The Fourier Series!!

[robgb]

Hi guys, i'm having a bit a trouble helping my daughter with this question on the fourier series approximation:

The fourier series for a real, odd function, f(t) can be written as:
f(t) = [SUM to infinity, n=1, of]: b[subscipt n] sin(nwt)
where f(t=T)=f(t) and w=(2[pie])/T

Prove that b[subscipt n] = 2/T [integral between T/2 & -T/2 of]: f(t)sin(nwt)dt

Sorry about not knowing how to do all the symbols etc, but if you write it out i'm pretty sure it makes sence.

Anyway, if anyone could give me a hand with how to go about proving this, it would be greatly appreciated.

Many thanks, Robert.

[chroot]

Hey Robert,

You have a function of t represented by the series

f(t) = \sum_{n=1}^\infty b_n \sin{n \omega t}

And you'd like to know what the \inline{b_n} are. You're also given the condition \inline{f(t + T) = f(t)}, which just indicates the function's period is \inline{T}.

You're not even required to figure out what the \inline{b_n} are; you're just asked to demonstrate that they are, in fact

b_n = \int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt.

To do this, first note the following fact:

\begin{equation*}
\begin{split}
\int_{-T/2}^{T/2} \sin^2{\left( \frac{2 \pi n t}{T} \right) }\ dt &= \frac{T}{2}
\end{split}
\end{equation*}


for all n, and use it to perform the following steps:

\begin{equation*}
\begin{split}
\int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt &= \int_{-T/2}^{T/2} \left[ \sum_{n=1}^\infty b_n \sin{n \omega t} \right] \sin{n \omega t}\ dt\\
&= \sum_{n=1}^\infty \int_{-T/2}^{T/2} b_n \sin^2{n \omega t}\ dt\\
&= \frac{T}{2} b_n
\end{split}
\end{equation*}


Does this make sense?

- Warren

[robgb]

Thanks mate, yep i essentially understand, just 1 question though:

How did you get from the second line to the third line in the following:

\begin{equation*}
\begin{split}
\int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt &= \int_{-T/2}^{T/2} \left[ \sum_{n=1}^\infty b_n \sin{n \omega t} \right] \sin{n \omega t}\ dt\\
&= \sum_{n=1}^\infty \int_{-T/2}^{T/2} b_n \sin^2{n \omega t}\ dt\\
&= \frac{T}{2} b_n
\end{split}
\end{equation*}


Could you work me though that bit in a little more detail mate?

Thanks a lot,
Robert.

[lethe]

Originally posted by robgb
Thanks mate, yep i essentially understand, just 1 question though:

How did you get from the second line to the third line in the following:

Could you work me though that bit in a little more detail mate?

Thanks a lot,
Robert.
i think chroot pulled a fast one on you. there is a bit more to the trick that you have to realize to make the infinite summation turn into a single term, with one integration.

to see this, first let me remind you of some trigonometric identities:


\cos(a\pm b)=\cos a\cos b \mp\sin a\sin b

so

\begin{align}
&\frac{1}{2}(\cos(a-b)-\cos(a+b))\\
&=-\frac{1}{2}\cos a\cos b+\frac{1}{2}\sin a\sin b\\
& -\frac{1}{2}\cos a\cos b+\frac{1}{2}\sin a\sin b\\
&= \sin a\sin b
\end{align}


we need to use this identity to evaluate some integrals:


\begin{align}
&\int_{-T/2}^{T/2}\sin(\frac{2\pi mt}{T})\sin(\frac{2\pi nt}{T})\ dt=\\
&\frac{1}{2}\int_{-T/2}^{T/2}\cos\frac{2\pi(n-m)t}{T}\ dt-\frac{1}{2}\int_{-T/2}^{T/2}\cos\frac{2\pi(n+m)t}{T}\ dt=\\
&\left\frac{T}{4\pi(n-m)}\sin\frac{2\pi(n-m)t}{T}\right]^{T/2}_{-T/2}\\
&-\left\frac{T}{4\pi(n+m)}\sin\frac{2\pi(n+m)t}{T}\r ight]^{T/2}_{-T/2}\\
&=0
\end{align}

note that this integration is not valid when n=m, since it involves a division by zero. in the n=m case, i have:

\begin{align}
\int_{-T/2}^{T/2}\sin^2\frac{2\pi nt}{T}\! dt\\
=\frac{1}{2}\int_{-T/2}^{T/2}dt-\frac{1}{2}\int_{-T/2}^{T/2}\cos\frac{4\pi nt}{T}dt\\
=T/2
\end{align}


so, now, armed with these integrals, we can do the problem.

i am going to rename the dummy index n, in the infinite summation to m. you will see why.

f(t) = \sum_m^\infty b_m \sin\frac{2\pi mt}{T}


then


\begin{equation*}
\begin{split}
\int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt &= \int_{-T/2}^{T/2} \left[ \sum_m^\infty b_m \sin\frac{2\pi mt}{T}\right] \sin\frac{2\pi nt}{T}dt\\
&= \sum_m^\infty b_m\int_{-T/2}^{T/2} \sin\frac{2\pi nt}{T}\sin\frac{2\pi mt}{T}\ dt
\end{split}
\end{equation*}


this summation contains an infinite number of terms, one for every value of m, but only when m=n is the term nonzero, so i will drop the summation, and change m to n, so i am only considering one integral, and the value of that integral is T/2. this is the only nonzero term of the infinite summation.


\int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt =\frac{T}{2}b_n

or

b_n=\frac{2}{T}\int_{-T/2}^{T/2} f(t) \sin{n \omega t}\ dt


and that s it. any questions?