Solving the Helmholtz equation is a standard problem in almost any course in mathematical physics, and a simple way to introduce the notion of Green functions. I solve it here in full detail, explaining every step of the solution. Originally written on February 22, 2005.
I'm sharing this work under the Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license. See the LICENSE file for more information.