ABSTRACT
It is known [114] that the Helmholtz equation () https://www.w3.org/1998/Math/MathML"> ( ∇ 2 + k 2 ) E ( x , y , z ) = 0 , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429321610/47b35bc2-617c-4920-b4c9-1c82a672da14/content/eq235.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
