# How to check Fourier series solution (complex)

## Homework Statement

Find the complex Fourier series for:

$$f(t)=t(1-t), 0<t<1$$

## Homework Equations

$$f(t)=\sum_{n=-\infty}^{\infty}c_n{e^{i\omega_n{t}}}$$

$$c_n=\frac{1}{\tau}\int_{t_0}^{t_0+\tau}e^{-i\omega_n{t}}f(t)dt$$

$$\omega_n=2\pi{n}\quad\tau=1$$

## The Attempt at a Solution

I solved for c_n. I want to check my answer. I can only think of checking it by graphing it out to a few (50 or so) terms. I tried to graph this in Maple with my value for c_n and it couldn't do it. After that, I tried to solve the entire problem in Maple and that also did not work.

I have a few more of these to do, and I'd like to make sure I am doing this correctly before I move on. Does anyone know how to check my value for the coefficient?

gabbagabbahey
Homework Helper
Gold Member
Do your coefficients depend on your choice of $t_0$? Because the relevant equations you posted are not correct.

I found the coefficients, c_n, by integrating:

$$c_n=\int_{0}^{1}t(1-t)e^{-i2\pi{n}t}dt$$

Are you saying that this is not the correct method to find c_n?

gabbagabbahey
Homework Helper
Gold Member
No, it isn't.

The Fourier coefficients are given by

$$c_n=\int_{-\frac{1}{2}}^{\frac{1}{2}}f(2\pi t)e^{-i\omega_n t}dt=\frac{1}{2\pi}\int_{-\pi}^{\pi}f(t)e^{-int}dt$$

which is not equivalent to what you've posted. Where did you find that incorrect equation for the coefficients?

Thanks for pointing that out. I'll recalculate my coefficient value later today.

My tutorial lists the equation for c_n as:

$$c_n=\frac{1}{\lambda}\int_{x_0}^{x_0+\lambda}e^{-i{k_n}x}f(x)dx$$

In a paragraph above this equation, it states that:

"...But most applications involve either functions of position or of time. In the former case, the period of the function, $$\lambda$$, is more conventionally called the wavelength, and $$k_n=\frac{2\pi{n}}{\lambda}$$ is the wave number for the n'th mode. If time is the variable, however, the period is called, indeed, the period, and is usually represented by $$\tau$$. The n'th mode has the angular frequency, or often simply the frequency, $$\omega_n=\frac{2\pi{n}}{\tau}$$."

It did not explicitly give the equation for c_n in this case. I must have made a mistake converting from a spatial to time variable.

vela
Staff Emeritus
Homework Helper
No, it isn't.

The Fourier coefficients are given by

$$c_n=\int_{-\frac{1}{2}}^{\frac{1}{2}}f(2\pi t)e^{-i\omega_n t}dt=\frac{1}{2\pi}\int_{-\pi}^{\pi}f(t)e^{-int}dt$$

which is not equivalent to what you've posted. Where did you find that incorrect equation for the coefficients?
Those expressions are for a function with period 2π. If you rescale for a function with period τ and allow for a shift (since it really doesn't matter which particular cycle you integrate over), you get PhysicsMark's integral.

Those expressions are for a function with period 2π. If you rescale for a function with period τ and allow for a shift (since it really doesn't matter which particular cycle you integrate over), you get PhysicsMark's integral.

Thanks for the clarification, Vela. I spoke with my professor today, and he also said the original integral should be correct.

He went on to say that I should be able to plot it in Maple. So, I guess that means I need some more practice in Maple (That should be no surprise to Vela...https://www.physicsforums.com/showthread.php?t=391887).

Thanks to Vela and Gabbagabbahey for replying.

vela
Staff Emeritus
Homework Helper
In case you want to check your answer, here's what I found for cn for $n\ne 0$:
$$c_n=-\frac{1}{2n^2\pi^2}$$