Linearity and superpositions
13.1 Wave motions in linear systems
We have already on several occasions mentioned the superposition principle for linear systems. It is, indeed, reasonably intuitive that different wave motions may be combined and will propagate without interacting in the linear systems that form most of the cases we meet. The principle is nevertheless so important that we should address it directly.
Pain  pp. 128–132French  pp. 213–216 and 230–234Main  pp. 37–72 and 213–220Feynman  Vol. I, pp. 25-1–25-5
By no means, though, are all physical systems linear: the breaking waves of a wind-torn ocean provide a ...