- #1
Dustinsfl
- 2,281
- 5
Calculate the Fourier series of the function $f$ defined on the interval $[\pi, -\pi]$ by
$$
f(\theta) =
\begin{cases} 1 & \text{if} \ 0\leq\theta\leq\pi\\
-1 & \text{if} \ -\pi < \theta < 0
\end{cases}.
$$
$f$ is periodic with period $2\pi$ and odd since $f$ is symmetric about the origin.
So $f(-\theta) = -f(\theta)$.
Let $f(\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{in\theta}$.
Then $f(-\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{-in\theta} = \cdots + a_{-2}e^{2i\theta} + a_{-1}e^{i\theta} + a_0 + a_{1}e^{-i\theta} + a_{2}e^{-2i\theta}+\cdots$
$-f(\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{-in\theta} = \cdots - a_{-2}e^{-2i\theta} - a_{-1}e^{-i\theta} - a_0 - a_{1}e^{i\theta} - a_{2}e^{2i\theta}-\cdots$
$a_0 = -a_0 = 0$
I have solved many Fourier coefficients but I can't think today.
What do I need to do next?
$$
f(\theta) =
\begin{cases} 1 & \text{if} \ 0\leq\theta\leq\pi\\
-1 & \text{if} \ -\pi < \theta < 0
\end{cases}.
$$
$f$ is periodic with period $2\pi$ and odd since $f$ is symmetric about the origin.
So $f(-\theta) = -f(\theta)$.
Let $f(\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{in\theta}$.
Then $f(-\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{-in\theta} = \cdots + a_{-2}e^{2i\theta} + a_{-1}e^{i\theta} + a_0 + a_{1}e^{-i\theta} + a_{2}e^{-2i\theta}+\cdots$
$-f(\theta) = \sum\limits_{n = -\infty}^{\infty}a_ne^{-in\theta} = \cdots - a_{-2}e^{-2i\theta} - a_{-1}e^{-i\theta} - a_0 - a_{1}e^{i\theta} - a_{2}e^{2i\theta}-\cdots$
$a_0 = -a_0 = 0$
I have solved many Fourier coefficients but I can't think today.
What do I need to do next?