Quasi-periodic Green's functions of the Helmholtz and Laplace equations

TL;DR

This paper develops exponentially convergent series representations for quasi-periodic Green's functions of the Helmholtz and Laplace equations, applicable in various dimensions and periodicities, with implications for physics, engineering, and computational mathematics.

Contribution

It introduces new exponentially convergent series for quasi-periodic Green's functions, including the 1D periodicity case in 3D, and derives universal formulas for cylindrical and spherical Hankel functions.

Findings

New series representations for quasi-periodic Green's functions

Universal formulas for Hankel functions of any integer order

Applications in numerical boundary integral methods and wave diffraction

Abstract

A classical problem of free-space Green's function $G_{0\Lambda}$ representations of the Helmholtz equation is studied in various quasi-periodic cases, i.e., when an underlying periodicity is imposed in less dimensions than is the dimension of an embedding space. Exponentially convergent series for the free-space quasi-periodic $G_{0\Lambda}$ and for the expansion coefficients $D_{L}$ of $G_{0\Lambda}$ in the basis of regular (cylindrical in two dimensions and spherical in three dimension (3D)) waves, or lattice sums, are reviewed and new results for the case of a one-dimensional (1D) periodicity in 3D are derived. From a mathematical point of view, a derivation of exponentially convergent representations for Schl\"{o}milch series of cylindrical and spherical Hankel functions of any integer order is accomplished. The quasi-periodic Green's functions of the Laplace equation are obtained from the corresponding representations of $G_{0\Lambda}$ of the Helmholtz equation by taking the limit of the wave vector magnitude going to zero. The derivation of relevant results in the case of a 1D periodicity in 3D highlights the common part which is universally applicable to any of remaining quasi-periodic cases. The results obtained can be useful for numerical solution of boundary integral equations for potential flows in fluid mechanics, remote sensing of periodic surfaces, periodic gratings, in many contexts of simulating systems of charged particles, in molecular dynamics, for the description of quasi-periodic arrays of point interactions in quantum mechanics, and in various ab-initio first-principle multiple-scattering theories for the analysis of diffraction of classical and quantum waves.