The linear BBM-equation on the half-line, revisited

This note is concerned with the linear BBM equation on the half-line. Its nonlinear counterpart originally arose as a model for surface water waves in a channel. This model was later shown to have considerable predictive power in the context of waves generated by a periodically moving wavemaker at one end of a long channel. Theoretical studies followed that dealt with qualitative properties of solutions in the idealized situation of periodic Dirichlet boundary conditions imposed at one end of an infinitely long channel. One notable outcome of these works is the property that solutions become asymptotically periodic as a function of time at any fixed point x in the channel, a property that was suggested by the experimental outcomes. The earlier theory is here generalized using complex-variable methods. The approach is based on the rigorous implementation of the Fokas unified transform method. Exact solutions of the forced linear problem are written in terms of contour integrals and analyzed for more general boundary conditions. For \(\mathcal C^\infty \)-data satifisying a single compatibility condition, global solutions obtain. For Dirichlet and Neumann boundary conditions, asymptotic periodicity still holds. However, for Robin boundary conditions, we find not only that solutions lack asymptotic periodicity, but they in fact display instability, growing in amplitude exponentially in time.