Math Problem Statement
Example 4.4. Find the Fourier series to represent the function f(x)=x - x ^ 2 ln| interval -< x < π. Hence show that pi^ 2 6 = 1 + 1/(2 ^ 2) + 1/(3 ^ 2) + 1/(4 ^ 2) +...
Solution
Let's break down this problem into two parts: first, finding the Fourier series for the given function, and second, deriving the famous series for .
Part 1: Fourier Series Representation
Given the function on the interval , we need to find the Fourier series.
Step 1: Define the Fourier series
The Fourier series of a function on the interval is given by: where the coefficients and are given by:
Step 2: Set up the integrals
We'll need to calculate the Fourier coefficients for .
For now, let's focus on to work through the basic form and simplify the series. (Note that full calculation for would involve integration by parts.)
-
because is an odd function, and integrating over gives zero.
-
.
Using integration by parts for this integral:
- , because is an odd function, and the integral of an odd function over a symmetric interval is zero.
Thus, the Fourier series for is:
Part 2: Deriving
To derive the value , we consider the Fourier series for and evaluate it at a particular point. Specifically, we use Parseval's theorem, which states that the sum of the squares of the Fourier coefficients equals the mean square value of the function over the interval.
For the function , Parseval's theorem leads to the following identity: This is the famous Basel problem, solved by Euler in the 18th century.
Conclusion
- The Fourier series for on the interval is:
- Using Parseval's theorem, we can show that:
Would you like more detailed steps on the integral calculations or further clarification on how the series expansion works?
Here are five related questions to expand on this topic:
- How do you find the Fourier series of a function over a different interval (e.g., )?
- What is the significance of Parseval's theorem in Fourier analysis?
- Can the Fourier series be used to approximate non-periodic functions?
- How does the convergence of Fourier series behave for discontinuous functions?
- How would you compute the Fourier series for higher-order polynomial functions?
Tip: When dealing with Fourier series, always check the symmetry of the function (even/odd) to simplify the calculations of coefficients.
Ask a new question for Free
By Image
Drop file here or Click Here to upload
Math Problem Analysis
Mathematical Concepts
Fourier Series
Integral Calculus
Parseval's Theorem
Formulas
Fourier series: f(x) = a0/2 + ∑(an cos(nx) + bn sin(nx))
Fourier coefficients: a0 = (1/π) ∫ f(x) dx, an = (1/π) ∫ f(x) cos(nx) dx, bn = (1/π) ∫ f(x) sin(nx) dx
Parseval's theorem: ∑(1/n^2) = π^2 / 6
Theorems
Parseval's Theorem
Basel Problem Solution
Suitable Grade Level
Undergraduate Mathematics
Related Recommendation
Fourier Series Expansion of f(x) = x + x^2 and Basel Problem Deduction
Fourier Series Representation of x^2 and Basel Problem Deduction
Fourier Series of x^2 and Deduction of Series Representation
Fourier Series for f(x) = x^2 and Series Deduction
Fourier Series for Piecewise Function and Deduction of Pi^2/8 Series