The quadratic-form identity for constructing Hamiltonian structures of the NLS–MKdV hierarchy and multi-component Levi hierarchy

The trace identity is extended to the quadratic-form identity. The Hamiltonian structures of the NLS–MKdV hierarchy, and integrable coupling of multi-component Levi hierarchy are obtained by the quadratic-form identity. The method can be used to produce the Hamiltonian structures of the other integrable couplings or multi-component hierarchies.