This problem is designed to guide you through a proof of Plancherels theorem, by starting with the
Question:
This problem is designed to guide you through a “proof” of Plancherel’s theorem, by starting with the theory of ordinary Fourier series on a finite interval, and allowing that interval to expand to infinity.
(a) Dirichlet’s theorem says that “any” function f(x) on the interval [-α +α] can be expanded as a Fourier series:
Show that this can be written equivalently as
What is cn , in terms of αn and bn ?
(b) Show (by appropriate modification of Fourier’s trick) that
(c) Eliminate n and cn in favor of the new variables k = (nπ/α) and
Show that (a) and (b) now become
where Δk is the increment in k from one n to the next.
(d) Take the limit to obtain Plancherel’s theorem. Comment: In view of their quite different origins, it is surprising (and delightful) that the two formulas—one for F(k) in terms of f(x), the other for f(x) in terms of F(k) —have such a similar structure in the limit
.
Step by Step Answer:
Introduction To Quantum Mechanics
ISBN: 9781107189638
3rd Edition
Authors: David J. Griffiths, Darrell F. Schroeter