Linear theory represents a first-order approximation
in satisfying the free-surface conditions.
It can be improved by introducing higher-order
terms in a consistent manner–a Stokes expansion.
The next approximation would solve the problem
to second order in the parameter ζa /λ characterizing
the wave amplitude/wavelength ratio
of the linear (first-order) solution. Second-order
theory means that we keep in a consistent way
all terms proportional to O((ζa /λ)2) and O(ζa /λ).
For sinusoidal unidirectional progressive deepwater
waves for which the solution in Table 3.1