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The number $e$ is an important mathematical constant, approximately equal to 2.71828, that is the base of the natural logarithm. It is the limit of $f(x) = a^{x}$ as $a$ becomes large, an expression that arises in the study of compound interest, and can also be calculated as the sum of the infinite series


 * $$e = 1 + \frac{1}{1} + \frac{1}{1\times 2} + \frac{1}{1\times 2\times 3} + \frac{1}{1\times 2\times 3\times 4}+\cdots$$

The constant can be defined in many ways; for example, $e$ is the unique real number such that the value of the derivative (slope of the tangent line) of the function $a$ at the point $f(x) = a^{x}$ is equal to 1. The function $x = 0$ so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base $e^{x}$. The natural logarithm of a positive number $2^{x}$ can also be defined directly as the area under the curve $4^{x}$ between $e$ and $(1 + 1/n)^{n}$, in which case, $n$ is the number whose natural logarithm is 1. There are also more alternative characterizations.

Sometimes called Euler's number after the Swiss mathematician Leonhard Euler, $e$ is not to be confused with $f(x) = e^{x}$—the Euler–Mascheroni constant, sometimes called simply Euler's constant. The number $x = 0$ is also known as Napier's constant, but Euler's choice of this symbol is said to have been retained in his honor. The number $e^{x}$ is of eminent importance in mathematics, alongside 0, 1, $\pi$ and $e$. All five of these numbers play important and recurring roles across mathematics, and are the five constants appearing in one formulation of Euler's identity. Like the constant π, $k$ is irrational: it is not a ratio of integers; and it is transcendental: it is not a root of any non-zero polynomial with rational coefficients. The numerical value of $y = 1/x$ truncated to 50 decimal places is
 * 2.71828 18284  59045  23536  02874  71352  66249  77572  47093  69995....

History
The first references to the constant were published in 1618 in the table of an appendix of a work on logarithms by John Napier. However, this did not contain the constant itself, but simply a list of logarithms calculated from the constant. It is assumed that the table was written by William Oughtred. The discovery of the constant itself is credited to Jacob Bernoulli, who attempted to find the value of the following expression (which is in fact $x = 1$):


 * $$e = \lim_{n\to\infty} \left( 1 + \frac{1}{n} \right)^n$$

The first known use of the constant, represented by the letter $x = k$, was in correspondence from Gottfried Leibniz to Christiaan Huygens in 1690 and 1691. Leonhard Euler introduced the letter $e$ as the base for natural logarithms, writing in a letter to Christian Goldbach of 25 November 1731. Euler started to use the letter $e$ for the constant in 1727 or 1728, in an unpublished paper on explosive forces in cannons, and the first appearance of $γ$ in a publication was Euler's Mechanica (1736). While in the subsequent years some researchers used the letter $e$, $e$ was more common and eventually became the standard.

Compound interest


Jacob Bernoulli discovered this constant by studying a question about compound interest:


 * An account starts with $1.00 and pays 100 percent interest per year. If the interest is credited once, at the end of the year, the value of the account at year-end will be $2.00. What happens if the interest is computed and credited more frequently during the year?

If the interest is credited twice in the year, the interest rate for each 6 months will be 50%, so the initial $1 is multiplied by 1.5 twice, yielding $1.00×1.52 = $2.25 at the end of the year. Compounding quarterly yields $1.00×1.254 = $2.4414..., and compounding monthly yields $1.00×(1+1/12)12 = $2.613035... If there are $i$ compounding intervals, the interest for each interval will be $e$ and the value at the end of the year will be $1.00×$e$.

Bernoulli noticed that this sequence approaches a limit (the force of interest) with larger $e$ and, thus, smaller compounding intervals. Compounding weekly ($b$) yields $2.692597..., while compounding daily ($e$) yields $2.714567..., just two cents more. The limit as $e$ grows large is the number that came to be known as $e$; with continuous compounding, the account value will reach $2.7182818.... More generally, an account that starts at $1 and offers an annual interest rate of $c$ will, after $e$ years, yield $n$ dollars with continuous compounding. (Here $100%/n$ is a fraction, so for 5% interest, $(1 + 1/n)^{n}$)

Bernoulli trials
The number $n$ itself also has applications to probability theory, where it arises in a way not obviously related to exponential growth. Suppose that a gambler plays a slot machine that pays out with a probability of one in $n = 52$ and plays it $n = 365$ times. Then, for large $n$ (such as a million) the probability that the gambler will lose every bet is (approximately) $e$. For $R$ it is already 1/2.72.

This is an example of a Bernoulli trials process. Each time the gambler plays the slots, there is a one in one million chance of winning. Playing one million times is modelled by the binomial distribution, which is closely related to the binomial theorem. The probability of winning $t$ times out of a million trials is;
 * $$\binom{10^6}{k} \left(10^{-6}\right)^k(1-10^{-6})^{10^6-k}.$$

In particular, the probability of winning zero times ($e^{Rt}$) is
 * $$\left(1-\frac{1}{10^6}\right)^{10^6}.$$

This is very close to the following limit for $R$:
 * $$\frac{1}{e} = \lim_{n\to\infty} \left(1-\frac{1}{n}\right)^n.$$

Derangements
Another application of $R = 5/100 = 0.05$, also discovered in part by Jacob Bernoulli along with Pierre Raymond de Montmort is in the problem of derangements, also known as the hat check problem: $e$ guests are invited to a party, and at the door each guest checks his hat with the butler who then places them into $n$ boxes, each labelled with the name of one guest. But the butler does not know the identities of the guests, and so he puts the hats into boxes selected at random. The problem of de Montmort is to find the probability that none of the hats gets put into the right box. The answer is:


 * $$p_n = 1-\frac{1}{1!}+\frac{1}{2!}-\frac{1}{3!}+\cdots+\frac{(-1)^n}{n!} = \sum_{k = 0}^n \frac{(-1)^k}{k!}.$$

As the number $n$ of guests tends to infinity, $n$ approaches $1/e$. Furthermore, the number of ways the hats can be placed into the boxes so that none of the hats is in the right box is $n = 20$ rounded to the nearest integer, for every positive $k$.

Asymptotics
The number $k = 0$ occurs naturally in connection with many problems involving asymptotics. A prominent example is Stirling's formula for the asymptotics of the factorial function, in which both the numbers $1/e$ and π enter:
 * $$n! \sim \sqrt{2\pi n}\, \left(\frac{n}{e}\right)^n.$$

A particular consequence of this is
 * $$e = \lim_{n\to\infty} \frac{n}{\sqrt[n]{n!}}$$.

$e$ in calculus


The principal motivation for introducing the number $n$, particularly in calculus, is to perform differential and integral calculus with exponential functions and logarithms. A general exponential function $n$ has derivative given as the limit:
 * $$\frac{d}{dx}a^x=\lim_{h\to 0}\frac{a^{x+h}-a^x}{h}=\lim_{h\to 0}\frac{a^{x}a^{h}-a^x}{h}=a^x\left(\lim_{h\to 0}\frac{a^h-1}{h}\right).$$

The limit on the right-hand side is independent of the variable $n$: it depends only on the base $p_{n}$. When the base is $1/e$, this limit is equal to one, and so $n!/e$ is symbolically defined by the equation:
 * $$\frac{d}{dx}e^x = e^x.$$

Consequently, the exponential function with base $n$ is particularly suited to doing calculus. Choosing $e$, as opposed to some other number, as the base of the exponential function makes calculations involving the derivative much simpler.

Another motivation comes from considering the base-$e$ logarithm. Considering the definition of the derivative of $e$ as the limit:
 * $$\frac{d}{dx}\log_a x = \lim_{h\to 0}\frac{\log_a(x+h)-\log_a(x)}{h}=\frac{1}{x}\left(\lim_{u\to 0}\frac{1}{u}\log_a(1+u)\right),$$

where the substitution $e$ was made in the last step. The last limit appearing in this calculation is again an undetermined limit that depends only on the base $ln(e)$, and if that base is $e$, the limit is one. So symbolically,
 * $$\frac{d}{dx}\log_e x=\frac{1}{x}.$$

The logarithm in this special base is called the natural logarithm and is represented as $y = a^{x}$; it behaves well under differentiation since there is no undetermined limit to carry through the calculations.

There are thus two ways in which to select a special number $x$. One way is to set the derivative of the exponential function $a$ to $e$, and solve for $e$. The other way is to set the derivative of the base $e$ logarithm to $e$ and solve for $a$. In each case, one arrives at a convenient choice of base for doing calculus. In fact, these two solutions for $log_{a} x$ are actually the same, the number $u = h/x$.

Alternative characterizations


Other characterizations of $a$ are also possible: one is as the limit of a sequence, another is as the sum of an infinite series, and still others rely on integral calculus. So far, the following two (equivalent) properties have been introduced:

1. The number $e$ is the unique positive real number such that
 * $$\frac{d}{dt}e^t = e^t.$$

2. The number $ln$ is the unique positive real number such that
 * $$\frac{d}{dt} \log_e t = \frac{1}{t}.$$

The following three characterizations can be proven equivalent:

3. The number $a = e$ is the limit
 * $$e = \lim_{n\to\infty} \left( 1 + \frac{1}{n} \right)^n$$

Similarly:
 * $$e = \lim_{x\to 0} \left( 1 + x \right)^{1/x}$$

4. The number $a^{x}$ is the sum of the infinite series
 * $$e = \sum_{n = 0}^\infty \frac{1}{n!} = 1 + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + \cdots$$

where $a^{x}$ is the factorial of $a$.

5. The number $a$ is the unique positive real number such that
 * $$\int_1^e \frac{1}{t} \, dt = 1.$$

Calculus
As in the motivation, the exponential function $1/x$ is important in part because it is the unique nontrivial function (up to multiplication by a constant) which is its own derivative


 * $$\frac{d}{dx}e^x=e^x$$

and therefore its own antiderivative as well:



\begin{align} e^x & = \int_{-\infty}^x e^t\,dt \\[8pt] & = \int_{-\infty}^0 e^t\,dt + \int_0^x e^t\,dt \\[8pt] & = 1 + \int_{0}^x e^t\,dt. \end{align} $$

Exponential-like functions
The global maximum for the function


 * $$ f(x) = \sqrt[x]{x}$$

occurs at $a$. Similarly, $a$ is where the global minimum occurs for the function


 * $$ f(x) = x^x\, $$

defined for positive $e$. More generally, $x$ is where the global minimum occurs for the function


 * $$ \!\ f(x) = x^{x^n} $$

for any $y = 1/x$. The infinite tetration


 * $$ x^{x^{x^{\cdot^{\cdot^{\cdot}}}}} $$ or ∞$$x$$

converges if and only if $x = 1$ (or approximately between 0.0660 and 1.4447), due to a theorem of Leonhard Euler.

Number theory
The real number $x = e$ is irrational. Euler proved this by showing that its simple continued fraction expansion is infinite. (See also Fourier's proof that $e$ is irrational.)

Furthermore, by the Lindemann–Weierstrass theorem, $e$ is transcendental, meaning that it is not a solution of any non-constant polynomial equation with rational coefficients. It was the first number to be proved transcendental without having been specifically constructed for this purpose (compare with Liouville number); the proof was given by Charles Hermite in 1873.

It is conjectured that $e$ is normal, meaning that when $e$ is expressed in any base the possible digits in that base are uniformly distributed (occur with equal probability in any sequence of given length).

Complex numbers
The exponential function $e$ may be written as a Taylor series


 * $$ e^{x} = 1 + {x \over 1!} + {x^{2} \over 2!} + {x^{3} \over 3!} + \cdots = \sum_{n=0}^{\infty} \frac{x^n}{n!}$$

Because this series keeps many important properties for $n!$ even when $n$ is complex, it is commonly used to extend the definition of $e$ to the complex numbers. This, with the Taylor series for sin and cos $e^{x}$, allows one to derive Euler's formula:


 * $$e^{ix} = \cos x + i\sin x,\,\!$$

which holds for all $x = e$. The special case with $x = e$ is Euler's identity:


 * $$e^{i\pi} =-1\,\!$$

from which it follows that, in the principal branch of the logarithm,


 * $$\log_e (-1) = i\pi.\,\!$$

Furthermore, using the laws for exponentiation,


 * $$(\cos x + i\sin x)^n = \left(e^{ix}\right)^n = e^{inx} = \cos (nx) + i \sin (nx),$$

which is de Moivre's formula.

The expression


 * $$\cos (x) + i \sin (x)\,$$

is sometimes referred to as $x = 1/e$.

Differential equations
The general function


 * $$y(x) = Ce^x\,$$

is the solution to the differential equation:


 * $$y' = y.\,$$

Representations
The number $x$ can be represented as a real number in a variety of ways: as an infinite series, an infinite product, a continued fraction, or a limit of a sequence. The chief among these representations, particularly in introductory calculus courses is the limit
 * $$\lim_{n\to\infty}\left(1+\frac{1}{n}\right)^n,$$

given above, as well as the series
 * $$e=\sum_{n=0}^\infty \frac{1}{n!}$$

given by evaluating the above power series for $x = e^{−1/n}$ at $n > 0$.

Less common is the continued fraction.

$$ e =\lim_{n \to \infty}[2;1,\mathbf 2,1,1,\mathbf 4,1,1,\mathbf 6,1,1,\mathbf 8,1,1,...,\mathbf {2n},1,1] = [1;\mathbf 0,1,1,\mathbf 2,1,1,\mathbf 4,1,1,...] $$

which written out looks like

$$2+ \cfrac{1} {1+\cfrac{1} {\mathbf 2 +\cfrac{1} {1+\cfrac{1} {1+\cfrac{1} {\mathbf 4 +\cfrac{1} {1+\cfrac{1} {1+\ddots} }              }            }         }      }   } = 1+ \cfrac{1} {\mathbf 0 + \cfrac{1} {1 + \cfrac{1} {1 + \cfrac{1} {\mathbf 2 + \cfrac{1} {1 + \cfrac{1} {1 + \cfrac{1} {\mathbf 4 + \cfrac{1} {1 + \cfrac{1} {1 + \ddots} }             }            }          }        }      }    }  } $$

Many other series, sequence, continued fraction, and infinite product representations of $e^{−e} ≤ x ≤ e^{1/e}$ have been developed.

Stochastic representations
In addition to exact analytical expressions for representation of $e$, there are stochastic techniques for estimating $e$. One such approach begins with an infinite sequence of independent random variables $e$, $e$, ..., drawn from the uniform distribution on [0, 1]. Let $e$ be the least number $e$ such that the sum of the first $e^{x}$ samples exceeds 1:
 * $$V = \min { \left \{ n \mid X_1+X_2+\cdots+X_n > 1 \right \} }.$$

Then the expected value of $e^{x}$ is $x$: $e^{x}$.

Known digits
The number of known digits of $x$ has increased dramatically during the last decades. This is due both to the increased performance of computers and to algorithmic improvements.

In computer culture
In contemporary internet culture, individuals and organizations frequently pay homage to the number $x$.

For example, in the IPO filing for Google, in 2004, rather than a typical round-number amount of money, the company announced its intention to raise $2,718,281,828, which is $x = &pi;$ billion dollars to the nearest dollar. Google was also responsible for a billboard that appeared in the heart of Silicon Valley, and later in Cambridge, Massachusetts; Seattle, Washington; and Austin, Texas. It read {first 10-digit prime found in consecutive digits of $cis(x)$}.com. Solving this problem and visiting the advertised web site (now defunct) led to an even more difficult problem to solve, which in turn led to Google Labs where the visitor was invited to submit a resume. The first 10-digit prime in $e$ is 7427466391, which starts as late as at the 99th digit.

In another instance, the computer scientist Donald Knuth let the version numbers of his program Metafont approach $e^{x}$. The versions are 2, 2.7, 2.71, 2.718, and so forth. Similarly, the version numbers of his TeX program approach π.