We study the question of existence of time-periodic, spatially periodic solutions
for dispersive evolution equations, and in particular, we introduce a framework
for demonstrating the nonexistence of such solutions. We formulate the problem
so that doubly periodic solutions correspond to fixed points of a certain operator.
We prove that this operator is locally contracting, for almost every temporal
period, if the Duhamel integral associated to the evolution exhibits a weak smoothing
property. This implies the nonexistence of nontrivial, small-amplitude time-periodic
solutions for almost every period if the smoothing property holds. This can be viewed
as a partial analogue of scattering for dispersive equations on periodic intervals,
since scattering in free space implies the nonexistence of small coherent structures.
We use a normal form to demonstrate the smoothing property on specific examples,
so that it can be seen that there are indeed equations for which the hypotheses
of the general theorem hold. The nonexistence result is thus established through the
novel combination of small-divisor estimates and dispersive smoothing estimates. The
examples treated include the Korteweg–de Vries equation and the Kawahara equation.
Keywords
doubly periodic solutions, dispersive equations, small
divisors, smoothing