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       <td width="20" height="25"></td>         <td width="580" height="25" colspan="2"></td>         <td width="42" height="25"></td>       </tr>       <tr>         <td width="20"></td>         <td width="175"><p align="center"><img src="../../images/biograph/lagrange.gif"         width="125" height="125" alt="Lagrange Joseph Louis"></td>         <td width="405"><font face="Tahoma" size="5" color="#535353">Lagrange Joseph Louis</font><p><font         face="Arial" size="3" color="#FF8000">Mathmaticien et astronome franais (Turin, 1736 - Paris, 1813)</font></td>         <td width="42"></td>       </tr>       <tr>         <td width="20" height="20"></td>         <td width="580" height="20" colspan="2"></td>         <td width="42" height="20"></td>       </tr>       <tr>         <td width="20" height="15"></td>         <td width="580" colspan="2"><font face="Arial" size="2" color="#535353"><p align="justify">Jospeh Louis Lagrange est l'un des plus grands scientifiques du XVIIIe sicle. N  Turin (Italie) en 1736, il est un lve brillant  l'universit de sa ville natale avant d'tre nomm professeur de l'Ecole royale d'Artillerie  seulement dix-neuf ans. Avec quelques amis, il fonde en 1757 une socit scientifique, future Acadmie des sciences de Turin. Deux mathmaticiens vont jouer dans la vie de Lagrange un rle important. Le premier, <a href="biograph.php3?Ref=23">Euler</a>, avec qui il entretient une importante correspondance, l'encourage et l'inspire dans ses travaux mathmatiques. Le second, d'Alembert, est son ami. Grce  lui, Lagrange rejoint la cour de Frdric II  Berlin en 1766. Il y est nomm directeur de la section mathmatique de l'Acadmie des sciences,  la suite de son mentor parti pour Saint-Ptersbourg. A la mort de l'empereur, aprs vingt annes de travail fertile, Lagrange accepte l'invitation du gouvernement franais et part pour Paris. Devenu pensionnaire vtran de l'Acadmie des sciences, il participe, pendant la Rvolution,  l'laboration du systme mtrique en collaboration avec <a href="../portrait/lavoisier.html">Lavoisier</a>, puis contribue  la fondation de l'Ecole normale, de l'Ecole Polytechnique et du Bureau des longitudes. Admir par Napolon 1<sup>er</sup>, il meurt honor (snateur, comte d'Empire et grand officier de la Lgion d'honneur), en avril 1813.</p>  <p align="justify">Lagrange est  l'origine de nombreuses avances dans tous les domaines des mathmatiques. Il est, avec Euler, l'un des fondateurs du calcul des variations. Il dmontre galement les thormes de Wilson et de Bachet et fonde la thorie des formes quadratiques (<i>Recherches arithmtiques</i>, 1775). Mais l'uvre majeure de Lagrange est sans aucun doute sa <i>Mcanique analytique</i>, publie en 1788. Dans cet ouvrage, il systmatise l'utilisation des quations diffrentielles qu'il applique  des problmes de mcanique pure. Sa mthode sera  l'origine de nombreuses recherches ultrieures dont celles de Hamilton. Enfin, en mcanique cleste, Lagrange dveloppe galement des recherches importantes : sur le problme des trois corps (voir l'article <a href="../../articles/articles_aff.php3?Ref=336">"Trios dans l'espace"</a>), la libration de la Lune et le mouvement des plantes.</p></font></td>         <td width="42" height="15"></td>       </tr>       <tr>         <td width="20" height="50"></td>         <td width="580" height="50" colspan="2"></td>         <td width="42" height="50"></td>       </tr>     </table>     <table border="0" cellpadding="0" cellspacing="0" width="642" height="30">       <tr>         <td align="center" width="150"></td>         <td align="center" width="30"><img src="../../images/logos/trait.gif" alt="trait"         WIDTH="20" HEIGHT="25"></td>         <td align="center" width="140"><a href="biograph_som.html" style="text-decoration: none"><font         face="Arial" size="1"><b>nouvelle recherche</b></font></a></td>         <td align="center" width="30"><img src="../../images/logos/trait.gif" alt="trait"         WIDTH="20" HEIGHT="25"></td>         <td align="center" width="292"></td>       </tr>     </table>         <p>&nbsp;</td>   </tr>   <tr>     <td width="780" valign="top" colspan="3"><p align="center">&nbsp;</p>     <p align="center"><a href="../../info.html#copyright"><font face="Arial" size="1">Copyright</font></a><font     face="Arial" size="1" color="#535353"> Info Science 1998-2003 - Tous droits rservs<br>     Usage strictement personnel. 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