The Continuous-Time Fourier Series (CTFS), often called just Fourier Series (FS), tells us that we can write a signal as a linear combination of orthogonal functions, or bases. In general, we can use any bases of orthogonal functions, but we will mainly use complex exponentials. 

\begin{align*} 
x(t) &= \sum \limits_{k=-\infty}^{\infty} X_k e^{ik\omega_0 t} \\
x(t) &= \sum \limits_{k=-\infty}^{\infty} X_k \Psi_k(t) \\
\end{align*}

This brings us to our synthesis equation:
$$
x = \sum \limits_{k=-\infty}^{\infty}X_k\Psi_k
$$
We use the inner product to find the spectral coefficient $X_k$, which brings us to our analysis equation:

$$
X_k = \frac{\langle x,\Psi_k\rangle}{\langle \Psi_k,\Psi_k\rangle}
$$

But how do we define our inner product? In discrete-time, we used

\begin{nicebox}
$$
\langle f,g\rangle = \sum \limits_{n=\langle p\rangle} f(n)g^*(n) \quad \mbox{(Discrete-time inner product)}
$$
\end{nicebox}

We can simplify this using linear algebraic language. 

\begin{align*}
f &= \left[ \begin{array}{c} f(0) \\ f(1) \\ \vdots \\ f(p-1) \\ \end{array} \right] \\
g &= \left[ \begin{array}{c} g(0) \\ g(1) \\ \vdots \\ g(p-1) \\ \end{array} \right] 
\end{align*}

Therefore, we can write $\langle f,g \rangle$ as multiplication of a tranposed vector with another vector:

$$ \langle f,g\rangle = f^Tg^* $$

But now that we are in continuous-time, we need more. We have a continuum of values, not a discrete, countable set. Hence, we define the inner product as an integral:

\begin{nicebox}
$$
\langle f,g\rangle = \int_{\langle p\rangle} f(t)g^*(t) dt \quad \mbox{(Continuous-time inner product)}
$$
\end{nicebox}

Now lets look at $\langle \Psi_k, \Psi_\ell\rangle$:
\begin{align*}
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} \Psi_k(t)\Psi_\ell^*(t) dt \\
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} e^{ik\omega_0 t}e^{-i\ell \omega_0 t} dt \\
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} e^{i(k-\ell)\omega_0 t} dt \\
\end{align*}

We have two cases, where the exponent is zero when $k=\ell$, and then when it is non-zero and $k\neq \ell$.


I) $k=\ell$


This one is more simple. We are integrating 1 over a contiguous interval of length $p$, hence the value is $p$:

\begin{align*}
\langle \Psi_\ell, \Psi_\ell\rangle &= \int_{\langle p\rangle} dt = p
\end{align*}


II) $k\neq\ell$


This case requires a bit more work. Lets integrate.

\begin{align*}
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} e^{i(k-\ell)\omega_0 t} dt \\
\langle\Psi_k, \Psi_\ell\rangle &= \left.\frac{e^{i(k-\ell)\omega_0 t}}{i(k-\ell)\omega_0} \right|_{\langle p\rangle} \\
\langle\Psi_k, \Psi_\ell\rangle &= \frac{e^{i(k-\ell)\omega_0 p}- e^{0}}{i(k-\ell)\omega_0} \\
\langle\Psi_k, \Psi_\ell\rangle &= \frac{e^{i(k-\ell)2\pi}- e^{0}}{i(k-\ell)\omega_0} \\
\langle\Psi_k, \Psi_\ell\rangle &= \frac{0}{i(k-\ell)\omega_0} = 0\\
\end{align*}

Lets look at this in another perspective and change to sines and cosines instead of integrating:
\begin{align*}
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} e^{i(k-\ell)\omega_0 t} dt \\
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} \cos((k-\ell)\omega_0 t) + i\sin((k-\ell)\omega_0 t) dt \\
\langle\Psi_k, \Psi_\ell\rangle &= \int_{\langle p\rangle} \cos((k-\ell)\omega_0 t) dt + \int_{\langle p\rangle}i\sin((k-\ell)\omega_0 t) dt \\
\end{align*}
Consider the case where $k-\ell=1$. We are summing over a full period of the cosine function:

% \img{images/trigonometric/cos1/cos.ps}

Notice that there is an equal amount of positive and negative area, hence the value is zero. If $k-\ell=2$, then we are integrating $\cos(2\omega_0 t)$, which has a period $p/2$, hence we would integrate over two full periods of the cosine function giving us zero:

% \img{images/trigonometric/cos2/cos.ps}

If you were to let $k-\ell=N$, where $N\in \Z$, then you would be integrating over $N$ periods since the function would have a period of $p/N$. This same phenomenon occurs with the sine function.

% \img{images/trigonometric/sin1/sin.ps}

Therefore, any time we have $k-\ell \neq 0$, we are integrating over integer multiples of the period, hence

$$\langle \Psi_k, \Psi_\ell\rangle = p\delta(k-\ell)$$

Looking back at the analysis equation, we have

$$ X_k = \frac{\langle x,\Psi_k\rangle}{\langle\Psi_k, \Psi_k\rangle} = \frac{1}{p} \int_{\langle p\rangle}x(t)e^{-ik\omega_0 t} dt $$

Note that since $\omega_0$ and $k$ are both required, the equation relies on them, and sometimes you may see $X(k\omega_0)$.

To sum everything up, we have two important equations regarding the continuous-time fourier series (FS):

\begin{nicebox}
\begin{align*}
x(t) &= \sum \limits_{k=-\infty}^{\infty}X_ke^{ik\omega_0t} \quad &\mbox{(synthesis equation)} \\
X_k &= \frac{1}{p} \int_{\langle p\rangle}x(t)e^{-ik\omega_0t}dt \quad &\mbox{(analysis equation)}
\end{align*}
\end{nicebox}

Because we can write $x(t)$ as an infinite number of $X_k$s, $X_k$ is not necessarily periodic, but $x(t)$ is $p$-periodic in continuous-time. Note that we can write $x(t)$ using any orthogonal set of basis vectors:

\begin{align*}
x(t) &= \sum \limits_{k=-\infty}^{\infty}X_k\Psi_k(t) \\
x(t) &= \sum \limits_{k=0}^{\infty}A_k \cos(k\omega_0 t) + \sum \limits_{k=1}^{\infty}B_k \sin(k\omega_0 t)
\end{align*}

Consider the following bases:
\begin{align*}
\Psi_k(t) &= \cos(\omega_0 t) \quad k\in\Z_{\oplus} \\
\Phi_\ell(t) &= \sin(\omega_0 t) \quad \ell \in \Z_+ 
\end{align*}
In order to use these as bases, we need to show the following:

\begin{align*}
\langle \Psi_k, \Psi_m\rangle &= 0 \quad \mbox{if } k\neq m \\
\langle \Psi_k, \Phi_\ell\rangle &= 0 \quad \forall\mbox{ } k,\ell \\
\langle \Phi_\ell, \Phi_r\rangle &= 0 \quad \mbox{if } \ell\neq r \\
\end{align*}

Then to determine the coefficients, we can use the inner product.

Continuous-time Fourier Series