Unique continuation problems and stabilised finite element methods - Mihai Nechita

Numerical analysis for partial differential equations PDEs traditionally considers problems that are well-posed in the continuum for example the boundary value problem for Poisson s equation. Computational methods such as the finite element method FEM then discretise the problem and provide numerical solutions. However when a part of the boundary is inaccessible for measurements or no information is given on the boundary at all the continuum problem might be ill-posed and solving it in this case requires regularisation. In this thesis we consider the unique continuation problem with possibly noisy data given in an interior subset of the domain. This is an ill-posed problem also known as data assimilation and is related to the elliptic Cauchy problem. It arises often in inverse problems and control theory. We will focus on two PDEs for which the stability of this problem depends on the physical parameters the Helmholtz and the convection diffusion equations. We first prove conditional stability estimates that are explicit in the wave number and in the P clet number respectively by using Carleman inequalities. Under a geometric convexity assumption we obtain that for the Helmholtz equation the stability constants grow at most linearly in the wave number.