# Lecture 3 | The Fourier Transforms and its Applications | Summary and Q&A

207.2K views
July 3, 2008
by
Stanford
Lecture 3 | The Fourier Transforms and its Applications

## Summary

This video provides an introduction to the analysis of periodic phenomena using Fourier series. It covers how to represent a general periodic function as a sum of simpler periodic functions using complex exponentials and the formula for the coefficients. The video highlights the limitations of representing discontinuous or non-smooth functions using finite sums and introduces the concept of convergence in the mean as a more general and satisfactory approach to analyzing periodic functions.

### Q: What is the first step in analyzing general periodic phenomena using Fourier series?

The first step is to represent a general periodic function as a sum of simpler periodic functions using complex exponentials. This is done by writing the function as a linear combination of complex exponential terms, with coefficients determined by integration.

### Q: How are the coefficients for the complex exponential terms calculated?

The coefficients are calculated using an explicit formula, which involves integrating the function multiplied by a complex exponential term over the period of the function. The integral is evaluated from 0 to 1 and the coefficients are obtained as the result of the integration.

### Q: Why does the sum for the coefficients go from -n to n instead of 1 to n?

The sum goes from -n to n because the complex exponential terms exhibit symmetry. The values of the coefficients for negative frequencies (negative terms in the sum) are related to the values for positive frequencies (positive terms in the sum) through complex conjugation. This symmetry allows for representing the periodic function using a combination of sines and cosines.

### Q: What is the significance of using complex exponentials instead of real sines and cosines?

Using complex exponentials makes the algebraic work in the analysis much easier. The calculations involving complex exponentials are simpler and more efficient compared to using real sines and cosines. Complex exponentials also provide a more general representation, allowing for the analysis of complex systems using simple building blocks.

### Q: Can any periodic function be represented using a finite sum of complex exponentials?

No, a finite sum of complex exponentials can only represent functions that are infinitely smooth. If a function has any form of discontinuity or lack of smoothness, it cannot be accurately represented using a finite sum of complex exponentials. In such cases, infinite sums have to be considered.

### Q: How does the convergence of a Fourier series differ for continuous and smooth functions?

For continuous functions, the Fourier series converges pointwise, meaning that for each point in the interval, the sum of the series approaches the value of the function. However, for smooth functions, the convergence is even stronger, known as uniform convergence. This means the approximation given by the finite sum of terms is uniformly close to the function over the entire interval.

### Q: What happens when a function has a jump discontinuity?

In the case of a jump discontinuity, where the function jumps from one value to another at a specific point, the Fourier series converges at that point. The convergence is to the average value of the jump, which is the midpoint between the two values.

### Q: How is convergence in the mean different from pointwise convergence?

Convergence in the mean, also known as convergence in the average or convergence in energy, provides a more general and satisfactory approach to analyzing periodic functions. It avoids the limitations of pointwise convergence. Convergence in the mean considers the overall behavior of the series and examines the average or energy of the function over the entire interval.

### Q: What assumption is made about the function in convergence in the mean?

The only assumption made about the function for convergence in the mean is that its square is finite over the interval of interest. This assumption is relatively minimal and not overly restrictive.

### Q: Why is convergence in the mean a better approach to analyzing periodic functions?

Convergence in the mean allows for a more general and satisfactory analysis of periodic functions, including those that are discontinuous or non-smooth. It provides a broader framework for understanding the behavior of Fourier series and yields more accurate approximations of the original function. Convergence in the mean is a significant advancement in the field of Fourier analysis.