Weak-nonlinearity expansion.

Consider weakly nonlinear dispersive waves in a periodic box with a dispersion relation $\omega_k$ which allow three-wave interactions. Example of such systems include surface capillary waves [2,], Rossby waves [9] and internal waves in the ocean [8]. In Fourier space, we have the following Hamiltonian equations,

$$i \, \dot a_l = \epsilon \sum_{m,n=1}^\infty \left( V^l_{mn} a_{m} a_{n}e^{i\omeg... ...ar{V}^{m}_{ln} \bar a_{n} a_{m} e^{-i\omega^m_{ln}t } \, \delta^m_{l+n}\right),$$ (11)

where $a_l=a(k_l)$ is the complex wave amplitude in the interaction representation, $k_l = 2 \pi l /L$ is the wavevector, $L$ is the box side length, $\omega^l_{mn}\equiv\omega_{k_l}-\omega_{k_m}-\omega_{k_m}$, $\omega_l=\omega_{k_l}$ is the wave frequency, $\epsilon \ll 1$ is a formal nonlinearity parameter. Here, the interaction coefficient $V^l_{mn}$ is obviously symmetric with respect to $m$ and $n$ but we do not assume any further symmetries.²

In order to filter out fast oscillations at the wave period, let us seek for the solution at time $T$ such that $2 \pi / \omega \ll T \ll 1/\omega \epsilon^2$. The second condition ensures that $T$ is a lot less than the nonlinear evolution time. Now let us use a perturbation expansion in small $\epsilon$,

$$a_l(T)=a_l^{(0)}+\epsilon a_l^{(1)}+\epsilon^2 a_l^{(2)}.$$ (12)

Substituting this expansion in (11) we get in the zeroth order $a_l^{(0)}(T)=a_l(0)$, i.e. the zeroth order term is time independent. This corresponds to the fact that in the interaction representation, wave amplitudes are constant in the linear approximation. For simplicity, we will write $a^{(0)}_l(0)= a_l$, understanding that a quantity is taken at $T=0$ if its time argument is not mentioned explicitly. The first order is given by

$$a^{(1)}_l (T) = -i \sum_{m,n=1}^\infty \left( V^l_{mn} a_m a_n \D... ...l_{m+n} + 2 \bar{V}^m_{ln}a_m\bar{a}_n \bar\Delta^m_{ln}\delta^m_{l+n} \right),$$ (13)

where $\Delta^l_{mn}=\int_0^T e^{i\omega^l_{mn}t}d t = ({e^{i\omega^l_{mn}T}-1})/{i \omega^l_{mn}}.$ Iterating one more time we get

$$a_l^{(2)} (T) = \sum_{m,n, \mu, \nu}^\infty \left[ 2 V^l_{mn} \left( -V^m_{\mu \n... ...l \nu}_{n \mu},\omega^l_{mn}]\delta^\mu_{m + \nu}\right) \delta^l_{m+n} \right.$$

$$\left. + 2 \bar V^m_{ln} \left(-V^m_{\mu \nu}\bar a_n a_\mu a_\nu... ...{n \nu l},-\omega^m_{l n}] \delta^\mu_{m + \nu} \right) \delta^m_{l+ n} \right.$$

$$\left. + 2 \bar V^m_{ln} \left( \bar V^n_{\mu \nu}a_m \bar a_\mu ... ...u l}_{\nu m}, -\omega^m_{ln}]\delta^\mu_{n + \nu}\right)\delta^m_{l+n} \right],$$

where we introduced $E(x,y)=\int_0^T \Delta(x-y)e^{i y t} d t .$