How Do You Calculate the Fourier Series of a Piecewise Function?

In summary, the function is an even function of x, and the harmonics are cosines with an argument of nx.
  • #1
agata78
139
0

Homework Statement



Determine the Fourier series for the periodic function of period 2∏ defined by:



-2 when (-∏ ) ∠ x ∠ (-∏/2)
f(x)= 2 when ( -∏/2) ∠ x ∠ (∏ /2)
-2 when (∏/2) ∠ x ∠ (∏)

how to start i?. I have already drawn it but what next.
thank you

Homework Equations





The Attempt at a Solution

 
Physics news on Phys.org
  • #2
Agata 78 said:

Homework Statement



Determine the Fourier series for the periodic function of period 2∏ defined by:



-2 when (-∏ ) ∠ x ∠ (-∏/2)
f(x)= 2 when ( -∏/2) ∠ x ∠ (∏ /2)
-2 when (∏/2) ∠ x ∠ (∏)

how to start i?. I have already drawn it but what next.
thank you

Homework Equations





The Attempt at a Solution

The function runs from -π to +π. As yourself: is the function even in x or odd in x? If it is odd in x, then you only need to include the sin terms in the Fourier Series. if it is odd in x, then you only need to include the constant term and the cosine terms in the Fourier Series. What is the argument of sin or cosine going to be if the function runs from -π to +π?
 
  • #3
yes, the function runs from -∏ to ∏. its from (-∏ to ∏/2) ( -∏/2 to ∏/2) and ( ∏/2 to ∏). There is no period in it, it doesn't repeat. Than it has to be even in. even in is cosinus than yes?
 
  • #4
dont know what you mean by argument?
 
  • #5
Agata 78 said:
yes, the function runs from -∏ to ∏. its from (-∏ to ∏/2) ( -∏/2 to ∏/2) and ( ∏/2 to ∏). There is no period in it, it doesn't repeat. Than it has to be even in. even in is cosinus than yes?

Oh yes it does repeat. That's what 'periodic' means.
 
  • #6
Agata 78 said:
dont know what you mean by argument?

f(x) the argument is x.
 
  • #7
Agata 78 said:
yes, the function runs from -∏ to ∏. its from (-∏ to ∏/2) ( -∏/2 to ∏/2) and ( ∏/2 to ∏). There is no period in it, it doesn't repeat. Than it has to be even in. even in is cosinus than yes?

Yes, it's an even function in the argument x.
 
  • #8
Agata 78 said:
dont know what you mean by argument?
I was thinking that the terms in the Fourier Series should be Ancos(nx) where x runs from -π to +π, and n is the sequence of integers 1,2,3,... So, the argument is nx.
 
  • #9
Chestermiller said:
I was thinking that the terms in the Fourier Series should be Ancos(nx) where x runs from -π to +π, and n is the sequence of integers 1,2,3,... So, the argument is nx.

You're right, the argument of each of the harmonics is nx, not x.

I wanted to say that his function was an even function of x, x being the argument of f(x).

Sorry Chet!
 
Last edited:
  • #10
rude man said:
You're right, the argument of each of the harmonics is nx, not x.

I wanted to say that his function was an even function of x, x being the argument of f(x).

Sorry Chet!
No apology necessary RM. You're the man!

Chet
 
  • #11
Is this function is odd or even, cause I am confused. When it repeats itself its called odd when its not its even.
This one repeats it self then its odd yes?

And using co- efficient formulae with 2L= 2π, than L= π.
after calculating
[a][/0] =0
[a][/ 1] = 0

dont know how to calculate [/0] ?
 
  • #12
Agata 78 said:
Is this function is odd or even, cause I am confused. When it repeats itself its called odd when its not its even.
This one repeats it self then its odd yes?

And using co- efficient formulae with 2L= 2π, than L= π.
after calculating
[a][/0] =0
[a][/ 1] = 0

dont know how to calculate [/0] ?

No. A function is odd if f(-x)=-f(x), and it is even if f(-x)=+f(x). Your function is even. So, it will be described by a cosine series. Do you have a formula for calculating the coefficients of a Fourier cosine series? If so, please show us, and show us in detail how you applied it to the case of arbitrary n.

Chet
 
  • #13
∞ n∏x n∏x
Sf(x) = a0 +Ʃ [ an cos L + bn sin L ]
n=1

1 ∏
a0= 2∏ ∫ f(x) dx =O
-∏
1 ∏
a1= ∏ ∫ f(x) cos nx dx= 0,
-∏

what happenes if even times even = is it odd? I know when odd times odd = even.

Im sorry i don't have a clue how to use latex, i tried so many times and still doesn't work. Its hard
 
  • #14
Do you know, how to calculate the Fourier coefficients? If so, just calculate them ;-)).
 
  • #15
dont know how to calculate b1
 
  • #16
Agata 78 said:
∞ n∏x n∏x
Sf(x) = a0 +Ʃ [ an cos L + bn sin L ]
n=1

1 ∏
a0= 2∏ ∫ f(x) dx =O
-∏
1 ∏
a1= ∏ ∫ f(x) cos nx dx= 0,
-∏

what happenes if even times even = is it odd? I know when odd times odd = even.

Im sorry i don't have a clue how to use latex, i tried so many times and still doesn't work. Its hard

Your result for a1 is incorrect. The integral in the right hand side is not zero. Also, the value of n in the cosine term should be n=1. Try doing the integral for n =1 again, and please be more careful.
 
  • #17
I don't know how to do that, could you please help me?
 
  • #18
Agata 78 said:
I don't know how to do that, could you please help me?

[tex]\int_{-\pi}^{+\pi}{f(x)\cos{x}dx}=\int_{-\pi}^{-\pi/2}{(-2)\cos{x}dx}+\int_{-\pi/2}^{+\pi/2}{(2)\cos{x}dx}+\int_{\pi/2}^{\pi}{(-2)\cos{x}dx}[/tex]

Are you able to evaluate the integrals on the right hand side of this equation?

Chet
 
  • #19
It looks complicated, wouldn't know where to start!
 
  • #20
Agata 78 said:
It looks complicated, wouldn't know where to start!

Before you can start working with Fourier Series, you will first have to learn how to integrate simple functions.

Chet
 

FAQ: How Do You Calculate the Fourier Series of a Piecewise Function?

What is the Fourier series?

The Fourier series is a mathematical representation of a periodic function as a sum of sinusoidal functions. It is named after the French mathematician Joseph Fourier and is used to analyze and approximate various types of signals and waveforms.

What is the importance of the Fourier series?

The Fourier series is a powerful mathematical tool that is used in various fields such as physics, engineering, and signal processing. It allows us to break down complex functions into simpler components, making it easier to analyze and understand them. It also allows us to approximate any periodic function with a combination of sinusoidal functions.

How do you determine the coefficients of the Fourier series?

The coefficients of the Fourier series can be determined by using the Fourier series formula, which involves integrating the given function with respect to the variable of interest. The coefficients represent the amplitudes and frequencies of the sinusoidal functions that make up the Fourier series.

What is the difference between the Fourier series and the Fourier transform?

The Fourier series is used to represent a periodic function, while the Fourier transform is used to represent a non-periodic function. The Fourier series uses discrete frequencies, while the Fourier transform uses continuous frequencies. Additionally, the Fourier transform is more general and can be applied to a wider range of functions.

What are some real-world applications of the Fourier series?

The Fourier series has numerous applications in various fields, such as audio and image processing, telecommunications, and signal analysis. It is used to compress audio and image files, filter out noise from signals, and analyze the frequency components of a signal. It is also used in the design of electronic circuits and in solving differential equations.

Similar threads

Back
Top