"Spectral theory is connected with the investigation of localized vibrations of a variety of different objects, from atoms and molecules in chemistry to obstacles in acoustic waveguides. These vibrations have frequencies, and the issue is to decide when such localized vibrations occur, and how to go about computing the frequencies. This is a very complicated problem since every object has not only a fundamental tone but also a complicated series of overtones, which vary radically from one body to another." - Wikipedia

And I still don't understand it at all! :D.

Thanks for a difficult puzzle.

Great.

Ah, spectral theory, I learned a little at university :) Really enjoyable, thank you Tehniobium.