F. Helein. (2020)cite arxiv:2006.01870Comment: 44 pages, 3 figures. Chapter from the book: " Systemes integrables et theorie des quantique des champs", Paul Baird, Frédéric Hélein, Joseph Kouneiher, Franz Pedit, Volodya Roubtsov, editors, Hermann Travaux en cours N. 75, 2009.