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- A numerical algorithm to reduce ill-conditioning in meshless methods for the Helmholtz equationPublication . Antunes, Pedro R. S.Some meshless methods have been applied to the numerical solution of boundary value problems involving the Helmholtz equation. In this work, we focus on the method of fundamental solutions and the plane waves method. It is well known that these methods can be highly accurate assuming smoothness of the domains and the boundary data. However, the matrices involved are often ill-conditioned and the effect of this ill-conditioning may drastically reduce the accuracy. In this work, we propose a numerical algorithm to reduce the ill-conditioning in both methods. The idea is to perform a suitable change of basis. This allows to obtain new basis functions that span exactly the same space as the original meshless method, but are much better conditioned. In the case of circular domains, this technique allows to obtain errors close to machine precision, with condition numbers of order O(1), independently of the number of basis functions in the expansion.
- Improving the conditioning of the method of fundamental solutions for the Helmholtz equation on domains in polar or elliptic coordinatesPublication . Antunes, Pedro R. S.; Calunga, Hernani; Serranho, PedroA new approach to overcome the ill-conditioning of the Method of Fundamental Solutions (MFS) combining Singular Value Decomposition (SVD) and an adequate change of basis was introduced in [1] as MFS-SVD. The original formulation considered polar coordinates and harmonic polynomials as basis functions and is restricted to the Laplace equation in 2D. In this work, we start by adapting the approach to the Helmholtz equation in 2D and later extending it to elliptic coordinates. As in the Laplace case, the approach in polar coordinates has very good numerical results both in terms of conditioning and accuracy for domains close to a disk but does not perform so well for other domains, such as an eccentric ellipse. We therefore consider the MFS-SVD approach in elliptic coordinates with Mathieu functions as basis functions for the latter. We illustrate the feasibility of the approach by numerical examples in both cases.