  Nonlinear integrable equations, such as the KdV equation, the Boussinesq
equation and the KP equation, have the close relation with many-body problem.
The solutions of such equations are the same as the restricted flows of the
classical Calogero model, which is one-dimensional particle system with inverse
square interactions. The KP hierarchy and the Calogero model share the same
structure called ``additional symmetry''. This symmetry plays a crucial role in
this relation.
