= Wave equation
{wiki}
Describes perfect lossless waves on the surface of a string, or on a water surface.
Uniqueness: https://math.stackexchange.com/questions/1113622/uniqueness-of-solutions-to-the-wave-equation
As mentioned at: https://math.stackexchange.com/questions/579453/real-world-application-of-fourier-series/3729366\#3729366[] from <solving partial differential equations with the Fourier series> citing https://courses.maths.ox.ac.uk/node/view_material/1720[], analogously to the <heat equation>, the wave linear equation can be be solved nicely with <separation of variables>.