# Relation between “harmonic form” and fourier series?

I am currently prepping for uni having been a few years out of the studying loop (programming as it happens). Anyway, I've been re-reading my A-level notes/exercises and looking through OpenCourseWare stuff and I noticed this assertation in my notes:

Given $a \cos\theta \pm b \sin\theta = c$ then:

$a \sin\theta \pm b\cos\theta = r\sin(\theta\pm\alpha)+c$

$a \cos\theta \pm b\sin\theta = r\cos(\theta\pm\alpha)+c$

Given $r \in \mathbb{N}$, $0 \leq \theta < 2\pi$.

Now, I've heard of fourier series which have a very similar form to these equestions.

So, my question is, is there a relation between the two?

Please bear in mind I know I'm stepping off a cliff and into "unknown unknowns" territory if they are - I have absolutely no idea what harmonic analysis is and I don't (yet) understand fourier series fully, although I grasp roughly how they work. I'm just interested to know if they are related and of course any further information / direction / reading I should follow up. I ask because this was one of those "odd topics" at A-level that we derived in class from a geometric argument then I've never seen again.

Thanks.

-
I'd only comment that you have to read up on de Moivre's formula $(\cos(t)+i\sin(t))^n=\cos(nt)+i\sin(nt)$ and Euler's formula $\exp(it)=\cos(t)+i\sin(t)$. – J. M. Sep 2 '10 at 21:59
The assertion is unclear. What are those supposed to mean? – Aryabhata Sep 2 '10 at 22:27
You want to know if there is a relation between Fourier series and what, exactly? – Mariano Suárez-Alvarez Sep 2 '10 at 22:49

I can't really make sense of those equations, but it looks something like the statement that for any $a, b \in \mathbb{R}$, then for $r = \sqrt{a^2 + b^2 }$ and $\phi = \arctan(b/a)$ we have

$a \sin(\theta) + b \cos(\theta) = r \sin(\theta + \phi).$

If you visualize x and y axes with $cos(\theta)$ along the x-axis and $sin(\theta)$ along the y-axis, then this says we can identify any sinusoidal, i.e. any function like $r \sin(\theta + \phi)$, by its cartesian coordinates $(a,b)$ in this plane, or its polar coordinate $(r, \phi )$. $r$ is called the magnitude of the sinusodial, and $\phi$ the phase.

This is directly related to Fourier Analysis. The main idea in Fourier Analysis is to decompose a function into its sinusodial components. For instance, if $f$ is a real-valued function on the interval $[0, 2\pi]$ that is suitably regular, it turns out you can write $f$ uniquely as an infinite sum of sine and cosines, called the Fourier Series:

$f(x) = a_0 + \sum_{n=1}^{\infty} (a_n \cos(nx) + b_n \sin(nx)).$

The numbers $a_n$ and $b_n$ are called the Fourier coefficients of your function.

I think this representation of Fourier series is sometimes difficult to grasp. But using the relationship above, we see we could have also wrote:

$f(x) = a_0 + \sum_{n=1}^{\infty} r_n \sin(nx + \phi_n)$

In this form I think you can see more clearly what's going on: For each $n$, there is only one sinusoidal component of $f$ of frequency $n$; $r_n = \sqrt{a_n^2 +b_n^2}$ tells you its magnitude and $\phi_n$ tells you its phase.

-
You need to enclose latex inside dollar signs like this: $x^n + y^n = z^n$ which looks like $x^n + y^n = z^n$. – Aryabhata Sep 3 '10 at 2:08
@Moron. Thanks for the tip. Should be fixed now. – Greg O. Sep 3 '10 at 2:51
That makes sense. The constraints for $r$ and $\varphi$ don't exist in my notes but the basis of the work was to do the manipulations I assume in preparation for exactly what you've detailed above. Thanks for the answer, +1. – user892 Sep 3 '10 at 19:30