Integrable systems [electronic resource] : twistors, loop groups, and Riemann surfaces : based on lectures given at a conference on integrable systems organized by N.M.J. Woodhouse and held at the Mathematical Institute, University of Oxford, in September 1997 / N.J. Hitchin, Savilian, professor of Geometry, University of Oxford, G.B. Segal, Lowndean, professor of Astronomy and Geometry, University of Cambridge, R.S. Ward, professor of Mathematics, University of Durham.