Preprint A131/2002
On the Cauchy problem for a nonlocal perturbation of the KdV equation

Borys Alvarez Samaniego

**Keywords: **
Cauchy problem | Hilbert transform | KdV equation.

Let $\H$ denote the Hilbert transform and $\eta \ge 0$. We show
that the initial value problems
$ u_t + u u_x + u_{xxx} +
\eta(\mathcal{H} u_x + \mathcal{H} u_{xxx}) = 0, \,
u(\cdot , 0) = \phi (\cdot)$ and
$ u_t + \frac{1}{2} (u_x)^2 + u_{xxx} +
\eta(\mathcal{H} u_x + \mathcal{H} u_{xxx}) = 0, \,
u(\cdot , 0) = \phi (\cdot)$
are globally well-posed in $H^s(\mathbb{R})$, $s \ge 1$, $\eta>0$. We study
the limiting behavior of the solutions of the first
equation as $\eta$ tends to zero in $H^s(\mathbb{R})$ and $s \ge 2$.
Moreover, we prove a unique
continuation theorem for the first equation in
$\F_{3,3}(\mathbb{R})=H^3(\mathbb{R}) \cap L^2_3(\mathbb{R})$, $\eta >0$,
which implies that the persistence property does not hold.