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Dispersion of electromagnetic waves is usually described in terms of an integro-differential equation. In this paper we show that whenever a differential operator can be found that annihilates the susceptibility kernel of the medium, then dispersion can be modeled by a partial differential equation without nonlocal operators.

Let D be a bounded, simply connected domain in the plane R2 that is starlike with respect to the origin and has C2, α boundary, ∂D, described by the equation in polar coordinateswhere C2, α denotes the space of twice Hölder continuously differentiable functions of index α. In this paper, it is shown that any solution of the Helmholtz equationin D can be approximated in the space by an entire Herglotz wave functionwith kernel g ∈ L2[0,2π] having support in an interval [0, η] with η chosen arbitrarily in 0 > η < 2π.

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