Question - Is someone aware of a published work proving the following: Every bounded, differentiable, periodic function of $n$ variables has a Fourier Series expansion with exactly $m$ terms ($m$ is finite).
I suspect this is true for n = 1, and suspect it is false for n > 1. If this is not already in a textbook somewhere, I'll attempt to (dis)prove it to myself... feels like it is "common knowledge" in a PDE textbook somewhere.
I tried searching through the links in Is there a canonical database of theorems? and there's plenty on series, but this question might be too specific.