First of all, you should understand the meaning of Fourier series. In physics, we often encounter some periodic movements, such as alternating current and sound waves. Some are actually sine functions or cosine functions, such as alternating voltage, but some are special, such as this function f (x) = (- 1) [n], which is also a periodic function, where [n] is a Gaussian function (integer function). For this function, we want to further study how to do it. In the physical sense, we call it the superposition of waves, so Fourier series is produced. So for functions similar to trigonometric functions, we don't use trigonometric series expansion, because they are themselves, but we can also calculate them if we want to expand (reduce power and expand angle, expand angle). But for some special functions, it is more convenient for us to study their properties by Fourier series, which has great application in physics.

We can't learn mathematics, several formulas and theorems. Mathematics is a tool, such as Newton, who is both a mathematician and a physicist. Only by truly understanding the significance of mathematical application can we make good use of this tool.

Let me talk about it-what happens if you superimpose a finite number of trigonometric functions? For example, if the first 100 items of this series are superimposed, the result is still e(x) which is very similar to f(x). Moreover, this German function curve formed by the superposition of finite terms is also a periodic curve ...-You are wrong in this sentence, which shows that you are interested in finite terms and infinite terms.

Infinity is infinity. Many mathematicians have been puzzled in the history of mathematics, and the second mathematical crisis has also occurred. Not much to say here, relevant information can be found.