# Need help plotting Fourier series in matlab

1. Sep 24, 2013

### BigDave11

Hi all my first post as I need to seek help!

I have just learned some simple fourier series stuff and would like to be able to plot my answers in matlab.

Assuming this is correct I was wondering if someone would be able to walk me through plotting this equation in Matlab?

$$y=\sum_{n=1}^\infty (3sin(2∏nx))/(n∏)$$

2. Sep 25, 2013

### kreil

Here is some quick and dirty code to plot this over the domain $-2\pi \le x \le 2\pi$ for n up to 10,000

Code (Text):

%Use linspace() for the domain, it generates 100 evenly spaced points over
%a specified interval
x = linspace(-2*pi,2*pi);

%Define your function anonymously with two variables, X and N
y = @(X,N) (3./N*pi)*sin(2*pi*N*X);

%Loop through the n values and evaluate the function over the domain x for
%each value of n. Store those values in the rows of Y such that the first
%row of Y is n=1, the second is n=2, etc...
for n = 1:10000
Y(n,:) = y(x,n);
end

%Plot all rows of Y
figure;
plot(x,Y)

%Plot the summation over n
figure(2);
plot(x,sum(Y))

See the plots in the attached files.

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3. Oct 2, 2013

### BigDave11

Maybe it is the linespace that is the problem is there a way to evenly space more than 100 points in the domain?

edit: no come to think of it I don't think this will help do you know why this doesn't resemble a Fourier series?

Last edited: Oct 2, 2013
4. Oct 2, 2013

### Dr Transport

linspace(-2*pi, 2*pi, N) gives N evenly spaced points.

5. Oct 3, 2013

### BigDave11

Wow just realised the equation must be wrong as it tends to 0 as n tends to infinity which should not happen.

6. Oct 3, 2013

### kreil

So where did you derive the equation from? It sounds like you need to start over.

7. Oct 3, 2013

### BigDave11

yeh it was a simple example used to teach me about Fourier series. I thought would be fun to see how the series works out but I fear I may have done it wrong

f(x)=3 ; 0<x<1
=-3 ; -1<x<0

Sorry about the formatting still new to me