# Lighthill generalized function book

• rar0308
In summary, this textbook-style proof is very common in mathematics, and it's something that you might not be able to come up with on your own. However, it's something that you can practice and eventually become good at.
rar0308

see the pictures

## The Attempt at a Solution

one.
In the third picture.
3rd line and 4th line. i don't understand why 1-e^(-t^2/n^2) become (1+t^2)/n^2
two.
I'm not familiar with proving equality by showing difference going to zero.
for example, I prove equality by showing that A equal B, B equal C, so A equal C.
What is a good book to learn this way of proof ?

rar0308 said:

## Homework Statement

see the pictures
View attachment 66493View attachment 66494View attachment 66492

## The Attempt at a Solution

one.
In the third picture.
3rd line and 4th line. i don't understand why 1-e^(-t^2/n^2) become (1+t^2)/n^2

It's an inequality. ##1-e^{-t^2/n^2}## doesn't "become" ##\frac{1+t^2}{n^2}##; they're saying that ##1-e^{-t^2/n^2}\leq\frac{1+t^2}{n^2}##, which is provable using elementary calculus (hint: what is the global min of ##g(t)=\frac{1+t^2}{n^2}+e^{-t^2/n^2}-1##?).
two.
I'm not familiar with proving equality by showing difference going to zero.
for example, I prove equality by showing that A equal B, B equal C, so A equal C.
What is a good book to learn this way of proof ?

This method of proof, using ##\lim(f-a)=0## to show that ##\lim f=a## (please pardon the awful notation), is very common in analysis. I wouldn't characterize it as something that is "learned"; you become aware of it (which you apparently have) and then you recognize when it's being used in other proofs and try to use it in your own proofs. I would say that it is comparable to using ##a-b=0## to show that ##a=b##, which is something you probably wouldn't think twice about using (though someday you should ). As long as you're comfortable with the reasoning behind why it works, there's not much else to know about it. Just do it.

Thanks.
What I always feel after seeing a solution of a problem is a regret that why I haven't come up with this idea.
Do you have some way to prevent this regret?

First off, just about any decent proof is obvious once you've seen it. But most of them aren't obvious at all until you've seen it. It's the whole "hindsight is 20/20" phenomenon. So don't feel bad if something that was out of reach suddenly becomes obvious; that's how it works. That's how it's supposed to work. It's completely normal.

This particular proof looks like it came from a textbook. Most textbook authors leave most proofs that are doable by their intended audience as exercises. The fact that this one was done for you is an indication that you maybe aren't supposed to be able to come up with the ideas on your own. Also keep in mind that the proof was most likely originally discovered by someone more experienced (and maybe even more intelligent *gasp*) than you and I, and it probably took them much longer than you think to come up with it.

Also, mathematics, especially at the higher levels, is a discipline that is practiced. You're not likely going to be good at it right off the bat, and no one really expects you to be. Just keep at it. The more work you do, the better you will be. The tricks/techniques only come with experience, and experience is, unfortunately, something that you can't rush. The good news is that you can learn a lot of math in a very short period of time if you just do it.

## 1. What is the "Lighthill generalized function book" about?

The "Lighthill generalized function book" is a book written by Michael James Lighthill, a British mathematician, about generalized functions, also known as distributions. It covers the theory, applications, and properties of these functions, which are used to generalize the concept of a function in mathematics and physics.

## 2. Who is the intended audience for this book?

The book is primarily intended for mathematicians and physicists who are interested in the theory and applications of generalized functions. It is also suitable for graduate students and researchers in these fields.

## 3. What are some key topics covered in the book?

The book covers topics such as the definition and properties of generalized functions, their representation and convolution, applications in differential equations and Fourier analysis, and the historical development of the theory. It also includes exercises and examples to aid in understanding the material.

## 4. How is this book different from other books on generalized functions?

The "Lighthill generalized function book" is considered a classic in the field and is known for its clear and concise presentation of the theory. It also includes a comprehensive historical account of the development of the theory and its applications, making it a valuable resource for researchers and students.

## 5. Is this book suitable for self-study?

While the book is primarily used as a textbook for graduate courses, it is also suitable for self-study. It provides a thorough introduction to the subject and includes exercises and examples for self-assessment. However, a basic understanding of mathematics, including calculus and differential equations, is recommended before attempting to study this book.

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