Integrable systems and metrics of constant curvature

In this article we present a Lagrangian representation for evolutionary systems with a Hamiltonian structure determined by a differential-geometric Poisson bracket of the first order associated with metrics of constant curvature. Kaup-Boussinesq system has three local Hamiltonian structures and one nonlocal Hamiltonian structure associated with metric of constant curvature. Darboux theorem (reducing Hamiltonian structures to canonical form "d/dx" by differential substitutions and reciprocal transformations) for these Hamiltonian structures is proved. Copyright © 2002 by M Pavlov.