 In order to understand the substance of the correspondence principle it is helpful to consider a simplified model of the atom as a one-dimensional system, where the electron is undergoing simply periodic motion. Classically the trajectory of the electron is given by \(x(t)\), which is the solution to Newton’s equation of motion, and is periodic, which means it simply retraces its steps over and over again with a frequency, \(\omega\), known as the fundamental frequency. Because the motion is periodic, the position of the electron can be represented by a Fourier series as  follows:[3]