Sommerfeld identity
# Sommerfeld identity

The Sommerfeld identity is a mathematical identity, due Arnold Sommerfeld, used in the theory of propagation of waves,

    ${\frac {e^{ikR}}{R}}=\int \limits _{0}^{\infty }I_{0}(\lambda r)e^{-\mu \left|z\right|}{\frac {\lambda d\lambda }{\mu }}$

where

    $\mu ={\sqrt {\lambda ^{2}-k^{2}}}$

is to be taken with positive real part, to ensure the convergence of the integral and its vanishing in the limit $z\rightarrow \pm \infty$ and

    $R^{2}=r^{2}+z^{2}$.

Here, $R$ is the distance from the origin while $r$ is the distance from the central axis of a cylinder as in the $(r,\phi ,z)$ cylindrical coordinate system. Here the notation for Bessel functions follows the German convention, to be consistent with the original notation used by Sommerfeld. The function $I_{0}(z)$ is the zeroth-order Bessel function of the first kind, better known by the notation $I_{0}(z)=J_{0}(iz)$ in English literature. This identity is known as the Sommerfeld identity.

In alternative notation, the Sommerfeld identity can be more easily seen as an expansion of a spherical wave in terms of cylindrically-symmetric waves:

    ${\frac {e^{ik_{0}r}}{r}}=i\int \limits _{0}^{\infty }{dk_{\rho }{\frac {k_{\rho }}{k_{z}}}J_{0}(k_{\rho }\rho )e^{ik_{z}\left|z\right|}}$

Where

    $k_{z}=(k_{0}^{2}-k_{\rho }^{2})^{1/2}$

The notation used here is different form that above: $r$ is now the distance from the origin and $\rho$ is the radial distance in a cylindrical coordinate system defined as $(\rho ,\phi ,z)$. The physical interpretation is that a spherical wave can be expanded into a summation of cylindrical waves in $\rho$ direction, multiplied by a two-sided plane wave in the $z$ direction; see the Jacobi-Anger expansion. The summation has to be taken over all the wavenumbers $k_{\rho }$.

The Sommerfeld identity is closely related to the two-dimensional Fourier transform with cylindrical symmetry, i.e., the Hankel transform. It is found by transforming the spherical wave along the in-plane coordinates ($x$,$y$, or $\rho$, $\phi$) but not transforming along the height coordinate $z$.

