A survey of different representations for lattice sums for the Helmholtz equation is made. These sums arise naturally when dealing with wave scattering by periodic structures. One of the main objectives is to show how the various forms depend on the dimension d of the underlying space and the lattice dimension d Λ . Lattice sums are related to, and can be calculated from, the quasi-periodic Green's function and this object serves as the starting point of the analysis.
LATTICE SUMS FOR THE HELMHOLTZ EQUATION 631to mathematically challenging problems. Attempts to evaluate the sum in (1.1) in closed form have so far been unsuccessful, though some remarkable identities have been derived in the process [23,113].The lattice sum in (1.1) arises from a sum of singular solutions of Laplace's equation. Here we will be concerned with related sums in which the underlying physics is governed by the Helmholtz equationwhich arises naturally in many contexts, particularly when investigating linear wave phenomena at a given frequency. For example, in acoustics u represents fluctuations in pressure, whereas in elasticity it would be some component of the displacement vector, and in electromagnetism a component of the electric or magnetic field. The quantity k (assumed real and positive) is the wavenumber, related to the frequency ω via some appropriate dispersion relation. In diffraction theory we are often faced with trying to solve (1.2) in a region exterior to some scatterer(s) (and maybe inside as well) subject to boundary conditions on the surface of the scatterer(s). The case where the scatterer is periodic represents an important class of such problems; examples include the study of diffraction gratings, scattering by periodic surfaces, and wave propagation through composite materials. Equation (1.2) is also equivalent to the time-independent Schrödinger equation in the presence of a constant potential, in which case k is related to the total energy of the particle under consideration, and as a result the Helmholtz equation finds application in the study of electron scattering in solids. A common simplification used is the so-called muffin-tin approximation (due to Slater [99]), in which the potential is supposed to be spherically symmetric within spheres surrounding each atom and constant in the region inbetween these spheres. This then leads naturally to the need to solve (1.2) on a periodic domain.The study of wave propagation in periodic structures has a long history and leads to a whole range of interesting mathematical problems. The classic text by Brillouin [11] arguably still serves as the best introduction to the subject, though many significant advances have been made since it was written. For example, the mathematical theory of diffraction gratings (based on functional analysis) is developed in books such as [120] and there is an ever-growing literature describing the rapidly developing field of photonic and phononic crystals; an excellent text is [39]. One of the consequences of periodicity is that a problem on...