Discrete Fourier transform - Norms of complex input signals and their transformation
I may consider them as L^2 then following theorem holds.

https://en.wikipedia.org/wiki/Plancherel_theorem

https://de.wikipedia.org/wiki/Satz_von_Plancherel

https://ms.mcmaster.ca/craig/craig-pde-chap5.pdf

Also keen to learn if there is any other theorems.