On the Additional Symmetry; Many-Body Problem Related to the KP Hierarchy

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.