar {\ooverline40Q{\mkern 1mu}{}}
\def \Rbar {\ooverline40R{\mkern 1mu}{}}
\def \Sbar {\ooverline40S{\mkern 1mu}{}}
\def \Tbar {\ooverline10T{\mkern 1mu}{}}
\def \Ubar {\ooverline31U{\mkern 1mu}{}}
\def \Vbar {\ooverline21V{\mkern 1mu}{}}
\def \Wbar {\ooverline21W{\mkern 1mu}{}}
\def \Xbar {\ooverline30X{\mkern 1mu}{}}
\def \Ybar {\ooverline11Y{\mkern 1mu}{}}
\def \Zbar {\ooverline40Z{\mkern 1mu}{}}

% locutions et abréviations
\newcommand{\loccit}{\textit{loc. cit.}}
\newcommand{\cf}{\textit{cf }}
\newcommand{\ie}{\textit{i.e. }}
\newcommand{\idem}{\textit{idem }}
\newcommand{\ops}{on peut supposer }
\newcommand{\ssi}{si et seulement si }
\newcommand{\etc}{\textit{etc}}

\begin{document}

blabla



\end{document}