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On accuracy conditions for the numerical computation of waves. (English) Zbl 0647.65072
The Helmholtz equation $$(\Delta +K^ 2n^ 2)u=f$$ with a variable index of refraction n and a suitable radiation condition at infinity serves as a model for a wide variety of wave propagation problems. Such problems can be solved numerically by first truncating the given unbounded domain, imposing a suitable outgoing radiation condition on an artificial boundary and then solving the resulting problem on the bounded domain by direct discretization (for example, using a finite element method). In practical applications, the mesh size h and the wave number K are not independent but are constrained by the accuracy of the desired computation. It will be shown that the number of points per wavelength, measured by $$(Kh)^{-1}$$, is not sufficient to determine the accuracy of a given discretization. For example, the quantity $$K^ 3h^ 2$$ is shown to determine the accuracy in the $$L^ 2$$ norm for a second-order discretization method applied to several propagation models.

##### MSC:
 65N30 Finite element, Rayleigh-Ritz and Galerkin methods for boundary value problems involving PDEs 35J05 Laplace operator, Helmholtz equation (reduced wave equation), Poisson equation
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