Covariant symplectic structure of the complex Monge-Amp\`ere equation
  The complex Monge-Amp\`ere equation admits covariant bi-symplectic structure for complex dimension 3, or higher. The first symplectic 2-form is obtained from a new variational formulation of complex Monge- Amp\`ere equation in the framework of the covariant Witten-Zuckerman approach to symplectic structure. We base our considerations on a reformulation of the Witten-Zuckerman theory in terms of holomorphic differential forms. The first closed and conserved Witten-Zuckerman symplectic 2-form for the complex Monge-Amp\`ere equation is obtained in arbitrary dimension and for all cases elliptic, hyperbolic and homogeneous.   The connection of the complex Monge-Amp\`ere equation with Ricci-flat K\"ahler geometry suggests the use of the Hilbert action. However, we point out that Hilbert's Lagrangian is a divergence for K\"ahler metrics. Nevertheless, using the surface terms in the Hilbert Lagrangian we obtain the second Witten-Zuckerman symplectic 2-form for complex dimension>2. 
Y. Nutku