 which is the Fourier decomposition of the classical periodic motion of the electron in an allowed stationary state \(n\). For this stationary state \(n\), the fundamental frequency (i.e., periodicity of the electron motion) is \(\omega = n^{\frac{1}{2}}\). Note that there are only the first \((\tau = 1)\) and third \((\tau = 3)\) harmonics present in the classical motion (Equation 6). According to Bohr’s selection rule, this means that there can only be quantum jumps between stationary states that are one or three stationary states apart. So, for example, there can be transitions from the \(n = 100\) stationary state to the \(n = 99\) or \(n = 97\) stationary states; but there cannot be transitions from the \(n = 100\) stationary state to the \(n = 98\) stationary state, because there is no second harmonic in the classical electron orbit.