In the context of wave-like equations, an open-boundary condition is one that "lets the wave go through without reflection".

This condition is very useful when we want to simulate infinite domains with a numerical method. Ciro Santilli wants to do this all the time when trying to come up with demos for his physics writings.

Here are some resources that cover such boundary conditions:

- https://www.asc.tuwien.ac.at/~arnold/pdf/graz/graz.pdf lots of slides
- http://hplgit.github.io/wavebc/doc/pub/._wavebc_cyborg002.html mentions them and gives a 1D formula. It mentions that things get complicated in 2D and 3D TODO why.The other page: http://hplgit.github.io/wavebc/doc/pub/._wavebc_cyborg003.html shows solution demos.