  In this chapter we present a general method of constructing the effective
equation which describes the behaviour of small-amplitude solutions for a
nonlinear PDE in finite volume, provided that the linear part of the equation
is a hamiltonian system with a pure imaginary discrete spectrum. The effective
equation is obtained by retaining only the resonant terms of the nonlinearity
(which may be hamiltonian, or may be not); the assertion that it describes the
limiting behaviour of small-amplitude solutions is a rigorous mathematical
theorem. In particular, the method applies to the three-- and four--wave
systems. We demonstrate that different possible types of energy transport are
covered by this method, depending on whether the set of resonances splits into
finite clusters (this happens, e.g. in case of the Charney-Hasegawa-Mima
equation), or is connected (this happens, e.g. in the case of the NLS equation
if the space-dimension is at least two). For equations of the first type the
energy transition to high frequencies does not hold, while for equations of the
second type it may take place. In the case of the NLS equation we use next some
heuristic approximation from the arsenal of wave turbulence to show that under
the iterated limit "the volume goes to infinity", taken after the limit "the
amplitude of oscillations goes to zero", the energy spectrum of solutions for
the effective equation is described by a Zakharov-type kinetic equation.
Evoking the Zakharov ansatz we show that stationary in time and homogeneous in
space solutions for the latter equation have a power law form. Our method
applies to various weakly nonlinear wave systems, appearing in plasma,
meteorology and oceanology.
