For the small minority of readers who are not entertained by ancient Jazz clips, here is a quick puzzle. Consider the quantum mechanics of a particle moving in a plane, paramatrized by coordinates x and y, and feeling the effect of the potential , is the spectrum continuous or discrete?
(For a certain generation of string theorists this may bring back memories…)
The answer will come much later, as I am now hopping on a plane. Shame to leave really, looks like it is going to snow sometime soon.