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{{DISPLAYTITLE:Proof that ''e'' is irrational}}
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{{E (mathematical constant)}}


The number ''e'' was introduced by [[Jacob Bernoulli]] in 1683. More than half a century later, [[Leonhard Euler|Euler]], who had been a student of Jacob's younger brother [[Johann Bernoulli|Johann]], proved that ''e'' is irrational, that is, that it can not be expressed as the quotient of two integers.
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==Euler's proof==
Euler wrote the first proof of the fact that ''e'' is irrational in 1737 (but the text was published only seven years later).<ref>{{cite journal | last = Euler | first = Leonhard | date = 1744 | title = De fractionibus continuis dissertatio | url = http://www.math.dartmouth.edu/~euler/docs/originals/E071.pdf | journal = Commentarii academiae scientiarum Petropolitanae | volume = 9 | pages = 98–137 | trans_title = A dissertation on continued fractions}}</ref><ref>{{cite journal | last = Euler | first = Leonhard | title = An essay on continued fractions | journal = Mathematical Systems Theory | volume = 18 | pages = 295–398 | url = https://kb.osu.edu/dspace/handle/1811/32133 | publication-date = 1985}}</ref><ref>{{cite book | last1 = Sandifer | first1 = C. Edward | title = How Euler did it | chapter = Chapter 32: Who proved ''e'' is irrational? | publisher = [[Mathematical Association of America]] | pages = 185–190 | year = 2007 | isbn = 978-0-88385-563-8 | lccn = 2007927658}}</ref> He computed the representation of ''e'' as a [[simple continued fraction]], which is
:<math>e = [2; 1, 2, 1, 1, 4, 1, 1, 6, 1, 1, 8, 1, 1, \ldots, 2n, 1, 1, \ldots]. \,</math>
Since this continued fraction is infinite, ''e'' is irrational. A short proof of the previous equality is known.<ref>{{cite journal | last = Cohn | first = Henry | journal = [[American Mathematical Monthly]] | volume = 113 | issue = 1 | pages = 57–62 | publisher = [[Mathematical Association of America]] | date = 2006 | title = A short proof of the simple continued fraction expansion of ''e'' | jstor = 27641837}}</ref> Since the simple continued fraction of ''e'' is not [[Periodic continued fraction|periodic]], this also proves that ''e'' is not a root of second degree polynomial with rational coefficients; in particular, ''e''<sup>2</sup> is irrational.
 
==Fourier's proof==
The most well-known proof is [[Joseph Fourier]]'s [[reductio ad absurdum|proof by contradiction]],<ref>{{Cite book | last1 = de Stainville | first1 = Janot | date = 1815 | title = Mélanges d'Analyse Algébrique et de Géométrie | trans_title = A mixture of Algebraic Analysis and Geometry | publisher = Veuve Courcier | pages = 340–341}}</ref> which is based upon the equality
 
: <math>e = \sum_{n = 0}^{\infty} \frac{1}{n!}\cdot</math>
 
Initially ''e'' is assumed to be a rational number of the form <sup>''a''</sup>&frasl;<sub>''b''</sub>. We then analyze a blown-up difference ''x'' of the series representing ''e'' and its strictly smaller {{nowrap|''b''<sup>&thinsp;th</sup>}} partial sum, which approximates the limiting value ''e''. By choosing the magnifying factor to be the [[factorial]] of&nbsp;''b'', the fraction <sup>''a''</sup>&frasl;<sub>''b''</sub> and the {{nowrap|''b''<sup>&thinsp;th</sup>}} partial sum are turned into integers, hence ''x'' must be a positive integer. However, the fast convergence of the series representation implies that the magnified approximation error  ''x'' is still strictly smaller than&nbsp;1. From this contradiction we deduce that ''e'' is irrational.
 
Suppose that ''e'' is a [[rational number]]. Then there exist positive integers ''a'' and ''b'' such that ''e''&nbsp;=&nbsp;<sup>''a''</sup>&frasl;<sub>''b''</sub>. Define the number
 
:<math>
x = b!\,\biggl(e - \sum_{n = 0}^{b} \frac{1}{n!}\biggr)\!
</math>
 
To see that if ''e'' is rational, then ''x'' is an integer, substitute ''e'' = <sup>''a''</sup>&frasl;<sub>''b''</sub> into this definition to obtain
 
:<math>
x = b!\,\biggl(\frac{a}{b} - \sum_{n = 0}^{b} \frac{1}{n!}\biggr)
= a(b - 1)! - \sum_{n = 0}^{b} \frac{b!}{n!}\,.
</math>
 
The first term is an integer, and every fraction in the sum is actually an integer because ''n''&nbsp;≤&nbsp;''b'' for each term. Therefore ''x'' is an integer.
 
We now prove that {{nowrap|0 < ''x'' < 1}}. First, to prove that ''x'' is strictly positive, we insert the above series representation of ''e'' into the definition of ''x'' and obtain
 
:<math>x =  b!\,\biggl(\sum_{n = 0}^{\infty} \frac{1}{n!} - \sum_{n = 0}^{b} \frac{1}{n!}\biggr) = \sum_{n = b+1}^{\infty} \frac{b!}{n!}>0\,,\!</math>
 
because all the terms are strictly positive.
 
We now prove that ''x''&nbsp;<&nbsp;1. For all terms with {{nowrap|''n'' ≥ ''b'' + 1}} we have the upper estimate
:<math>\frac{b!}{n!}
=\frac1{(b+1)(b+2)\cdots(b+(n-b))}
\le\frac1{(b+1)^{n-b}}\,.\!
</math>
This inequality is strict for every ''n''&nbsp;≥&nbsp;''b''&nbsp;+&nbsp;2. Changing the index of summation to ''k''&nbsp;=&nbsp;''n''&nbsp;–&nbsp;''b'' and using the formula for the [[Geometric series#Infinite geometric series|infinite geometric series]], we obtain
:<math>
x
=\sum_{n = b+1}^\infty \frac{b!}{n!}
< \sum_{n=b+1}^\infty \frac1{(b+1)^{n-b}}
=\sum_{k=1}^\infty \frac1{(b+1)^k}
=\frac{1}{b+1} \biggl(\frac1{1-\frac1{b+1}}\biggr)
= \frac{1}{b}
\le 1.
</math>
 
Since there is no integer strictly between 0 and 1, we have reached a contradiction, and so ''e'' must be irrational. [[Q.E.D.]]
 
==Alternate proofs==
Another proof<ref>{{Citation | last1 = MacDivitt | first1 = A. R. G. | last2 = Yanagisawa | first2 = Yukio | title = An elementary proof that ''e'' is irrational | journal = [[The Mathematical Gazette]] | volume = 71 | issue = 457 | pages = 217 | year = 1987 | publisher =[[Mathematical Association]] | place = London | jstor = 3616765}}</ref> can be obtained from the previous one by noting that
:<math>(b+1)x=1+\frac1{b+2}+\frac1{(b+2)(b+3)}+\cdots<1+\frac1{b+1}+\frac1{(b+1)(b+2)}+\cdots=1+x,</math>
and this inequality is equivalent to the assertion that ''bx''&nbsp;<&nbsp;1. This is impossible, of course, since ''b'' and ''x'' are natural numbers.
 
Still another proof<ref>{{cite journal | last = Penesi | first = L. L. | date = 1953 | title = Elementary proof that ''e'' is irrational | journal = [[American Mathematical Monthly]] | publisher = [[Mathematical Association of America]] | volume = 60 | issue = 7 | pages = 474 | jstor = 2308411 }}</ref> can be obtained from the fact that
:<math>\frac1e=e^{-1}=\sum_{n=0}^\infty\frac{(-1)^n}{n!}\cdot</math>
==Generalizations==
In 1840, [[Joseph Liouville|Liouville]] published a proof of the fact that ''e''<sup>2</sup> is irrational<ref>{{cite journal | last = Liouville | first = Joseph | journal = [[Journal de Mathématiques Pures et Appliquées]] | title = Sur l'irrationalité du nombre ''e'' = 2,718&hellip; | series = 1 | volume = 5 | pages = 192 | year = 1840 | language = french}}</ref> followed by a proof that ''e''<sup>2</sup> is not a root of a second degree polynomial with rational coefficients.<ref>{{cite journal | last = Liouville | first = Joseph | journal = [[Journal de Mathématiques Pures et Appliquées]] | title = Addition à la note sur l'irrationnalité du nombre ''e'' | series = 1 | volume = 5 | pages = 193–194 | year = 1840 | language = french}}</ref> This last fact implies that ''e''<sup>4</sup> is irrational. His proofs are similar to Fourier's proof of the irrationality of ''e''.  
 
More generally, ''e''<sup>''q''</sup> is irrational for any non-zero rational ''q''.<ref>{{Citation | last1=Aigner | first1=Martin | author1-link = Martin Aigner | last2=Ziegler | first2=Günter M. | author2-link=Günter M. Ziegler | title=[[Proofs from THE BOOK]] | publisher=[[Springer-Verlag]] | location=Berlin, New York | year=1998|pages=27–36|isbn=978-3-642-00855-9|doi=10.1007/978-3-642-00856-6|edition=4th}}.</ref>
 
==See also==
*[[Characterizations of the exponential function]]
*[[Transcendental number]], including a [[Transcendental_number#Sketch_of_a_proof_that_e_is_transcendental|proof that ''e'' is transcendental]]
*[[Lindemann–Weierstrass theorem]]
 
==References==
<references/>
 
[[Category:Diophantine approximation]]
[[Category:Exponentials]]
[[Category:Article proofs]]
[[Category:E (mathematical constant)]]

Revision as of 18:49, 2 March 2014

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