Since the frequencies that the particle emits are the same as the frequencies in the Fourier description of its motion, this suggests that something in the time-dependent description of the particle is oscillating with frequency (E n − E m)/ h. Heisenberg called this quantity X nm, and demanded that it should reduce to the classical Fourier coefficients in the classical limit. For large values of n, m but with n − m relatively small, X nm is the (n − m) th Fourier coefficient of the classical motion at orbit n. Since X nm has opposite frequency to X mn, the condition that X is real becomes X n m = X m n ∗ ∗ .