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e (mathematical constant)
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{{Redirect|Euler's number|other uses|List of things named after Leonhard Euler#Euler's numbers}}{{Redirect|E (number)|the codes representing food additives|E number}}{{pp-move-indef|small=yes}}(File:hyperbola E.svg|thumb|237px|right|Graph of the equation {{Math|1=y = 1/x}}. Here, {{mvar|e}} is the unique number larger than 1 that makes the shaded area equal to 1.){{E (mathematical constant)}}The number {{mvar|e}} is a mathematical constant that is the base of the natural logarithm: the unique number whose natural logarithm is equal to one. It is approximately equal to 2.71828,Oxford English Dictionary, 2nd ed.: natural logarithm and is the limit of {{math|(1 + 1/n)n}} as {{mvar|n}} approaches infinity, an expression that arises in the study of compound interest. It can also be calculated as the sum of the infinite seriesEncyclopedic Dictionary of Mathematics 142.D
e = sumlimits_{n = 0}^{infty} frac{1}{n!} = frac{1}{1} + frac{1}{1} + frac{1}{1cdot 2} + frac{1}{1cdot 2cdot 3} + cdots
The constant can be characterized in many different ways. For example, it can be defined as the unique positive number {{mvar|a}} such that the graph of the function {{math|1=y = a'x}} has unit slope at {{math|1=x = 0}}.BOOK, Calculus, Jerrold E. Marsden, Alan Weinstein, Springer, 1985, 978-0-387-90974-5,weblink The function {{math|1=f(x) = e'x}} is called the (natural) exponential function, and is the unique exponential function equal to its own derivative. The natural logarithm, or logarithm to base {{mvar|e}}, is the inverse function to the natural exponential function. The natural logarithm of a number {{math|k > 1}} can be defined directly as the area under the curve {{math|1=y = 1/x}} between {{math|1=x = 1}} and {{math|1=x = k}}, in which case {{mvar|e}} is the value of k for which this area equals one (see image). There are alternative characterizations.{{mvar|e}} is sometimes called Euler's number after the Swiss mathematician Leonhard Euler (not to be confused with {{math|Î³}}, the Eulerâ€“Mascheroni constant, sometimes called simply Euler's constant), or as Napier's constant. However, Euler's choice of the symbol {{mvar|e}} is said to have been retained in his honor.WEB, Sondow, Jonathan, e,weblink MathWorld, Wolfram Mathworld, Wolfram Research, 10 May 2011, The constant was discovered by the Swiss mathematician Jacob Bernoulli while studying compound interest.WEB,weblink The number e, MacTutor History of Mathematics, J J, O'Connor, E F, Robertson, The number {{mvar|e}} is of eminent importance in mathematics,BOOK, An Introduction to the History of Mathematics, Howard Whitley Eves, 1969, Holt, Rinehart & Winston, 978-0-03-029558-4, alongside 0, 1, {{pi}} and {{mvar|i}}. 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 {{pi}}, {{mvar|e}} is irrational: it is not a ratio of integers. Also like {{pi}}, {{mvar|e}} is transcendental: it is not a root of any non-zero polynomial with rational coefficients. The numerical value of {{mvar|e}} truncated to 50 decimal places is{{block indent
| {{gaps|2.71828|18284|59045|23536|02874|71352|66249|77572|47093|69995...}} {{OEIS|A001113}}.
}}

## 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 in 1683,Jacob Bernoulli considered the problem of continuous compounding of interest, which led to a series expression for e. See: Jacob Bernoulli (1690) "QuÃ¦stiones nonnullÃ¦ de usuris, cum solutione problematis de sorte alearum, propositi in Ephem. Gall. A. 1685" (Some questions about interest, with a solution of a problem about games of chance, proposed in the Journal des Savants (Ephemerides Eruditorum GallicanÃ¦), in the year (anno) 1685.), Acta eruditorum, pp. 219â€“23. On page 222, Bernoulli poses the question: "Alterius naturÃ¦ hoc Problema est: QuÃ¦ritur, si creditor aliquis pecuniÃ¦ summam fÃ¦nori exponat, ea lege, ut singulis momentis pars proportionalis usurÃ¦ annuÃ¦ sorti annumeretur; quantum ipsi finito anno debeatur?" (This is a problem of another kind: The question is, if some lender were to invest [a] sum of money [at] interest, let it accumulate, so that [at] every moment [it] were to receive [a] proportional part of [its] annual interest; how much would he be owed [at the] end of [the] year?) Bernoulli constructs a power series to calculate the answer, and then writes: " â€¦ quÃ¦ nostra serie [mathematical expression for a geometric series] &c. major est. â€¦ si a=b, debebitur plu quam 2Â½a & minus quam 3a." ( â€¦ which our series [a geometric series] is larger [than]. â€¦ if a=b, [the lender] will be owed more than 2Â½a and less than 3a.) If a=b, the geometric series reduces to the series for a Ã— e, so 2.5 < e < 3. ( The reference is to a problem which Jacob Bernoulli posed and which appears in the Journal des SÃ§avans of 1685 at the bottom of page 314.)BOOK, Carl Boyer, Uta Merzbach, Uta Merzbach, A History of Mathematics, 419, Wiley, 1991, 2nd, who attempted to find the value of the following expression (which is in fact {{mvar|e}}):
lim_{ntoinfty} left( 1 + frac{1}{n} right)^n.
The first known use of the constant, represented by the letter {{math|b}}, was in correspondence from Gottfried Leibniz to Christiaan Huygens in 1690 and 1691. Leonhard Euler introduced the letter {{mvar|e}} as the base for natural logarithms, writing in a letter to Christian Goldbach on 25 November 1731.Lettre XV. Euler Ã  Goldbach, dated November 25, 1731 in: P.H. Fuss, ed., Correspondance MathÃ©matique et Physique de Quelques CÃ©lÃ¨bres GÃ©omÃ¨tres du XVIIIeme SiÃ¨cle â€¦ (Mathematical and physical correspondence of some famous geometers of the 18th century), vol. 1, (St. Petersburg, Russia: 1843), pp. 56â€“60, see especially p. 58. From p. 58: " â€¦ ( e denotat hic numerum, cujus logarithmus hyperbolicus est = 1), â€¦ " ( â€¦ (e denotes that number whose hyperbolic [i.e., natural] logarithm is equal to 1) â€¦ )BOOK, Remmert, Reinhold, Reinhold Remmert, Theory of Complex Functions, 136, Springer-Verlag, 1991, 978-0-387-97195-7, {{inconsistent citations, }} Euler started to use the letter {{mvar|e}} for the constant in 1727 or 1728, in an unpublished paper on explosive forces in cannons,Euler, Meditatio in experimenta explosione tormentorum nuper instituta. and the first appearance of {{mvar|e}} in a publication was in Euler's Mechanica (1736).Leonhard Euler, Mechanica, sive Motus scientia analytice exposita (St. Petersburg (Petropoli), Russia: Academy of Sciences, 1736), vol. 1, Chapter 2, Corollary 11, paragraph 171, p. 68. From page 68: Erit enim frac{dc}{c} = frac{dy ds}{rdx} seu c = e^{intfrac{dy ds}{rdx}} ubi e denotat numerum, cuius logarithmus hyperbolicus est 1. (So it [i.e., c, the speed] will be frac{dc}{c} = frac{dy ds}{rdx} or c = e^{intfrac{dy ds}{rdx}}, where e denotes the number whose hyperbolic [i.e., natural] logarithm is 1.) While in the subsequent years some researchers used the letter {{math|c}}, the letter {{mvar|e}} was more common and eventually became standard.{{citation needed|date=October 2017}}In mathematics, the standard is to typeset the constant as "{{mvar|e}}", in italics; the ISO 80000-2:2009 standard recommends typesetting constants in an upright style, but this has not been validated by scientific community.

## Applications

### Compound interest

(File:Compound Interest with Varying Frequencies.svg|thumb|right|350px|The effect of earning 20% annual interest on an {{nowrap|initial \$1,000}} investment at various compounding frequencies)Jacob Bernoulli discovered this constant in 1683 by studying a question about compound interest: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 {{nowrap|1=\$1.00 Ã— 1.52 = \$2.25}} at the end of the year. Compounding quarterly yields {{nowrap|1=\$1.00 Ã— 1.254 = \$2.4414â€¦}}, and compounding monthly yields {{nowrap|1=\$1.00 Ã— (1 + 1/12)12 = \$2.613035â€¦}} If there are {{math|n}} compounding intervals, the interest for each interval will be {{math|100%/n}} and the value at the end of the year will be \$1.00Ã—{{math|1=(1 + 1/n)n}}.Bernoulli noticed that this sequence approaches a limit (the force of interest) with larger {{math|n}} and, thus, smaller compounding intervals. Compounding weekly ({{math|1=n = 52}}) yields \$2.692597..., while compounding daily ({{math|1=n = 365}}) yields \$2.714567..., just two cents more. The limit as {{math|n}} grows large is the number that came to be known as {{mvar|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 {{math|R}} will, after {{math|t}} years, yield {{math|eRt}} dollars with continuous compounding. (Here {{math|R}} is the decimal equivalent of the rate of interest expressed as a percentage, so for 5% interest, {{math|1=R = 5/100 = 0.05}}.)

### Bernoulli trials

(File:Bernoulli trial sequence.svg|thumb|300px|Graphs of probability P of {{em|not}} observing independent events each of probability 1/n after n Bernoulli trials, and 1 âˆ’ P  vs n ; it can be observed that as n increases, the probability of a 1/n-chance event never appearing after n tries rapidly {{nowrap|converges to 1/e.}})The number {{mvar|e}} 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 {{math|n}} and plays it {{math|n}} times. Then, for large {{math|n}} (such as a million) the probability that the gambler will lose every bet is approximately {{math|1/e}}. For {{math|1=n = 20}} it is already approximately 1/2.79.This is an example of a Bernoulli trial 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 and Pascal's triangle. The probability of winning {{math|k}} times out of a million trials is:
binom{10^6}{k} left(10^{-6}right)^kleft(1 - 10^{-6}right)^{10^6-k}.
In particular, the probability of winning zero times ({{math|1=k = 0}}) is
left(1 - frac{1}{10^6}right)^{10^6}.
This is very close to the limit
lim_{ntoinfty} left(1 - frac{1}{n}right)^n = frac{1}{e}.

### Standard normal distribution

The normal distribution with zero mean and unit standard deviation is known as the standard normal distribution, given by the probability density function
phi(x) = frac{1}{sqrt{2pi}} e^{-frac{1}{2} x^2}.
The constraint of unit variance (and thus also unit standard deviation) results in the {{frac2|1|2}} in the exponent, and the constraint of unit total area under the curve {{math|Ï•(x)}} results in the factor textstyle 1/sqrt{2pi}.[proof] This function is symmetric around {{math|1=x = 0}}, where it attains its maximum value textstyle 1/sqrt{2pi}, and has inflection points at {{math|1=x = Â±1}}.

### Derangements

Another application of {{mvar|e}}, 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:Grinstead, C.M. and Snell, J.L.Introduction to probability theory (published online under the GFDL), p. 85. {{math|n}} guests are invited to a party, and at the door, the guests all check their hats with the butler, who in turn places the hats into {{math|n}} boxes, each labelled with the name of one guest. But the butler has not asked 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 {{math|n}} of guests tends to infinity, {{math|pn}} approaches {{math|1/e}}. Furthermore, the number of ways the hats can be placed into the boxes so that none of the hats are in the right box is {{math|n!/e}} rounded to the nearest integer, for every positive {{math|n}}.Knuth (1997) The Art of Computer Programming Volume I, Addison-Wesley, p. 183 {{isbn|0-201-03801-3}}.

### Optimal planning problems

A stick of length {{mvar|L}} is broken into {{mvar|n}} equal parts. The value of {{mvar|n}} that maximizes the product of the lengths is then eitherBOOK, Mathematical constants, Steven Finch, 2003, Cambridge University Press, 14,
n = leftlfloor frac{L}{e} rightrfloor or leftlfloor frac{L}{e} rightrfloor + 1.
The stated result follows because the maximum value of x^{-1}ln x occurs at x = e (Steiner's problem, discussed below). The quantity x^{-1}ln x is a measure of information gleaned from an event occurring with probability 1/x, so that essentially the same optimal division appears in optimal planning problems like the secretary problem.

### Asymptotics

The number {{mvar|e}} occurs naturally in connection with many problems involving asymptotics. An example is Stirling's formula for the asymptotics of the factorial function, in which both the numbers {{mvar|e}} and {{pi}} enter:
n! sim sqrt{2pi n} left(frac{n}{e}right)^n.
As a consequence,
e = lim_{ntoinfty} frac{n}{sqrt[n]{n!}} .

## In calculus

thumb|right|The graphs of the functions {{math|x â†¦ a{{sup|x}}}} are shown for {{math|1=a = 2}} (dotted), {{math|1=a = e}} (blue), and {{math|1=a = 4}} (dashed). They all pass through the point {{math|(0,1)}}, but the red line (which has slope {{math|1}}) is tangent to only {{math|e{{sup|x}}}} there.thumb|right|The value of the natural log function for argument {{mvar|e}}, i.e. {{math|ln(e)}}, equals {{math|1.}}The principal motivation for introducing the number {{mvar|e}}, particularly in calculus, is to perform differential and integral calculus with exponential functions and logarithms.Kline, M. (1998) Calculus: An intuitive and physical approach, section 12.3 "The Derived Functions of Logarithmic Functions.", pp. 337 ff, Courier Dover Publications, 1998, {{isbn|0-486-40453-6}} A general exponential {{nowrap|function {{math|y {{=}} ax}}}} has a derivative, given by a limit:
begin{align}
frac{d}{dx}a^x
&= lim_{hto 0}frac{a^{x+h} - a^x}{h} = lim_{hto 0}frac{a^x a^h - a^x}{h}
&= a^x cdot left(lim_{hto 0}frac{a^h - 1}{h}right).
end{align}The parenthesized limit on the right is independent of the {{nowrap|variable {{math|x}}:}} it depends only on the {{nowrap|base {{math|a}}.}} When the base is set {{nowrap|to {{mvar|e}},}} this limit is equal {{nowrap|to {{math|1}},}} and so {{mvar|e}} is symbolically defined by the equation:
frac{d}{dx}e^x = e^x.
Consequently, the exponential function with base {{mvar|e}} is particularly suited to doing calculus. {{nowrap|Choosing {{mvar|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 derivative of the base-{{math|a}} logarithm,This is the approach taken by Kline (1998). i.e., of {{math|loga x}} for {{math|x > 0}}:
begin{align}
frac{d}{dx}log_a x
&= lim_{hto 0}frac{log_a(x + h) - log_a(x)}{h}
&= lim_{hto 0}frac{log_a(1 + h/x)}{xcdot h/x}
&= frac{1}{x}log_aleft(lim_{uto 0}(1 + u)^frac{1}{u}right)
&= frac{1}{x}log_a e,
end{align}where the substitution {{math|u {{=}} h/x}} was made. The {{mvar|a}}-logarithm of {{mvar|e}} is 1, if {{mvar|a}} equals {{mvar|e}}. So symbolically,
frac{d}{dx}log_e x = frac{1}{x}.
The logarithm with this special base is called the natural logarithm and is denoted as {{math|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 such special numbers {{mvar|a}}. One way is to set the derivative of the exponential function {{math|a'x}} equal to {{math|a'x}}, and solve for {{math|a}}. The other way is to set the derivative of the base {{math|a}} logarithm to {{math|1/x}} and solve for {{math|a}}. In each case, one arrives at a convenient choice of base for doing calculus. It turns out that these two solutions for {{mvar|a}} are actually the same, the number {{mvar|e}}.

### Alternative characterizations

File:Part38figure02.gif|thumb|right|The five colored regions are of equal area, and define units of hyperbolic angle along the {{nowrap|hyperbolahyperbola{{See also|#Representations|Characterizations of the exponential function}}Other characterizations of {{mvar|e}} 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 {{mvar|e}} is the unique positive real number such that frac{d}{dt}e^t = e^t.
2. The number {{mvar|e}} is the unique positive real number such that frac{d}{dt} log_e t = frac{1}.
The following four characterizations can be proven equivalent:{hide}ordered list
| start=3
| The number {{mvar|e{edih} is the limit

e = lim_{ntoinfty} left( 1 + frac{1}{n} right)^n
Similarly:
e = lim_{tto 0} left( 1 + t right)^{frac{1}}
| The number {{mvar|e}} is the sum of the infinite series

e = sum_{n = 0}^infty frac{1}{n!} = frac{1}{0!} + frac{1}{1!} + frac{1}{2!} + frac{1}{3!} + frac{1}{4!} + cdots ,
where {{math|n!}} is the factorial of {{math|n}}. (By convention {0!} = 1.)
| The number {{mvar|e}} is the unique positive real number such that

int_1^e frac{1} , dt = 1.
| If {{math|f(t)}} is an exponential function, then the quantity tau = f(t)/f'(t) is a constant, sometimes called the time constant (it is the reciprocal of the exponential growth constant or decay constant). The time constant is the time it takes for the exponential function to increase by a factor of {{math|e}}: f(t+tau) = e f(t).
}}

## Properties

### Calculus

As in the motivation, the exponential function {{math|ex}} 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:
int e^x,dx = e^x + C .

### Inequalities

(File:Exponentials vs x+1.pdf|thumb|right|Exponential functions y = 2^x and y = 4^x intersect the graph of y = x + 1, respectively, at x = 1 and x = -1/2. The number e is the unique base such that y=e^x intersects only at x = 0. We may infer that e lies between 2 and 4.)The number {{math|e}} is the unique real number such that
left(1 + frac{1}{x}right)^x < e < left(1 + frac{1}{x}right)^{x+1}
for all positive {{math|x}}.BOOK, Dorrie, Heinrich, 100 Great Problems of Elementary Mathematics, 1965, Dover, 44â€“48, Also, we have the inequality
e^x ge x + 1
for all real {{math|x}}, with equality if and only if {{math|x {{=}} 0}}. Furthermore, {{math|e}} is the unique base of the exponential for which the inequality {{math|ax â‰¥ x + 1}} holds for all {{math|x}}.A standard calculus exercise using the mean value theorem; see for example Apostol (1967) Calculus, Â§6.17.41. This is a limiting case of Bernoulli's inequality.

### Exponential-like functions

Image:Xth root of x.svg|thumb|right|250px|The occurs at {{math|x {{=}} e}}.}}Steiner's problem asks to find the global maximum for the function
f(x) = x^frac{1}{x} .
This maximum occurs precisely at {{math|x {{=}} e}}. For proof, the inequality e^y ge y + 1, from above, evaluated at y = (x - e)/e and simplifying gives e^{x/e} ge x. So e^{1/e} ge x^{1/x} for all positive x.BOOK, Dorrie, Heinrich, 100 Great Problems of Elementary Mathematics, 1965, Dover, 359, Similarly, {{math|x {{=}} 1/e}} is where the global minimum occurs for the function
f(x) = x^x
defined for positive {{math|x}}. More generally, for the function
f(x) = x^{x^n}
the global maximum for positive {{math|x}} occurs at {{math|x {{=}} 1/e}} for any {{math|n < 0}}; and the global minimum occurs at {{math|x {{=}} eâˆ’1/n}} for any {{math|n > 0}}.The infinite tetration
x^{x^{x^{cdot^{cdot^{cdot}}}}} or {^infty}x
converges if and only if {{math|eâˆ’e â‰¤ x â‰¤ e1/e}} (or approximately between 0.0660 and 1.4447), due to a theorem of Leonhard Euler.Euler, L. "De serie Lambertina Plurimisque eius insignibus proprietatibus." Acta Acad. Scient. Petropol. 2, 29â€“51, 1783. Reprinted in Euler, L. Opera Omnia, Series Prima, Vol. 6: Commentationes Algebraicae. Leipzig, Germany: Teubner, pp. 350â€“369, 1921. (facsimile)

### Number theory

The real number {{mvar|e}} is irrational. Euler proved this by showing that its simple continued fraction expansion is infinite.WEB,weblink How Euler Did It: Who proved {{mvar, e, is Irrational?|last=Sandifer|first=Ed|date=Feb 2006|publisher=MAA Online|accessdate=2010-06-18|url-status=dead|archiveurl=https://web.archive.org/web/20140223072640weblink|archivedate=2014-02-23|df=}} (See also Fourier's proof that {{math|e}} is irrational.)Furthermore, by the Lindemannâ€“Weierstrass theorem, {{mvar|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 {{mvar|e}} is normal, meaning that when {{math|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 {{math|ex}} 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 is convergent for every complex value of {{mvar|x}}, it is commonly used to extend the definition of {{math|ex}} to the complex numbers. This, with the Taylor series for {{math|sin}} and {{math|cos x}}, allows one to derive Euler's formula:
e^{ix} = cos x + isin x ,
which holds for every complex {{math|x}}. The special case with {{math|x {{=}} Ï€}} is Euler's identity:
e^{ipi} + 1 = 0 ,
from which it follows that, in the principal branch of the logarithm,
ln (-1) = ipi .
Furthermore, using the laws for exponentiation,
(cos x + isin 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 {{math|cis(x)}}.The expressions of sin x and cos x in terms of the exponential function can be deduced:
sin x = frac{e^{ix} - e^{-ix}}{2i} , qquad
cos x = frac{e^{ix} + e^{-ix}}{2}.

### Differential equations

The general function
y(x) = Ce^x
is the solution to the differential equation:
y' = y .

## Representations

}}The number {{mvar|e}} 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_{ntoinfty}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 {{math|ex}} at {{math|x {{=}} 1}}.Less common is the continued fraction
e = [2; 1, 2, 1, 1, 4, 1, 1, 6, 1, ..., 1, 2n, 1, ...],
Hofstadter, D.R., "Fluid Concepts and Creative Analogies: Computer Models of the Fundamental Mechanisms of Thought" Basic Books (1995) {{isbn|0-7139-9155-0}}{{OEIS|id=A003417}}which written out looks like
e = 2 +
cfrac{1}
{1 + cfrac{1}
{2 + cfrac{1}
{1 + cfrac{1}
{1 + cfrac{1}
{4 + cfrac{1}
{1 + cfrac{1}
{1 + ddots}
}
}
}
}
}
}
.This continued fraction for {{mvar|e}} converges three times as quickly:{{citation needed|date=September 2018}}
e = 1 + cfrac{2}{1 + cfrac{1}{6 + cfrac{1}{10 + cfrac{1}{14 + cfrac{1}{18 + cfrac{1}{22 + cfrac{1}{26 + ddots}}}}}}}.
Many other series, sequence, continued fraction, and infinite product representations of {{mvar|e}} have been developed.

### Stochastic representations

In addition to exact analytical expressions for representation of {{mvar|e}}, there are stochastic techniques for estimating {{mvar|e}}. One such approach begins with an infinite sequence of independent random variables {{math|X1}}, {{math|X2}}..., drawn from the uniform distribution on [0, 1]. Let {{math|V}} be the least number {{math|n}} such that the sum of the first {{math|n}} observations exceeds 1:
V = minleft{ n mid X_1 + X_2 + cdots + X_n > 1 right}.
Then the expected value of {{math|V}} is {{mvar|e}}: {{math|E(V) {{=}} e}}.Russell, K.G. (1991) Estimating the Value of e by Simulation The American Statistician, Vol. 45, No. 1. (Feb., 1991), pp. 66â€“68.Dinov, ID (2007) Estimating e using SOCR simulation, SOCR Hands-on Activities (retrieved December 26, 2007).

### Known digits

The number of known digits of {{mvar|e}} has increased substantially during the last decades. This is due both to the increased performance of computers and to algorithmic improvements.Sebah, P. and Gourdon, X.; The constant e and its computationGourdon, X.; Reported large computations with PiFast{| class="wikitable" style="margin: 1em auto 1em auto"e}}! Date || Decimal digits || Computation performed by
1 Jacob Bernoulli
13 Roger CotesRoger Cotes (1714) "Logometria," Philosophical Transactions of the Royal Society of London, 29 (338) : 5â€“45; weblink" title="archive.is/20140410203227weblink">see especially the bottom of page 10. From page 10: "Porro eadem ratio est inter 2,718281828459 &c et 1, â€¦ " (Furthermore, by the same means, the ratio is between 2.718281828459â€¦ and 1, â€¦ )
23 Leonhard EulerLeonhard Euler, Introductio in Analysin Infinitorum (Lausanne, Switzerland: Marc Michel Bousquet & Co., 1748), volume 1, page 90.
137 William ShanksWilliam Shanks, Contributions to Mathematics, â€¦ (London, England: G. Bell, 1853), page 89.
205 William ShanksWilliam Shanks (1871) "On the numerical values of e, loge 2, loge 3, loge 5, and loge 10, also on the numerical value of M the modulus of the common system of logarithms, all to 205 decimals," Proceedings of the Royal Society of London, 20 : 27â€“29.
346 J. Marcus BoormanJ. Marcus Boorman (October 1884) "Computation of the Naperian base," Mathematical Magazine, 1 (12) : 204â€“205.
2,010 John von Neumann (on the ENIAC)
100,265 Daniel Shanks and John WrenchDANIEL SHANKS AND JOHN W WRENCH>QUOTE=WE HAVE COMPUTED E ON A 7090 TO 100,265D BY THE OBVIOUS PROGRAMJOURNAL =MATHEMATICS OF COMPUTATIONYEAR=1962 PAGES =76â€“99 (78)DOI=10.2307/2003813, 2003813,
116,000 Steve Wozniak on the Apple IIHTTPS://ARCHIVE.ORG/STREAM/BYTE-MAGAZINE-1981-06/1981_06_BYTE_06-06_OPERATING_SYSTEMS#PAGE/N393/MODE/2UP > TITLE=THE IMPOSSIBLE DREAM: COMPUTING E TO 116,000 PLACES WITH A PERSONAL COMPUTER DATE=JUNE 1981 AUTHOR=WOZNIAK, STEVE, 392,
Since that time, the proliferation of modern high-speed desktop computers has made it possible for amateurs, with the right hardware, to compute trillions of digits of e.WEB, e,weblink Alexander Yee,

## Notes

{{Reflist}}

• Maor, Eli; {{mvar|e}}: The Story of a Number, {{isbn|0-691-05854-7}}
• Commentary on Endnote 10 of the book Prime Obsession for another stochastic representation
• JOURNAL, McCartin, Brian J., e: The Master of All, The Mathematical Intelligencer, 28, 2006, 2, 10â€“21,weblink 10.1007/bf02987150,

{{Commons category|E (mathematical constant)}}
{{Irrational number}}{{Good article}}{{DISPLAYTITLE:{{mvar|e}} (mathematical constant)}}

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