This book includes a self-contained approach of the general theory of quadratic forms and integral Euclidean lattices, as well as a presentation of the theory of automorphic forms and Langlands' conjectures, ranging from the first definitions to the recent and deep classification results due to James Arthur. Its connecting thread is a question about lattices of rank 24: the problem of p-neighborhoods between Niemeier lattices. This question, whose expression is quite elementary, is in fact very natural from the automorphic point of view, and turns out to be surprisingly intriguing. We explain how the new advances in the Langlands program mentioned above pave the way for a solution. This study proves to be very rich, leading us to classical themes such as theta series, Siegel modular forms, the triality principle, L-functions and congruences between Galois representations. This monograph is intended for any mathematician with an interest in Euclidean lattices, automorphic forms or number theory. A large part of it is meant to be accessible to non-specialists.

La thA(c)orie classique des suites de Sturm fournit un algorithme pour dA(c)terminer le nombre de racines da (TM)un polynAme A coefficients rA(c)els contenues dans un intervalle donnA(c). La (TM)objet principal de ce mA(c)moire est de montrer qua (TM)une gA(c)nA(c)ralisation adA(c)quate de la thA(c)orie des suites de Sturm fournit entre autres choses:

• une notion da (TM)indice de Maslov pour un lacet algA(c)brique de lagrangiens dA(c)fini sur un anneau commutatif;
• une dA(c)monstration du thA(c)orA]me fondamental de la K-thA(c)orie (algA(c)brique) hermitienne, thA(c)orA]me dA A M. Karoubi;
• une dA(c)monstration des thA(c)orA]mes de pA(c)riodicitA(c) de Bott (topologique), dans la (TM)esprit des travaux de F. Latour;
• un calcul du groupe K2 relatif, symplectique-linA(c)aire, pour tous les anneaux commutatifs, dans la (TM)esprit des travaux de R. Sharpe.

Le livre est dans la mesure du possible A self-contained A et A(c)lA(c)mentaire: il met essentiellement en oeuvre des arguments da (TM)algA]bre linA(c)aire ou bilinA(c)aire. Il prA(c)sente une approche unifiA(c)e de la (TM)indice de Maslov en termes de suites de Sturm et de formes quadratiques.