The KdV equation on the half-line: The Dirichlet to Neumann map

We consider initial-boundary value problems for the KdV equation $u_t + u_x + 6uu_x + u_{xxx} = 0$onthehalf-line$x \geq 0$. For a well-posed problem, the initial data $u(x,0)$ as well as one of the three boundary values ${u(0,t), u_x(0,t), u_{xx}(0,t)}$ can be prescribed; the other two boundary values remain unknown. We provide a characterization of the unknown boundary values for the Dirichlet as well as the two Neumann problems in terms of a system of nonlinear integral equations. The characterizations are effective in the sense that the integral equations can be solved perturbatively to all orders in a well-defined recursive scheme.