[ E(x) = e^{i k x} \sum_{n=-\infty}^{\infty} E_n , e^{i n K x} ]
Let’s explore how engineers and physicists use Fourier series to model and solve real-world discontinuous periodic systems. Consider a perfect square wave—a signal that jumps instantly between +1 and -1. This is the poster child for discontinuity. Its Fourier series is: [ E(x) = e^{i k x} \sum_{n=-\infty}^{\infty} E_n
[ f(x) = \frac{4}{\pi} \sum_{n=1,3,5,\ldots} \frac{\sin(nx)}{n} ] e^{i m K x}
[ \varepsilon(x) = \sum_{m=-\infty}^{\infty} \varepsilon_m , e^{i m K x}, \quad K = \frac{2\pi}{a} ] step-index optical fibers
The surprising answer is that when analyzing physical structures with abrupt changes—think square waves, step-index optical fibers, digital signals, or phononic crystals.