How Did the Author Determine the Fourier Sine Series for x^2?

In summary, the author recognized that the expression given in the second to last paragraph must be the Fourier sine series for x^2. To verify this, one can find the Fourier sine series of x^2. However, this may not be easily recognizable for those who are just learning about Fourier series. The given expression is actually the correct sine series for f(x) = x(pi-x).
  • #1
iScience
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I'm having trouble understanding a part in my book.

second to last paragraph where it says 4.2 must be the Fourier sine series for x^2, how did the author arrive at that?


http://i.imgur.com/gLLUYXw.jpg
 
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  • #2
The author was familiar enough with Fourier series to recognize the expression arrived at is the sine expansion of ##x^2##. To verify, simply find the Fourier sine series of ##x^2##.

There is no reason why most people would recognize this, especially if you're just learning it, so I wouldn't worry about it too much.
 
  • #3
Something is wrong there. That is not the half range sine for series for ##x^2##. The odd periodic extension of ##x^2## is discontinuous so the coefficients could converge no faster than order of ##\frac 1 n##. It is in fact the correct sine series for ##f(x) = x(\pi -x)## as is stated.
 

Related to How Did the Author Determine the Fourier Sine Series for x^2?

1. What is a Fourier sine series?

A Fourier sine series is a mathematical representation of a periodic function using only sine functions. It is a type of Fourier series that is used to approximate functions that are odd or have odd symmetry.

2. How is a Fourier sine series calculated?

The coefficients of a Fourier sine series can be calculated using the Fourier sine transform, which involves integrating the function with respect to x and multiplying it by a sine function with the appropriate frequency. The series itself is then formed by summing these coefficients times their corresponding sine functions.

3. What is the main difference between a Fourier sine series and a Fourier cosine series?

The main difference between the two series lies in the type of functions they are used to approximate. A Fourier sine series is used for odd functions, while a Fourier cosine series is used for even functions. This is because sine functions are odd while cosine functions are even.

4. What are some applications of Fourier sine series?

Fourier sine series are commonly used in the field of signal processing and image analysis to approximate and analyze periodic signals and images. They are also used in physics and engineering for solving differential equations and modeling physical systems.

5. Can a Fourier sine series approximate any function?

No, Fourier sine series can only approximate functions that are odd or have odd symmetry. To approximate a function that is neither odd nor even, a combination of both Fourier sine and cosine series, known as a Fourier series, is needed.

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