One of the most striking development of the last decades in the study of
minimal surfaces, constant mean surfaces and harmonic maps is the
discovery that many classical problems in differential geometry -
including these examples - are actually integrable systems. This theory
grew up mainly after the important discovery of the properties of the
Korteweg-de Vries equation in the sixties. After C. Gardner, J. Greene,
M. Kruskal et R. Miura [44] showed that this equation could be solved
using the inverse scattering method and P. Lax [62] reinterpreted this
method by his famous equation, many other deep observations have been
made during the seventies, mainly by the Russian and the Japanese
schools. In particular this theory was shown to be strongly connected
with methods from algebraic geom- etry (S. Novikov, V. B. Matveev, LM.
Krichever. . . ), loop techniques (M. Adler, B. Kostant, W. W. Symes, M.
J. Ablowitz . . . ) and Grassmannian manifolds in Hilbert spaces (M.
Sato . . . ). Approximatively during the same period, the twist or
theory of R. Penrose, built independentely, was applied successfully by
R. Penrose and R. S. Ward for constructing self-dual Yang-Mills
connections and four-dimensional self-dual manifolds using complex
geometry methods. Then in the eighties it became clear that all these
methods share the same roots and that other instances of integrable
systems should exist, in particular in differential ge- ometry. This led
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