# Euler's number

**e** is a constant that appears in a variety of mathematical contexts. It has been shown to be both irrational and transcendental. Numerically, it is approximately equal to 2.71828.

## e and calculus

**e** is defined as the following limit: .
In calculus, the fact that is used often, based on the above definition and the Binomial Theorem.

### Where comes from

Suppose is a positive real number, and for all real numbers . Let's try to figure out what is.

.

Something special has almost happened. We found the derivative of , and what we got was *almost* . In other words, the derivative of this function is *almost* the same function that we started with. However, there is that annoying and kind of messy limit on the right that is messing things up.

It seems like it would be at least cool, if not downright useful, to have a function whose derivative is equal to itself. (In fact, it turns out that such a function is very useful indeed, for example, in finding solutions to certain differential equations.) So, let's ask this question: Is it possible to cleverly pick a special value of in order to make that annoying limit on the right turn out to be equal to ? If this were possible, then for that special value of , it would in fact be true that the derivative of is just , exactly the same function we started with.

Well, the fact is that there is a special value of which accomplishes this goal. It is approximately , and it is called . ("" stands for *exponential* and not *Euler*, despite the fact that Leonhard Euler was one of the first mathematicians to use it and the one to name it. is sometimes (perhaps incorrectly) called "Euler's number." However, Napier came close to discovering (or, rather, ) before Euler did.)

So, how do we choose so that ? Could we pick in some clever way to make this expression simplify? As a first rough idea, imagine what would happen to the expression if were equal to . Everything cancels out nicely, and we are left with just . This suggests that we should select to be . Here we have our definition of . This limit is approximately .

To make this rigorous, it should be possible to prove that this limit actually exists. Once we define to be this limit, it should be possible to prove that .

(This definition of is equivalent to the one given at the beginning of this article. Just let . Then .)