Integrable nonlinear evolution equations on a finite interval

Let q(x, t) satisfy an integrable nonlinear evolution PDE on the interval 0 < x < L, and let the order of the highest x-derivative be n. For a problem to be at least linearly well-posed one must prescribe N boundary conditions at x = 0 and n − N boundary conditions at x = L, where if n is even, N = n/2, and if n is odd, N is either (n − 1)/2 or (n + 1)/2, depending on the sign of ∂n x q. For example, for the sine-Gordon (sG) equation one must prescribe one boundary condition at each end, while for the modified Korteweg-de Vries (mKdV) equations involving qt + qxxx and qt − qxxx one must prescribe one and two boundary conditions, respectively, at x = 0. We will refer to these two mKdV equations as mKdV I and mKdV II, respectively. Here we analyze the Dirichlet problem for the sG equation, as well as typical boundary value problems for the mKdV I and mKdV II equations. We first show that the unknown boundary values at each end (for example, qx(0, t) and qx(L, t) in the case of the Dirichlet problem for the sG equation) can be expressed in terms of the given initial and boundary conditions through a system of four nonlinear ODEs. For the sG and the focusing versions of mKdV I and mKdV II equations, this system has a global solution, while for the defocusing versions of mKdV I and mKdV II equations, the global existence remains open. We then show that q(x, t) can be expressed in terms of the solution of a 2 × 2 matrix Riemann-Hilbert problem formulated in the complex k-plane. This problem has explicit (x, t) dependence in the form of an exponential; for example, for the case of the sG this exponential is exp{i(k− 1/k)x+ i(k+1/k)t}. Furthermore, the relevant jump matrices are explicitly given in terms of the spectral functions {a(k), b(k)}, {A(k), B(k)}, and {A(k),B(k)}, which in turn are defined in terms of the initial conditions, of the boundary values of q and of its x-derivatives at x = 0, and of the boundary values of q and of its x-derivatives at x = L, respectively. This Riemann-Hilbert problem has a global solution.