The number Template:Mvar is an important mathematical constant that is the base of the natural logarithm. It is approximately equal to 2.71828,^{[1]} and is the limit of (1 + 1/n)^{n} as Template:Mvar approaches infinity, an expression that arises in the study of compound interest. It can also be calculated as the sum of the infinite series^{[2]}
 $e\; =\; 1\; +\; \backslash frac\{1\}\{1\}\; +\; \backslash frac\{1\}\{1\backslash cdot\; 2\}\; +\; \backslash frac\{1\}\{1\backslash cdot\; 2\backslash cdot\; 3\}\; +\; \backslash frac\{1\}\{1\backslash cdot\; 2\backslash cdot\; 3\backslash cdot\; 4\}+\backslash cdots$
The constant can be defined in many ways; for example, Template:Mvar is the unique real number such that the value of the derivative (slope of the tangent line) of the function f(x) = e^{x} at the point x = 0 is equal to 1.^{[3]} The function e^{x} so defined is called the exponential function, and its inverse is the natural logarithm, or logarithm to base Template:Mvar. The natural logarithm of a positive number k can also be defined directly as the area under the curve y = 1/x between x = 1 and x = k, in which case, Template:Mvar 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, Template:Mvar is not to be confused with γ—the Euler–Mascheroni constant, sometimes called simply Euler's constant. The number Template:Mvar is also known as Napier's constant, but Euler's choice of this symbol is said to have been retained in his honor.^{[4]} The number Template:Mvar is of eminent importance in mathematics,^{[5]} alongside 0, 1, and . 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 Template:Pi, Template:Mvar is irrational: it is not a ratio of integers; and it is transcendental: it is not a root of any nonzero polynomial with rational coefficients. The numerical value of Template:Mvar truncated to 50 decimal places is
 2.71828182845904523536028747135266249775724709369995... (sequence OEIS).
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.^{[6]} 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 Template:Mvar):
 $\backslash lim\_\{n\backslash to\backslash infty\}\; \backslash left(\; 1\; +\; \backslash frac\{1\}\{n\}\; \backslash right)^n$
The first known use of the constant, represented by the letter b, was in correspondence from Gottfried Leibniz to Christiaan Huygens in 1690 and 1691. Leonhard Euler introduced the letter Template:Mvar as the base for natural logarithms, writing in a letter to Christian Goldbach of 25 November 1731.^{[7]} Euler started to use the letter Template:Mvar for the constant in 1727 or 1728, in an unpublished paper on explosive forces in cannons,^{[8]} and the first appearance of Template:Mvar in a publication was Euler's Mechanica (1736). While in the subsequent years some researchers used the letter c, Template:Mvar was more common and eventually became the standard.
Applications
Compound interest
Jacob Bernoulli discovered this constant by studying a question about compound interest:^{[6]}
 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 yearend 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 $1.00×1.5^{2} = $2.25 at the end of the year. Compounding quarterly yields $1.00×1.25^{4} = $2.4414..., and compounding monthly yields $1.00×(1+1/12)^{12} = $2.613035... If there are n compounding intervals, the interest for each interval will be 100%/n and the value at the end of the year will be $1.00×(1 + 1/n)^{n}.
Bernoulli noticed that this sequence approaches a limit (the force of interest) with larger n and, thus, smaller compounding intervals. Compounding weekly (n = 52) yields $2.692597..., while compounding daily (n = 365) yields $2.714567..., just two cents more. The limit as n grows large is the number that came to be known as Template:Mvar; 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 R will, after t years, yield e^{Rt} dollars with continuous compounding. (Here R is a fraction, so for 5% interest, R = 5/100 = 0.05)
Bernoulli trials
The number Template:Mvar 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 and plays it n times. Then, for large n (such as a million) the probability that the gambler will lose every bet is (approximately) 1/e. For n = 20 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 k times out of a million trials is;
 $\backslash binom\{10^6\}\{k\}\; \backslash left(10^\{6\}\backslash right)^k(110^\{6\})^\{10^6k\}.$
In particular, the probability of winning zero times (k = 0) is
 $\backslash left(1\backslash frac\{1\}\{10^6\}\backslash right)^\{10^6\}.$
This is very close to the following limit for 1/e:
 $\backslash frac\{1\}\{e\}\; =\; \backslash lim\_\{n\backslash to\backslash infty\}\; \backslash left(1\backslash frac\{1\}\{n\}\backslash right)^n.$
Derangements
Another application of Template:Mvar, 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:^{[9]} n 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:
 $p\_n\; =\; 1\backslash frac\{1\}\{1!\}+\backslash frac\{1\}\{2!\}\backslash frac\{1\}\{3!\}+\backslash cdots+\backslash frac\{(1)^n\}\{n!\}\; =\; \backslash sum\_\{k\; =\; 0\}^n\; \backslash frac\{(1)^k\}\{k!\}.$
As the number n of guests tends to infinity, p_{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!/e rounded to the nearest integer, for every positive n.^{[10]}
Asymptotics
The number Template:Mvar 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 Template:Mvar and enter:
 $n!\; \backslash sim\; \backslash sqrt\{2\backslash pi\; n\}\backslash ,\; \backslash left(\backslash frac\{n\}\{e\}\backslash right)^n.$
A particular consequence of this is
 $e\; =\; \backslash lim\_\{n\backslash to\backslash infty\}\; \backslash frac\{n\}\{\backslash sqrt[n]\{n!\}\}$.
Standard normal distribution
(from Normal distribution)
The simplest case of a normal distribution is known as the standard normal distribution, described by this probability density function:
 $\backslash phi(x)\; =\; \backslash frac\{1\}\{\backslash sqrt\{2\backslash pi\}\}\backslash ,\; e^\{\; \backslash frac\{\backslash scriptscriptstyle\; 1\}\{\backslash scriptscriptstyle\; 2\}\; x^2\}.$
The factor $\backslash scriptstyle\backslash \; 1/\backslash sqrt\{2\backslash pi\}$ in this expression ensures that the total area under the curve ϕ(x) is equal to one^{[proof]}. The
 REDIRECT Template:Sfrac in the exponent ensures that the distribution has unit variance (and therefore also unit standard deviation). This function is symmetric around x=0, where it attains its maximum value $1/\backslash sqrt\{2\backslash pi\}$; and has inflection points at +1 and −1.
Template:Mvar in calculus
The principal motivation for introducing the number Template:Mvar, particularly in calculus, is to perform differential and integral calculus with exponential functions and logarithms.^{[11]} A general exponential function y = a^{x} has derivative given as the limit:
 $\backslash frac\{d\}\{dx\}a^x=\backslash lim\_\{h\backslash to\; 0\}\backslash frac\{a^\{x+h\}a^x\}\{h\}=\backslash lim\_\{h\backslash to\; 0\}\backslash frac\{a^\{x\}a^\{h\}a^x\}\{h\}=a^x\backslash left(\backslash lim\_\{h\backslash to\; 0\}\backslash frac\{a^h1\}\{h\}\backslash right).$
The limit on the far right is independent of the variable x: it depends only on the base a. When the base is Template:Mvar, this limit is equal to one, and so Template:Mvar is symbolically defined by the equation:
 $\backslash frac\{d\}\{dx\}e^x\; =\; e^x.$
Consequently, the exponential function with base Template:Mvar is particularly suited to doing calculus. Choosing Template:Mvar, 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 basea logarithm.^{[12]} Considering the definition of the derivative of log_{a} x as the limit:
 $\backslash frac\{d\}\{dx\}\backslash log\_a\; x\; =\; \backslash lim\_\{h\backslash to\; 0\}\backslash frac\{\backslash log\_a(x+h)\backslash log\_a(x)\}\{h\}=\backslash frac\{1\}\{x\}\backslash left(\backslash lim\_\{u\backslash to\; 0\}\backslash frac\{1\}\{u\}\backslash log\_a(1+u)\backslash right),$
where the substitution u = h/x was made in the last step. The last limit appearing in this calculation is again an undetermined limit that depends only on the base a, and if that base is Template:Mvar, the limit is one. So symbolically,
 $\backslash frac\{d\}\{dx\}\backslash log\_e\; x=\backslash frac\{1\}\{x\}.$
The logarithm in this special base is called the natural logarithm and is represented as ln; 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 a = e. One way is to set the derivative of the exponential function a^{x} to a^{x}, and solve for a. The other way is to set the derivative of the base a logarithm to 1/x and solve for a. In each case, one arrives at a convenient choice of base for doing calculus. In fact, these two solutions for a are actually the same, the number Template:Mvar.
Alternative characterizations
Other characterizations of Template:Mvar 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 Template:Mvar is the unique positive real number such that
 $\backslash frac\{d\}\{dt\}e^t\; =\; e^t.$
2. The number Template:Mvar is the unique positive real number such that
 $\backslash frac\{d\}\{dt\}\; \backslash log\_e\; t\; =\; \backslash frac\{1\}\{t\}.$
The following three characterizations can be proven equivalent:
3. The number Template:Mvar is the limit
 $e\; =\; \backslash lim\_\{n\backslash to\backslash infty\}\; \backslash left(\; 1\; +\; \backslash frac\{1\}\{n\}\; \backslash right)^n$
Similarly:
 $e\; =\; \backslash lim\_\{x\backslash to\; 0\}\; \backslash left(\; 1\; +\; x\; \backslash right)^\{\backslash frac\{1\}\{x\}\}$
4. The number Template:Mvar is the sum of the infinite series
 $e\; =\; \backslash sum\_\{n\; =\; 0\}^\backslash infty\; \backslash frac\{1\}\{n!\}\; =\; \backslash frac\{1\}\{0!\}\; +\; \backslash frac\{1\}\{1!\}\; +\; \backslash frac\{1\}\{2!\}\; +\; \backslash frac\{1\}\{3!\}\; +\; \backslash frac\{1\}\{4!\}\; +\; \backslash cdots$
where n! is the factorial of n.
5. The number Template:Mvar is the unique positive real number such that
 $\backslash int\_1^e\; \backslash frac\{1\}\{t\}\; \backslash ,\; dt\; =\; 1.$
Properties
Calculus
As in the motivation, the exponential function e^{x} is important in part because it is the unique nontrivial function (up to multiplication by a constant) which is its own derivative
 $\backslash frac\{d\}\{dx\}e^x=e^x$
and therefore its own antiderivative as well:
 $\backslash int\; e^x\backslash ,dx\; =\; e^x\; +\; C.$
Exponentiallike functions
The global maximum for the function
 $f(x)\; =\; \backslash sqrt[x]\{x\}$
occurs at x = e. Similarly, x = 1/e is where the global minimum occurs for the function
 $f(x)\; =\; x^x\backslash ,$
defined for positive x. More generally, x = e^{−1/n} is where the global minimum occurs for the function
 $\backslash !\backslash \; f(x)\; =\; x^\{x^n\}$
for any n > 0. The infinite tetration
 $x^\{x^\{x^\{\backslash cdot^\{\backslash cdot^\{\backslash cdot\}\}\}\}\}$ or ^{∞}$x$
converges if and only if e^{−e} ≤ x ≤ e^{1/e} (or approximately between 0.0660 and 1.4447), due to a theorem of Leonhard Euler.
Number theory
The real number Template:Mvar is irrational. Euler proved this by showing that its simple continued fraction expansion is infinite.^{[13]} (See also Fourier's .)
Furthermore, by the Lindemann–Weierstrass theorem, Template:Mvar is transcendental, meaning that it is not a solution of any nonconstant 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 Template:Mvar is normal, meaning that when Template:Mvar 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^{x} may be written as a Taylor series
 $e^\{x\}\; =\; 1\; +\; \{x\; \backslash over\; 1!\}\; +\; \{x^\{2\}\; \backslash over\; 2!\}\; +\; \{x^\{3\}\; \backslash over\; 3!\}\; +\; \backslash cdots\; =\; \backslash sum\_\{n=0\}^\{\backslash infty\}\; \backslash frac\{x^n\}\{n!\}$
Because this series keeps many important properties for e^{x} even when x is complex, it is commonly used to extend the definition of e^{x} to the complex numbers. This, with the Taylor series for sin and cos x, allows one to derive Euler's formula:
 $e^\{ix\}\; =\; \backslash cos\; x\; +\; i\backslash sin\; x,\backslash ,\backslash !$
which holds for all x. The special case with x = π is Euler's identity:
 $e^\{i\backslash pi\}\; =1\backslash ,\backslash !$
from which it follows that, in the principal branch of the logarithm,
 $\backslash log\_e\; (1)\; =\; i\backslash pi.\backslash ,\backslash !$
Furthermore, using the laws for exponentiation,
 $(\backslash cos\; x\; +\; i\backslash sin\; x)^n\; =\; \backslash left(e^\{ix\}\backslash right)^n\; =\; e^\{inx\}\; =\; \backslash cos\; (nx)\; +\; i\; \backslash sin\; (nx),$
which is de Moivre's formula.
The expression
 $\backslash cos\; x\; +\; i\; \backslash sin\; x\; \backslash ,$
is sometimes referred to as cis(x).
Differential equations
The general function
 $y(x)\; =\; Ce^x\backslash ,$
is the solution to the differential equation:
 $y\text{'}\; =\; y.\backslash ,$
Representations
The number Template:Mvar 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
 $\backslash lim\_\{n\backslash to\backslash infty\}\backslash left(1+\backslash frac\{1\}\{n\}\backslash right)^n,$
given above, as well as the series
 $e=\backslash sum\_\{n=0\}^\backslash infty\; \backslash frac\{1\}\{n!\}$
given by evaluating the above power series for e^{x} at x = 1.
Less common is the OEIS).
 $$
e = [2;1,\mathbf 2,1,1,\mathbf 4,1,1,\mathbf 6,1,1,...,\mathbf {2n},1,1,...] = [1;\mathbf 0,1,1,\mathbf 2,1,1,\mathbf 4,1,1,...,\mathbf {2n},1,1,...],
^{[14]}
which written out looks like
 $e\; =\; 2+$
\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}
}
}
}
}
}
}
}
}.
This continued fraction for Template:Mvar converges three times as quickly:
 $e\; =\; [\; 1;\; 0.5\; ,\; 12\; ,\; 5\; ,\; 28\; ,\; 9\; ,\; 44\; ,\; 13\; ,\; \backslash ldots\; ,\; 4(4n1)\; ,\; (4n+1)\; ,\; \backslash ldots\; ],$
which written out looks like
 $e\; =\; 1+\backslash cfrac\{2\}\{1+\backslash cfrac\{1\}\{6+\backslash cfrac\{1\}\{10+\backslash cfrac\{1\}\{14+\backslash cfrac\{1\}\{18+\backslash cfrac\{1\}\{22+\backslash cfrac\{1\}\{26+\backslash ddots\backslash ,\}\}\}\}\}\}\}.$
Many other series, sequence, continued fraction, and infinite product representations of Template:Mvar have been developed.
Stochastic representations
In addition to exact analytical expressions for representation of Template:Mvar, there are stochastic techniques for estimating Template:Mvar. One such approach begins with an infinite sequence of independent random variables X_{1}, X_{2}..., drawn from the uniform distribution on [0, 1]. Let V be the least number n such that the sum of the first n samples exceeds 1:
 $V\; =\; \backslash min\; \{\; \backslash left\; \backslash \{\; n\; \backslash mid\; X\_1+X\_2+\backslash cdots+X\_n\; >\; 1\; \backslash right\; \backslash \}\; \}.$
Then the expected value of V is Template:Mvar: E(V) = e.^{[15]}^{[16]}
Known digits
The number of known digits of Template:Mvar has increased dramatically during the last decades. This is due both to the increased performance of computers and to algorithmic improvements.^{[17]}^{[18]}
Number of known decimal digits of Template:Mvar
Date 
Decimal digits 
Computation performed by

1748 
23 
Leonhard Euler^{[19]}

1853 
137 
William Shanks

1871 
205 
William Shanks

1884 
346 
J. Marcus Boorman

1949 
2,010 
John von Neumann (on the ENIAC)

1961 
100,265 
Daniel Shanks and John Wrench^{[20]}

1978 
116,000 
Steve Wozniak on the Apple II^{[21]}

1994 April 1 
1,000,000 
Robert Nemiroff & Jerry Bonnell ^{[22]}

1999 November 21 
1,250,000,000 
Xavier Gourdon ^{[23]}

2000 July 16 
3,221,225,472 
Colin Martin & Xavier Gourdon ^{[24]}

2003 September 18 
50,100,000,000 
Shigeru Kondo & Xavier Gourdon ^{[25]}

2007 April 27 
100,000,000,000 
Shigeru Kondo & Steve Pagliarulo ^{[26]}

2009 May 6 
200,000,000,000 
Rajesh Bohara & Steve Pagliarulo ^{[26]}

2010 July 5 
1,000,000,000,000 
Shigeru Kondo & Alexander J. Yee ^{[27]}

In computer culture
In contemporary internet culture, individuals and organizations frequently pay homage to the number Template:Mvar.
For example, in the IPO filing for Google, in 2004, rather than a typical roundnumber amount of money, the company announced its intention to raise $2,718,281,828, which is Template:Mvar billion dollars to the nearest dollar. Google was also responsible for a billboard^{[28]} that appeared in the heart of Silicon Valley, and later in Cambridge, Massachusetts; Seattle, Washington; and Austin, Texas. It read "{first 10digit prime found in consecutive digits of Template:Mvar}.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.^{[29]} The first 10digit prime in Template:Mvar is 7427466391, which starts at the 99th digit.^{[30]}
In another instance, the computer scientist Donald Knuth let the version numbers of his program Metafont approach Template:Mvar. The versions are 2, 2.7, 2.71, 2.718, and so forth. Similarly, the version numbers of his TeX program approach Template:Pi.^{[31]}
Notes
Further reading
 Maor, Eli; Template:Mvar: The Story of a Number, ISBN 0691058547
 Prime Obsession for another stochastic representation
External links
 for the nonmathematician
 2 and 5 million places (link obsolete)
 Approximations – Wolfram MathWorld
 Earliest Uses of Symbols for Constants Jan. 13, 2008
 Gresham College, 28 February 2007 (available for audio and video download)
 Search Engine 2 billion searchable digits of Template:Mvar, Template:Pi and √2
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