Bretherton equation


In mathematics, the Bretherton equation is a nonlinear partial differential equation introduced by Francis Bretherton in 1964:
with integer and While and denote partial derivatives of the scalar field
The original equation studied by Bretherton has quadratic nonlinearity, Nayfeh treats the case with two different methods: Whitham's averaged Lagrangian method and the method of multiple scales.
The Bretherton equation is a model equation for studying weakly-nonlinear wave dispersion. It has been used to study the interaction of harmonics by nonlinear resonance. Bretherton obtained analytic solutions in terms of Jacobi elliptic functions.

Variational formulations

The Bretherton equation derives from the Lagrangian density:
through the Euler–Lagrange equation:
The equation can also be formulated as a Hamiltonian system:
in terms of functional derivatives involving the Hamiltonian
with the Hamiltonian density – consequently The Hamiltonian is the total energy of the system, and is conserved over time.