# Euler's Number?

We all have studied about the Euler's Number $$e$$. We know it is the natural base. We know it is the value to which $$(1 + 1/n)^{n}$$ approaches as $$n$$ tends to infinity. We also know its Taylor expansion.

We learnt these things from our textbooks. But I never found one which explains all the point clearly. These textbooks simply teach these as facts.

I want to know why Euler mentioned this number? By what purpose did he invented (or discovered) it? I was told that its convenient to use $$e$$ as the base. Why this irrational is so convenient (actually, I haven't been convenient) ? How do we know its value and its taylor expansion?

Now when we know that

$e = 1 + 1 + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + ... = 2.7182...$

how did we discovered that

$e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!} + ...$?

Also, how Euler proved that

$e^{ix} = cos\theta + isin\theta$

Note by Avinash Pandey
5 years, 10 months ago

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## Comments

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If you write out the taylor/maclauren series expansion of e^x, cos, and sine, euler's formula is immediate.

As to why e is so famous, i'd say its because of the fact d/dx e^x = e^x, a property which no other function satisfies. This is why taking logarithms in base e are helpful.

In proving that d/dx e^x = e^x, you can apply the power rule on that very same expansion you have written

d/dx e^x = 1 + (2x)/2! + (3x^2)/3! + (4x3)/4! ...

when you cancel of the common factors, you will see that you end up with the same series you had started with, ie e^x

As to why its taylor series expansion is like this, its mainly due to the above fact, that d/dx e^x = e^x Just plug that in in the formula you have for taylor/maclauren series expansion - link

- 5 years, 10 months ago

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I remember being told that Euler created e as a number to satisfy the equation $$\frac{d}{dx}$$e$$^x$$ = e$$^x$$

- 5 years, 10 months ago

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i'm unsure as to how he could 'create e as a number' to me it seems more likely that he was seeing if the limit if 1/(n!) converges or not, and it did, since factorials get large pretty quickly

- 5 years, 9 months ago

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The proof that $$e = 1 + 1 + \frac{1}{2!} +\frac{1}{3!}\frac{1}{4!} + ...$$ is a very beautiful one, which comes from the binomial expansion of $$e = (1 + \frac{1}{N} )^N$$. The process for $$e^x$$ is very similar. However, I don't know where the $$e^{ix} = cosx+isinx$$ comes from.

- 5 years, 10 months ago

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What do you mean by $$e = ( 1 + \frac {1}{N})^N$$? Is $$N=2, 3$$? Otherwise, how do you take the binomial expansion?

Staff - 5 years, 10 months ago

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Although the original problem is solved now (almost), I have new series of questions on the same topic.

i) How did Euler knew (with reference to Harel D.) that an exponential function would be the one whose derivative is the function itself? ii) Aren't there other functions such that $$\frac{d}{dx} f(x) = f(x)$$ ? iii) For what values of x, the expansion of $$e^x$$ (except x=0) is rational? iv) Proof of Taylor's Theorem?

- 5 years, 10 months ago

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At last, after rigorous search on internet and libraries, I found answers of most of them. Actually, we use a theorem called Taylor's Theorem which can be used to represent any function as a series of polynomial terms.

$f(x)= \sum_{k=0}^{\infty} \frac{x^k}{k!} f^{(k)} (0)$

Now using the fact told by Harel D., Euler knew that "zeroth derivative of $$e^x$$ evaluated at zero is one and kth derivative, where k is natural number, of $$e^x$$ is $$e^x$$ itself". Now apply Taylor's Theorem to get the expansion. Now when we know $$e^x$$, then we can evaluate $$e=e^1$$.

Again using Taylor's Theorem, we know the expansion of sinx and cosx. And thus Euler's Theorem is immediate. Also, we can verify (I had little hassles evaluating it) :

$\lim_{x \to \infty} (1 + \frac{1}{x})^x = e$.

But still, the important one remains. Taking this irrational as the base, we can write the expansion easily. But still the expansion is infinite going. It never ends. So it doesn't really helps.

Also, I would now like to know for what values of x, the expansion of $$e^x$$ (except x=0) is rational.

- 5 years, 10 months ago

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it won't, if you're looking for real-valued x. extend your search to complex numbers, and you will be amazed :) google euler's identity

- 5 years, 10 months ago

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I found this page interesting: http://www-history.mcs.st-and.ac.uk/HistTopics/e.html

- 5 years, 10 months ago

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The text passage contains multiple questions related to the mathematical constant \( e \) and its properties. The questions are about the history and significance of \( e \), its Taylor series expansion, and Euler's formula.
multiple_questions

1. **why did euler mention this number?**
2. **what was the purpose of euler's invention (or discovery) of \( e \)?**
3. **why is \( e \) convenient to use as the base?**
4. **how do we know the value of \( e \) and its taylor expansion?**
5. **how did we discover the taylor expansion of \( e^x \)?**
6. **how did euler prove \( e^{ix} = \cos x + i \sin x \)?**

no answers to these questions are provided in the text.
Given a text passage, determine:
    1. If it contains a mathematical question (including word problems or calculations)
    2. If an answer to that mathematical question is provided in the text

First, describe the content structure you found in 1-2 sentences.

Then provide your analysis in the following format:
TYPE: <one of the following values>
- NO_QUESTION
- SINGLE_QUESTION
- SINGLE_QUESTION_WITH_ANSWER
- MULTIPLE_QUESTIONS
- MULTIPLE_QUESTIONS_WITH_ANSWERS
system
# Euler's Number?

We all have studied about the Euler's Number $$e$$. We know it is the natural base. We know it is the value to which $$(1 + 1/n)^{n}$$ approaches as $$n$$ tends to infinity. We also know its Taylor expansion.

We learnt these things from our textbooks. But I never found one which explains all the point clearly. These textbooks simply teach these as facts.

I want to know why Euler mentioned this number? By what purpose did he invented (or discovered) it? I was told that its convenient to use $$e$$ as the base. Why this irrational is so convenient (actually, I haven't been convenient) ? How do we know its value and its taylor expansion?

Now when we know that

$e = 1 + 1 + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + ... = 2.7182...$

how did we discovered that

$e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \frac{x^4}{4!} + ...$?

Also, how Euler proved that

$e^{ix} = cos\theta + isin\theta$

Note by Avinash Pandey
5 years, 10 months ago

MarkdownAppears as
*italics* or _italics_ italics
**bold** or __bold__ bold
- bulleted- list
• bulleted
• list
1. numbered2. list
1. numbered
2. list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1paragraph 2

paragraph 1

paragraph 2

[example link](https://brilliant.org)example link
> This is a quote
This is a quote
    # I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
# I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
MathAppears as
Remember to wrap math in $$...$$ or $...$ to ensure proper formatting.
2 \times 3 $$2 \times 3$$
2^{34} $$2^{34}$$
a_{i-1} $$a_{i-1}$$
\frac{2}{3} $$\frac{2}{3}$$
\sqrt{2} $$\sqrt{2}$$
\sum_{i=1}^3 $$\sum_{i=1}^3$$
\sin \theta $$\sin \theta$$
\boxed{123} $$\boxed{123}$$

## Comments

Sort by:

Top Newest

If you write out the taylor/maclauren series expansion of e^x, cos, and sine, euler's formula is immediate.

As to why e is so famous, i'd say its because of the fact d/dx e^x = e^x, a property which no other function satisfies. This is why taking logarithms in base e are helpful.

In proving that d/dx e^x = e^x, you can apply the power rule on that very same expansion you have written

d/dx e^x = 1 + (2x)/2! + (3x^2)/3! + (4x3)/4! ...

when you cancel of the common factors, you will see that you end up with the same series you had started with, ie e^x

As to why its taylor series expansion is like this, its mainly due to the above fact, that d/dx e^x = e^x Just plug that in in the formula you have for taylor/maclauren series expansion - link

- 5 years, 10 months ago

Log in to reply

I remember being told that Euler created e as a number to satisfy the equation $$\frac{d}{dx}$$e$$^x$$ = e$$^x$$

- 5 years, 10 months ago

Log in to reply

i'm unsure as to how he could 'create e as a number' to me it seems more likely that he was seeing if the limit if 1/(n!) converges or not, and it did, since factorials get large pretty quickly

- 5 years, 9 months ago

Log in to reply

The proof that $$e = 1 + 1 + \frac{1}{2!} +\frac{1}{3!}\frac{1}{4!} + ...$$ is a very beautiful one, which comes from the binomial expansion of $$e = (1 + \frac{1}{N} )^N$$. The process for $$e^x$$ is very similar. However, I don't know where the $$e^{ix} = cosx+isinx$$ comes from.

- 5 years, 10 months ago

Log in to reply

What do you mean by $$e = ( 1 + \frac {1}{N})^N$$? Is $$N=2, 3$$? Otherwise, how do you take the binomial expansion?

Staff - 5 years, 10 months ago

Log in to reply

Although the original problem is solved now (almost), I have new series of questions on the same topic.

i) How did Euler knew (with reference to Harel D.) that an exponential function would be the one whose derivative is the function itself? ii) Aren't there other functions such that $$\frac{d}{dx} f(x) = f(x)$$ ? iii) For what values of x, the expansion of $$e^x$$ (except x=0) is rational? iv) Proof of Taylor's Theorem?

- 5 years, 10 months ago

Log in to reply

At last, after rigorous search on internet and libraries, I found answers of most of them. Actually, we use a theorem called Taylor's Theorem which can be used to represent any function as a series of polynomial terms.

$f(x)= \sum_{k=0}^{\infty} \frac{x^k}{k!} f^{(k)} (0)$

Now using the fact told by Harel D., Euler knew that "zeroth derivative of $$e^x$$ evaluated at zero is one and kth derivative, where k is natural number, of $$e^x$$ is $$e^x$$ itself". Now apply Taylor's Theorem to get the expansion. Now when we know $$e^x$$, then we can evaluate $$e=e^1$$.

Again using Taylor's Theorem, we know the expansion of sinx and cosx. And thus Euler's Theorem is immediate. Also, we can verify (I had little hassles evaluating it) :

$\lim_{x \to \infty} (1 + \frac{1}{x})^x = e$.

But still, the important one remains. Taking this irrational as the base, we can write the expansion easily. But still the expansion is infinite going. It never ends. So it doesn't really helps.

Also, I would now like to know for what values of x, the expansion of $$e^x$$ (except x=0) is rational.

- 5 years, 10 months ago

Log in to reply

it won't, if you're looking for real-valued x. extend your search to complex numbers, and you will be amazed :) google euler's identity

- 5 years, 10 months ago

Log in to reply

I found this page interesting: http://www-history.mcs.st-and.ac.uk/HistTopics/e.html

- 5 years, 10 months ago

Log in to reply

×

Problem Loading...

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user
The text passage contains multiple questions related to the mathematical constant \( e \) and its properties. The questions are about the history and significance of \( e \), its Taylor series expansion, and Euler's formula.

TYPE: MULTIPLE_QUESTIONS

1. **Why did Euler mention this number?**
2. **What was the purpose of Euler's invention (or discovery) of \( e \)?**
3. **Why is \( e \) convenient to use as the base?**
4. **How do we know the value of \( e \) and its Taylor expansion?**
5. **How did we discover the Taylor expansion of \( e^x \)?**
6. **How did Euler prove \( e^{ix} = \cos x + i \sin x \)?**

No answers to these questions are provided in the text.